DIY build of a Tektronix SG502 Sinewave Generator

Because I sold all of my Tektronix gear, I was a bit sad to loose three particular instruments.
One was the DIY 5CT Curve Tracer with my readout modifications that was already covered in another post, the 5A22N Differential Amplifier, and the SG502 Sinewave Generator.

When I was at Tek, I build several instruments from parts, the SG502 being one of them.

I really liked the SG502, for its simplicity using analog(!) discrete parts only, and the overall specifications that made it perfect for most if not all of my applications. This instrument covers the frequency range from <5Hz to >500KH and has a pretty good distortion performance with only 0.0035% THD between 20Hz and 50Khz. The output is 5V RMS open circuit or 2.5V RMS into 50 Ohm. It also has a good step attenuation ranging from 0-70dB. Finally, it has a 5Vp-p square wave output that can also function as a trigger out.

The reason I seldom used it over the last years was that it comes as a TM500 plugin, and that uses a lot of real estate. The TM500 series are very deep, and they are heavy. I don't have room for them on my desk or counter anymore, so eventually I cut the umbilical cord and sold everything I had from Tek. Well, not really everything (sorry Raymond). I kept a few goodies.

I also have the so called Victor Mickevich Ultra Low Distortion 1kHz sinewave generator with a reported 0.00001% THD for special measurements (also described in another post: simple-but-precisice-1khz-distortion-tool), and I recently purchased the FeelTech FY6600S-30 14 bit DDS. That instrument is a Dual Channel Function/Arbitrary Waveform Generator, very versatile, but, it's digital...

If you consider building one, the SG502 has a few critical or rather special (unobtanium) parts. One is the dual N J-FET (Tek p/n 151-1054-00 or the 2N3958), used for the input differential amplification, another J-FET (Tek pn/ 151-1021-00 or the FN815) used in the AGC, and then the precision dual 10K tracking pot and satellite adjustment contraption, used to set the frequency. Less critical is the matched capacitor set (10uF, 1uF, 0.1uF 0.01uF and 0.001uF). They are used in the bridged T notch filter in a rather clever dual purpose way. The last rather special item is the SG3501D (156-0208-00) IC that is the center of the dual tracking +/- 20V power supply. I recently found that this IC died in my SG502, but I was able to order replacements on flea-bay.

I happened to have all these items as "spare" parts, and kept them since the early 70's, waiting to be used again. I did not have the special matched set of timing capacitors, but individual ones and testing revealed that they were so close and precise that they didn't need any further matching or adjusting.

Just to kinda take care of my guilt in letting my trusted and self build Tek gear go, I decided to rebuild the SG502, but in a much smaller enclosure. I use the TEKO KL22 enclosure in black/aluminum a lot for my projects. They cost less than 15 Euros, and have just the right size for most of my projects. I used them already for my three power supplies, my DC load, and now my SG502.

I first dabbled with the idea to upgrade the design with modern OpAmps and see if I could improve on the specifications. After thinking about this for a while, I decided not to. I could have build the SG505, Tek's own upgrade to the SG502 which uses OpAmps, but in my opinion, this classic should stay the way it was designed by Steve Stanger in the early 70's. Period! 

BTW, the PG505 has been described as a real masterpiece of analog design wizardry. The instrument was designed by Bruce Hofer who now is at Audio Precision, and is a true analog design genius. So, my statement "I could have built the SG505", must be taken with a handful grains of salt. It will not be easy to replicate that instrument. (well, it's now 2025 and that's exactly what I'm going to try)

In contrast, rebuilding the SG502 instrument turned out to be rather simple. If you follow some common sense design and layout rules, anybody with a little above average skills can do it. If you can't get your hands on the critical parts, you could try to find an SG502 unit on flea bay or on one of those surplus markets where they sell old electronics. Sometimes these SG's can be bought for less than 20 Euro's. Because all components are THT, it's real easy to harvest and use the most critical parts.

I'm not going to cover the design, you can find the Tektronix Instruction Manual online, here.  It has everything you ever wanted to know about this instrument. One thing you should note is that Tek made some important changes to the original design (especially the ACG, the voltage supply and the output attenuation circuit), and I used the latest available Change Reference (M34075 from 1-19-79) in my redesign. 

