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Friday, April 5, 2024

Build a DIY DC Dynamic Load Instrument


The conclusion of a very long time investigating the building of a combined AC-DC Dynamic Load (DL) led me to the realization that this is not really possible. At least not without making drastic compromises about accuracy and precision. The AC measurements do not need to be very precise and are, in my case, only occasionally needed, but DC measurements typically need to have the highest precision, stability and accuracy.

The attempts for this combined AC-DC DL are described in another Blog post. There is a lot of information there, so get some fresh Java and have a look.

The above picture shows the status of the last developments, with this AC-DC DL prototype as the basis.

To help design this new version of the instrument, I asked for the help of my friend Bud. He is a real designer, and we've done a number of projects together, also described on this Blog, like the VBA Curve Tracer and the Differential Probe.

He started a Blog post on Hackaday that can be found here : Dynamic Electronic Load |

At this moment, we have finished the design of the new version and the board is in production at my sponsor PCBWay.

I'm still working very hard on the software side, because this instrument will do some things a bit different, compared to many other DIY designs.

I'll explain that later in more details but here are two pictures to hopefully wet your appetite:

The initial target specifications:

Input voltage: 1..100VDC

Reverse polarity protection to -100V and a 15A fuse.

DUT disconnect by a relays

Maximum current of 10A @ 40V (needs to be verified)

Maximum power 150W (needs to be verified)

Volt Accuracy: 0.2%

Current Accuracy: 0.6%

Power input: 12VDC Wall-wart 1A with reverse polarity protection to -24V and PTC fuse

CC, CV, CP, CR and Battery discharge modes.

GUI: 128x128 OLED 1.5' color display and rotary encoder

Temperature controlled fan

More later...

Sunday, January 28, 2024

Transformer snubber design using Quasimodo test-jig

This is a little post about the transformer snubber design tool using the Quasimodo test-jig, designed by Mark Johnson.

The link to the overall post is here and the pcb's can be ordered here.

I've had this jig for almost 10 years and used it every time I need to add a transformer to my designs.

The test-jig helps you to create an optimum snubber configuration for transformer windings having critical damping, without requiring any calculations and without measuring the transformer's inductance or capacitance.

From the website: 
A power transformer snubber is a wonderful thing for reducing or eliminating RFI from rectifier-induced LCR ringing. Unfortunately it's a huge pain to design and optimize a snubber. First you have to measure the transformer's leakage inductance and secondary capacitance, at about 100 kHz, which is not especially easy. Then you have to estimate the capacitance of your rectifier(s), which does not always appear in datasheets. Finally you plug these numbers into a formula that spits out snubber values -- and then you hope it's all correct.

Here is the schematic of the Quasimodo test-jig:

The name Quasimodo is used because it was the bell ringer of the Notre Dame. I like that name, it rings a bell! In essence the test-jig creates a 555 generated frequency and a fast MOSFET to create a pulse with a very sharp edge, ringing the transformer winding. The snubber is used to limit the ringing.

Here is how you need to connect the transformer winding connections/shorts for various transformers. Make sure you short all windings except the one you want to measure:

Some real results on a Triad PP28-180 transformer with two independent primary and two independent secondary windings. See the first picture of the post for the setup.

First, banging one of the secondary windings without a snubber:

With a 10nF and 1K resistor snubber:

After tuning the snubber to 680 Ohm with 10nF:

No more ringing, the bell is silent now. 

An additional 150nF across the winding results in more damping:

Here is the result in my application (AC/DC Load):

Highly recommended!