Just recently, I found a note from Bruce Hofer, the designer of the SG505 with some additional low distortion modifications that will reduce the distortion to well under 0.002%. Modifications

I have not yet added these modifications myself.

One of the challenges is to get or replace the S50 push button switch set for the frequency selection and also the S160 push buttons used for the output attenuation. I used the same technique again that I already used for my DIY 5CT, and that is by using (reed) relays to do the switching. This will allow you to use inexpensive single deck rotary switches, in combination with diode matrices if required.

Here is the schematic of the range switching for the frequency selection:

Here is the schematic for the AGC damping (top) and the output attenuation:

After completing the unit, and testing it, I noticed a "design flaw" in my output attenuation switching design. When you switch between especially the higher attenuation settings, there is a short moment in-between the "clicks" that the output goes back to full scale. I need to add a delay to the relay fall-off times, to create the equivalent of a make-before-break action, change the rotary switch to make-before-break, or add a master output relays contact that prevents glitches to the DUT in-between setting changes.

I also redesigned the power supply somewhat. Normally, the SG502 uses the big transformer, diode bridge, electrolyte smoothing capacitors and the power transistors from the TM50X mainframe. I measured that the original SG502 uses about 70 mA on each 20V supply rail, so I could get away with a much simpler design.

First of all, I am a big fan of not putting mains transformers into the measurement enclosures. It keeps the hum out, and you don't have to deal with the bulky transformers, the main switch, filter, bridge etc. It allows me to use smaller enclosures, and put the transformer and the needed other stuff in a separate box that I can put someplace out of sight or away from my precious desk or bench space. Another benefit is that I can get multiple usages out of these transformers/supplies boxes.

Here is a photo of the 24-0-24V AC 160mA transformer box :

I didn't produce a schematic for the supply, so let me describe what I did. In the enclosure above, I put a 24-0-24VAC 160mA PCB transformer. I put a dual pole switch, a fuse and a neon indicator lamp on the primary side. On the secondary side, I connected the 24-0-24 AC outputs to 4mm binding posts.

Because the current demands are so small, I put the rectifier (1N4002) diodes directly on the 4 mm binding posts in the SG enclosure and also mounted the two smoothing caps (1000uF/50V) Manhatten style on them. On a little circuit board, I mounted the power section directly from the SG502 manual, and used two smallish power transistors that I had for the series transistors. I selected the voltage setting resistor (R348) to get as close as possible to the +/- 20V DC. Tek also hand selects this resistor, and I ended up with a value of 14K7, probably due to the fact that I used different power transistors.

To drive the two sets of (reed) relays, I wanted to balance the transformer and rectification loads a little, so I used an LM317/LM337 pair with 270 and 820 Ohm resistors to get + and - 5V rails. The +5V section is used to drive all the reed relays for the frequency selection and AGC dampening, and the -5V drives the 3 output attenuation relays. The grounds of both the 5V supplies are not connected to the analog ground on the analog circuit boards.

Here is a photo of the power board:

The left section on the board deals with the +/-5V and the right side with the +/-20V. I just happened to have the SG3501D chip, otherwise I could have used another set of LM317/337 to obtain the +/- 20V supplies. With these  more modern components, I really don't believe the supplies need to be tracking, because the LM317/337 are stable  and good enough.


The oscillator section is mounted on the main board, and looks like this:

Top left is the AGC damping section with 5 reed relays. On the right half is the frequency selection with the two sets of 5 reed relays. The large 10uF precision capacitor is mounted on top of the 1uF and 0.1uF capacitors to save some space. On this picture, the two output transistors(Q82 and Q83) are still the (isolating plastic!) 2N3904 and the 2N3906, they have been replaced by the 2N2222 and the 2N2907 metal can transistors after I was happy with the performance and took the picture. (Watch out for the different pin-out between these transistor types, as I forgot myself (;-o) )

I'm showing the backside with the rats nest, because it's a testament to the quality of the original design that I could stay well within the specifications without using a properly laid out PCB.

The output amplifiers for the sine wave and the output attenuation, in addition to the square wave generation and amplifier are on a separate board.

 

This board will be mounted through the output level potmeter to the front panel, and also on a stud to the main board.

The rest is mounted directly on the front panel:
The open hole on the left is for the output potmeter, and the hole on the right for the power LED.

To the left is the 5 position  rotary switch for the frequency multiplier then the special potmeter with the fine adjustment hardware contraption, and to the right the 8 position rotary switch with the diode matrix for the output attenuation (0 to -70dB in -10dB steps).

Together it looks a little bit cramped, but it fits easily.

The cool ribs you see on right at the outside of the back are not needed. I just stumbled on this old adhesive CPU cooler, and added it to the back initially, just in case.

And here is the front panel in detail:

I typically make a design of the front panel in PowerPoint, together with the drill map. I print the design on a color printer, using the best photo paper I can find, and in the highest resolution and best color quality.
I use double sided tape to secure the front panel and use a very sharp knife to cut the holes, I then carefully, without twisting the front layer of the paper, mount the hardware. Note that I try to use the same color scheme Tek used in the 70's. I really like it and use that for all my designs.

And here is the final unit.

While I was building the various sections, I was checking and verifying the results. When I was at Tek, I used to repair these instruments and I was amazed how well the original design worked, even with my modifications and wire nests and felt proud to have been part of this bit of T&M history. In retrospect, I'm glad I started on this project.

Using the procedure in the Instruction Manual, I verified everything as good as I could. I don't have a distortion analyzer or dedicated spectrum analyzer so I can't specify the distortion level. I used the FFT capability of my Rigol DS2302A scope, and that looked very good.

A small bit of info:
I went from using Tek equipment to a Rigol scope. Well, you probably didn't know this, but the Rigol subsidiary for the America's is located in Lake Oswego, which is only a few minutes from Beaverton, the home of Tek. You wonder why Rigol picked this location? (;-))

At a later date I will try to do a comparison with my Mickevic oscillator, my active double-T notchfilter together with an external sound card and my PC based analyzer software.

Everything else, except the rise/fall times for the square wave (>50nSec instead of <50)  is well within specification. I have not investigated this edge issue yet, for me it's good enough.

I did tweak the capacitor for the 50-500kHz range to match the other ranges. The 100Khz signal is the most critical, so I went back and force a few times to adjust the value of C55 the timing capacitor so when I switch ranges, the frequency setting is well within the specification. The original C565 value is 87pF, I ended up with 92pF.

What that means is that when I set the frequency setting to the (reference) 100.0MHz in the X100K setting, and switch to the X10K setting, the frequency is 10.17kHz, in the X1K setting 1.03kHz, in the X100 setting 101.7Hz and in the X10 setting 10.2Hz. That is excellent I think.

One caveat, and you may have already missed it. I don't have room for the frequency dial. First of all, I don't have one in the first place, but because I normally attach my scope anyway, it has a digital read out of the frequency setting, so I don't need the dial.

Even though I did not have the calculated precision resistors for the attenuation switch, I got really close by getting the closest E96 resistor value or selected a couple, and again, I was able to stay well within the specification.
If you're interested why Tek used these funny resistor value: have a look here :
matching-t-attenuator-calculator (use the 600 Ohm input/output setting and you'll see that the values match exactly to those that the Tek designers probably calculated with a slide ruler (;-))

One thing I need to do still, is to order copper sheet metal and use that on the inside of the (plastic) enclosure. Whenever I use my T12 solder iron, the high frequency pulses from the heater come right through. That's not just this unit alone though, but I want to create an extra barrier for this one.

All in all, I am mightily impressed with the design quality the Tek engineers at the time were able to pull off in the 70's. Rebuilding this unit and staying well within the original specification is again a testament to their skills. Hats off!

UPDATE 22-11-2017

After a lot of issues, I was finally able to create a vastly improved setup to measure FFT's, so I can now present the THD distortion number. A measured THD of 0.026% is even better than the specification for the original, which is 0.035% for the 20Hz to 50KHz range.
Here is the screenshot:

Enjoy!

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Designing a single Li-ion cell UPS for the Raspberry Pi

A friend and I set out on a quest to design a UPS for the RPi that really(!) worked for higher power applications.

Most designs, even the ones I have listed on this blog, suffer from a lack of true output power for the more power hungry RPi Models, like the Model 3, or when there are enough devices feeding off the USB port bus.

You will be hard pressed to find UPS designs, even commercial ones, that are able to supply and sustain more than 1 Amp to the Rpi. I purchased a couple, and all of them ended up in the "salvage for parts" bin. They were useless.

I designed several ones myself over the past years, and currently use the last version every day on several of my classic Model B's. I used it for my file server and web server and also while I'm doing designs and tests. It helps to protect my work and the SD card, of course.

This latest version is described here: rpi-power-supply-with-1-button-start
However, as with so many others, it is limited in the amount of power it can supply. As I said, most of them max out at about 1 Amp. I can only use it on an almost idling Model 3.

There are a few major challenges you need to overcome. One is the booster design, the amount of current and then there is the heat factor. Consider that at a minimal 3V Li-Ion cell voltage, the booster needs to get upwards of 5A from the cell to be able to supply 2.5A at a minimum of 4.8V to the RPi.

My mouse-pal friend Bud Bennet (an ex analog chip designer from Linear Technologies) and I (no qualifications to speak of), set out to design a solution. In the process, we actually designed a few solutions and stumbled several times to get it right.

Here is the blog with all the details:

single-cell-li-ion-powered-ups-for-raspberry-pi

After designing this one, and using it, Bud could not leave good enough alone and build the ultimate version:

https://hackaday.io/project/162653-u1liupsrpi

Enjoy!

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DIY build of my Tektronix gear and a 5CT1 Curve Tracer

When I worked at Tektronix in the 70's, I build my own 5440 Oscilloscope and TM503 mainframe.

What that means is that I collected every part and soldered each one of them myself. Some boards came from scrapped units, so I also scavenged parts.

This in itself was not a very unusual project at Tek, but I believe I was the only one at the time to build a 5440. Several others build portable scopes, but since I repaired the 5000's, I was very familiar with them. I purchased many parts from the Tek factory in Heerenveen, and in the employee shop in Beaverton. Some very good friends at Tek helped me to get the printed circuit boards, and several special hardware items, which were otherwise not available. I remember that one time I went on a business trip to Beaverton with one suitcase, and came back with two. Imagine the explanations I would have to do at customs at Schiphol...if I was stopped. The CRT was a reject from a customer (there was a dust spectacle on one of the plates that caused a funny light reflection effect. A colleague of mine spend days ticking on it and shaking the tube to get it off the plates. Eventually he gave up, and it was decided to scrap it. So, I had my tube. When I build my scope, I never saw that effect again... go figure)

I took the transformer from a unit that had been on fire, so that was another item I got. One of the 5000 frames was bent when a customer dropped his unit. I replaced the frame, and was lucky to be able to bent the old one in shape again without breaking it. It is now my TM503 frame.

The list goes on and all in all it took me several years, but here it is:

The measurement system consisted of the following parts:

Tektronix 5440 Plugin Oscilloscope System.

D40 Single Beam Display Unit
5403 3-slot Mainframe with Display Readout Option mainframe is typically 70MHz, could display up to 100MHz.

5A48 Dual Channel Amplifier with 50Mhz bandwidth

5B42 Delaying Timebase, to zoom in on a specific area of the trace.

5A22N Differential Amplifier (down to 10microVolt), with LF & HF filters and DC offset.

TM503 Mainframe
This is a TM503 with original parts, but build in a 5100 lower section scope frame.

It has no middle section, and therefore no handle. I usually have the scope on top of the TM503, so I don't miss the handle.

One the left:
DM502 Digital Multimeter, VAC, VDC, ACmA, DCmA, Ohms, Temperature(C/F), comes with 2 temperature probes and a spare tip

 

In the middle:

PS503A Dual Power Supply, (0 +20V, 0 -20V with tracking, 0-400mA and fixed 5V 1A)
This is a self-build copy of the original unit with lots of original Tek parts.

To the right:

SG502 RC Oscilator, 5Hz to 500KHz low distortion (<0.0035%) sine wave and square wave output

This is a fantastic unit for audio work, which is what I did most of the time, back then.

 

5CT1 Curve Tracer

After I left Tek, I had no time to spend on my electronic hobby, so everything was mostly stored, and followed along with the several moves we've made. Somewhere around 2000, I wanted to finish an instrument that was on my to-do list for quite some time, a Curve Tracer. Some parts were still waiting for this project, but since I left Tek, I had no access to parts anymore, at least not for reasonable prices.

I had no access to a front panel either, so I made my own, but missed the engravings for the settings. I also didn't have the customary drum rotary switch that Tek uses a lot, so I used normal single deck rotary switches. These were connected through diode matrices that drove little 5VDC DIP relays, and these mimicked the finger switches. I used the same technique to add a read-out capability for the Vertical Amperes/Div. and for the Step Amplitude. The ./. 1000 switch was added to the mix, so the display also changed from milli to micro etc.

The instrument uses a transformer to obtain voltages up to 300V. At first I used a normal 220V transformer in reverse, but I was not too happy about that. This transformer had no shielding either, so I got some extra switching noise. When I had an opportunity, I had a special transformer made. There was no information about the transformer Tek used, so I had to do some reverse engineering and the transformer company did the rest. In the end this trick worked really well and the unit is within specifications.

This is how it looks like (left compartment):
Note that since I added the readout capability at a later date, I should have renamed the 5CT1N to 5CT1. The "N" suffix in Tek parlor means "No Readout".

BTW, in case you wondered, the 7CT1N is exactly the same unit, but has a mainframe adapter board to make the circuit fit in the 7000 mainframes.

 Here I'm measuring the voltage drop of a 1N914 diode, showing 0.7V (at .5V/Div)

This is the breakdown voltage of the 1N914 at 20V/Div 

This is the voltage drop of a Schottkey diode.

Here is a trace of a 12V Zener diode (at 2V/Div)

This is the characteristic of a 2N2219 transistor.

This is the breakdown voltage of the 2N2219 at 20V/Div.
Note that the traces show in different intensity, but is caused by the sweep rate of the scope and the way I took the picture.

This is a screen shot of a tunnel diode, used quite a bit by the Tek designers in the 60's and 70's.

This is a BS170 MOSFET

Right now, I'm selling all my Tek equipment and parts. I don't have the space to keep them on my bench, and it's too cumbersome to create the needed space to use them for measurements. I have a new scope now, and I have no need for the DM501 and the PS503 anymore.

I will miss the SG502 because of the good quality sine wave forms (0.0035% THD). I may rebuild that instrument in a different enclosure at a later date, maybe with OpAmps. (one more item added to the already long Bucket List)
Update, I just finished that project, look for it on my blog.

And I will also miss the 5A22N differential amplifier capabilities and the Curve Tracer, although I'm using less and less older parts that need checking or verification. Parts are so inexpensive these days that swapping them out is a no brainer, certainly compared to the old days.

Update : I also started a project to replace the 5CT1/7CT1 Curve Tracer

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My New Power Supply Design Project Part 4

Before we dive in to the measurements, here are some pictures of the PCB and finished product.

As you can probably see, I used a combination of through hole parts with SMD. The reason is that I have a lot of THT parts, and I'm just starting with SMD.

Here are pictures of the finished supply for the 3A version:

The fan is mounted such that it will suck the air out. There are holes in the bottom of the enclosure, below the heatsink. It's a little hard to tell, but I added a number of holes in the top enclosure half to get as much airflow out as possible.

The parts on the backpanel are, from left to right, top to bottom:
Black negative supply, 3 pin DIN connector for the 9-0-9 VAC supply and the Red is the positive supply.
Yellow is the 12 or 15VAC or middle transformer tap for the panel meter supply.
Below it is for the 12VDC wallwar plug for the fan supply.
In the middle is the main supply fuse. (3.15Amp for the 3A version, which this is)
The two empty holes where used with the previous supply, but the thicker fan got in the way, so I can't use them anymore.

Here you can see how I mounted the two TIP142 transistors. In the middle is the NTC for the fan, it has a heatshrink to minimize effects of the airflow from the fan. This NTC is screwed into the heatsink with an M3 bolt.
Below the NTC is the 0.22Ohm shunt. The two green resistors are the 0.6Ohm balancing resistors for the TIP's. Sticking up are the 150 Ohm resistors in series with the Base. On top of the left TIP142 you can see the second NTC mounted on a lug, which is used for the thermal protection. It has a good heat transfer from the actual transistor die temperature.

Excuse the glue gobs for the LED's. I had dried out glue, which was needed to secure the 3mm LED panel holders. These holders suck, and the only way to secure them is to glue them. I ended up using super glue. Top right is the switch and the LED for the main power cut off.

And here is the frontpanel. I make a design in PowerPoint, and print it with a photoprinter on HP Premium Plus Photo Paper in the glossy type. I use double sided tape to secure it to the frontpanel, and carefully cut the openings with a small blade and very sharp knife.




Profiling and Measuring the Power Supply

Besides the obvious measurements, there is more information available that can be used to profile a lab power supply. After some searching around, I found a good application note that I'm following as much as I can, besides, it's a good read to get more information. The app note is from Agilent, number AN 372-1 and called Power Supply Testing.

One of the most important elements of any power supply is the dynamic load regulation, or as it is called in the app note, Load Transient Recovery Time.

In order to measure that, I first had to make some modifications to the pulsing of my Electronic Load. It did not have an adjustable base current load setting, so after adding that feature, the DC Load pulse has a DC offset capability. This is used to set a minimum load current, and the pulse itself raises it to the level you set with the output adjustment. You can follow the DC Load modifications in my post about that topic.

Before I took measurements, I twisted the leads to the DC Power Load as much as was feasible to reduce noise coupling. This is the setup I used for many of the measurements:

The DC load is connected to the supply output, and I use a 6digit bench multi meter to measure the output fluctuations. I use a scope to see any effects on the output itself.

When I finished the 3A version of the supply, my T3 box was still missing the 9-0-9 V transformer, so I did the following tests with the 1.5A T1 box first, and later with the T3 box.

Load Transient Recovery Time.

To  see the effect of the load transients on the output of the supply, I used a shunt resistor in series with the negative lead going to the DC Load. Accross the resistor is an (isolated) BNC connector that goes to another channel of my scope. The resistor is in a metal box to shield it from noise. The resistor is small (0.1 Ohm), so the volts across it are in the milli-volt region, so noise is a factor.

Here is a picture (taken with my new scope) that shows the effect of pulsing a 2.0A load at an output of the supply of just 1.8V. I used a setting of 1.8V, which realistically speaking, is the just about the lowest voltage you will encounter.  It is also the most strenuous on the supply because it means that the pass transistor is almost fully closed and the maximum voltage is across its C-E junction. It has to dissipate this large difference in heat.

The yellow trace (top) is the actual load pulse measured over the series shunt of 0.1 Ohm. The pulse is about 2V which means a 2A load. The blue trace is the transient measured on the supply output.
In the upper screen shot, the load is applied, which means that the current demand jumps from about 100mA to about 2A. It takes the supply about 30 uSec to react to this load change. At first the output goes low (blue trace), and then the regulation kicks in. This could be improved by adding a larger output capacitor, sometimes a few 100uF's is used, which is commonly the case.

Note that the output level of 1.8V after the initial transient is dropped a bit. I measured this to be 25mV with a more sensitive measurement. At a 1A load, the drop is only 10mV.

In the above screen shot, the load is removed, so the voltage over the shunt goes from about 2V (2A) to about 100mA. The blue trace is the measurement on the output of the supply. The output shoots up 300mV for a duration of 12uSec before regulation kicks in again. Note that this pulse is also an effect of the output capacitor dumping it's charge. This is why I keep it as small as possible. I personally believe that more damage can be done by the output capacitor dumping it's charge into the DUT, then the small sagging as the result of adding a load very quickly.

As you can see here also, when the load is removed the output level raises by about 50mV.

If you are looking at this for the first time, the pulse shooting up above the set output level may scare you. However, this is a normal behavior, and because the duration is so small, at about 12 uSec, it will cause no harm to the attached circuit.

The good news is, that the 300mV peak is about the same regardless of the output level. When I put the scope input to AC coupling, I can measure the pulse across the output range. With a 1.8V output (with AC coupling) the scope reports a pulse of 200mV, at 3.3V 195mV, at 5V 190mV, at 10V 186mV, at 15V 177mV and at 20V 173mV, etc.

For these measurements, I'm using the T2 transformer, because I don't have a 9-0-9VAC transformer for the T3 box. Initially, I used an old 6-0-6VAC transformer. The headroom for the 5V aux. supply is too small, and I need to change the zener diode value in the supply switch off transient circuit. This will stop me from interchanging these transformer boxes, so although thr supply is configured for 3A, I'm using the T2 transformer, not allowing me to go up to the full specifications at this moment.

One caveat though.

At this moment, I'm not sure if I'm looking at the maximum edge speed of the DC Load, or the Power Supply. The 555 Timer pulse that is driving the MOSFET's in the DC load has a rise time of 500nSec. I need to profile the DC Load a bit more to see where the limitations really are. For starters, it was never designed for speed, that requirement and modifications were added later on. For now, the Power Supply Transient Recovery Time looks pretty good to me, even though it could be limited by the DC Load. The Agilent app note mentions that this is typically measured in milliseconds or microseconds, so with 30 uSec rising and 12uSec falling, I can't be that far off.


Load Regulation
We already saw an example of the load regulation, where the output dropped 50mV from a set level of 1.8V, when a 1.5A pulsed load was applied.
I'm not sure that this is the correct way to measure load regulations, so for this measurement, I'm going to use my DC load in the static mode (DC only), and just dial in the current at various voltages..

The load regulation I measured is as follows. Although the panel meter is very precise (more later), I'm going to use my 5 1/2 digit DMM as a reference.

At an output of 1.8004V with no load, a 1 Amp load, dropped the output voltage to 1.7975, a drop of 2.9mV, or 0.16%
With a load of 2.0A, the output dropped to 1.7967V, 3.7mV, or 0,20%.

At an output of 24.998V, with no load, a 1 Amp load dropped the output to 24,997V or 1mV, or 0.004%.
With a load of 1.5A, the voltage dropped to 24,995 or 3mV or 0.011%


Current Limit Characterization
The Current Limit Characterization showed that this is a true Constant Current supply, as opposed to a less precise Current Limit supply. The set output current by the DC Load stays constant when the voltage is changed from minimum (at CC) to the maximum. The current deviates less than 1 mA (the least significant digit did not change) with a 5mA, 100mA, 500mA, 1A and 1.5A loads, while the output voltage is going through the range. Once set, the current limit staid stable.


PARD (Periodic and Random Deviation) Measurement
At this moment I don't have the ability to really measure this.
I can only use my scope, AC coupled, at the most sensitive vertical setting (5mV/div) and measure the noise on the output. I first established the "noise floor" of my measurement, by connecting both the tip of the probe and the ground lead to the negative output. The scope then tells me I have a 12 mV p-p noise level in the peak-detect mode.

Measuring directly on the output, I get 10mV p-p at 0V output,  and I get approx. 14 mV p-p at all levels from 1 to 30V.


Turning on (Startup) Transient
The output of the supply should not have any transients when it gets turned on by the main power switch.
Normally you should not really do this, but if there is a brown out event, the supply may turn off and on again.
There is a provision in the bias circuit (the 220uF capacitor) for the series transistor, that will delay the bias current, in effect holding off the series transistor from conducting.

Here is a screen shot of the turn on transient at an output voltage of 4V (bottom trace) with a 500mA load. The top trace is the raw positive auxiliary supply. The output gets turned on when the aux supplies are in regulation. There is no ringing or overshoot on the output.

Turning off Transient
If the supply gets turned off by the main switch, or a brown-out, there should be no transients at the output, to prevent damage to a (sensitive of expensive) DUT.
I concocted a little circuit that gets triggered by the sagging auxiliary supply, before the main regulators that are driving the Opamps come out of regulation. As soon as the unregulated supply gets below 10.6V DC, the bias for the pass transistor gets turned off. You can see that in the simulation result shown in part 1.
Here is the real thing:

The top trace is the raw positive auxiliary supply at about 15V.
Note that when that voltage is dropping to the 10.6V trigger level, the bias is cut and the supply output is gone before the series regulators are running out of headroom at about 7V. This results in a very clean turn off at the output.


Line Regulation (Source Effect)
I don't have the means (a variable transformer) to test this, but because our electric grid in the Netherlands is very reliable and stable, I'm not worried about this aspect, although a big factor is the transformer I use. The T2 transformer I used is at the limit, especially drawing higher currents at a high voltage, because the supply can get out of regulation. The good news is that the CV LED will go out as soon as the series transistor is out of headroom, so you have a visual warning.


Overvoltage Shutdown.
This is not implemented.


Short Circuit Output Current
I tried this three different ways. First by the Electronic Load by setting it to the maximum current draw. Then I used my DMM in the 10A current mode and put that across the output.
Finally I used a lead to short the output.
In all cases, the supply went straight into CC mode with no side effects.
I also did the "sparky" test by rapidly shortening the output and drawing sparks. No problem!


Drift
I did a little of that already, but only for relatively short periods of time. After a sufficient warm-up period, the output stayed at the set voltage level to within about 1 milli-Volt. This is without the 4.096V reference in pace.
From the 5V regulator, I used a series resistor of 10 Ohm and a 47uF Tantalum cap to create the reference voltage.

I just did another measurement, now with the 4.096V reference and the fully build supply. The supply was set at 5.0025V, and 4 1/2 hours later it is still at that level. So it stays within a 100micro Volt. I did not make a plot over time, still need to learn how to do that with my new scope, but I checked the value regularly. Only the last digit (100micro Volt) is sometimes flipping. For a power supply, this is more than adequate.


Measuring with the T3 box main transformer

OK, same stability measurement. Output at 1.8048V, with a 3A load, the output dropped to 1.7995, or 5.3mV, so a regulation accuracy of 0.29%.

With the output set at 20.005V, and a 3A load the supply is on the verge of current limiting, at which point the output is 19.750V, or a drop of  255mV, so a regulation accuracy of 1.27%.

With the output at 30.009V, the supply is getting out of regulation just over 2.6A.
At 2.5A, the output is at 29.910V, or a drop of 99mV so the regulation accuracy dropped to 0.33%, still respectable.

I should point out that the inability to get the full 3A at 30V is more a function of my transformer than the supply itself. Although I also should point out that the heatsink is not large enough to supply 3A for longer periods. The enclosure has no room for a larger one, but that was part of my constraints.

I seldom if ever need that much current, and if I do, I have other means. This supply will be used most of the time below 20V and well below 1A. Mission accomplished as far as I'm concerned.


PCB
I'm expecting this question so let me be frank and up front. The current PCB has some issues with the fan supply and the panel meter supply, as I pointed out earlier, and needed some surgery with trace cuts and wires. So, although I was initially planning to, I'm not going to publish the Gerber files, so please don't ask. I also used a lot of THT components, and that makes the board size larger than it needs to be, and therefore more expensive.

With the details in the schematics I provide you can build your own, it's not that difficult and not that critical. Just make sure you have a main center ground point to avoid loops, and keep things apart that needs to be separate.


Building the second supply for the 1.5A version
In the meantime, I finally found some time to scrap the old 30V 1.5A supply kit connected to the T1 box, and started building-up my second supply. I did some more work on the PCB trace cutting and isolation of the Fan Supply, so it's much cleaner.

With the exception of the maximum current resistors, all the parts are equal on this version. Obviously, I only needed one TIP142 series resistor. Look at the diagram in part 3 for details.

After carefully soldering all parts on the PCB and creating wire harnesses from all parts not on the PCB, I wired it all up and everything worked as it should. As mentioned in the testing section above, I now have a 30V at 1.45A version as well.

I hope you enjoyed this journey as much as I did.

If you like what you see, please support me by buying me a coffee: https://www.buymeacoffee.com/M9ouLVXBdw