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Friday, August 26, 2022

Dynamic AC/DC Load

This will be a description of my process to build a new Dynamic Load.

Below is the latest version of the prototype.

I already have a Dynamic Load, described here ( This instrument served me well while testing Power Supplies. It is a rather simple analog Constant Current mode Dynamic Load, but also has a pulsed output possibility and an offset capability so you can set the minimum and maximum current while pulsing.

For the new Dynamic Load, I wanted to add more modes like Constant Voltage, Constant Power (wattage), Constant Resistance and a Battery Discharge option. This will mean that it will have to be a digitally controlled instrument.

There are many designs that accomplish this already, but I could only find one that implemented an AC input, next to the usual DC input. The AC capability is useful for applying a load to non-DC inputs, like an AC sine-wave coming from transformers so you can profile them, and also other waveforms like triangle or pulsed like PWM signals.

The first challenge:

The first challenge is to see if I can combine a DC input with an AC input.

Adding an AC input goes hand-in-hand with a True RMS measurement capability which is an additional challenge for me. To top this off, I also want to use the new Raspberry Pi Pico board. It has a lot of interesting features and also has a dual core. This may be required to calculate the True RMS voltages if I decide to do that in software. Dedicated chips that can do that are pretty expensive.

I have not decided on a set of specifications yet. My initial requirements for the design phase will be if I can get up to 400VAC at say 1A. A lower voltage could be 60-80V at 3A and 30V at 5A. I don't need a larger current sink, this should do for my own use.

The unit should have a "pulsed" mode to test transients, and as I mentioned above, a battery discharge feature. I'm not interested to build a PC controlled unit, that's beyond my capabilities, but I could possibly create an interface that is compatible with existing PC-controlled software applications.

The user interface itself will be very simple. I envision a rotary encoder for the settings and menus, with maybe only one toggle switch or push-button to turn the load to the DUT on and off. I will try to use the rotary encoder push button to enter values and select menus. No key-pad! The display will most likely be a small 128x128 OLED display, most likely in color because the price difference between color and monochrome is very small these days.

The enclosure should be a small as possible, because the available real-estate in my lab is very mimimal.

So there you have it.

Standing on the shoulders of others

I have been collecting various designs from others and looked at the way they did things. Several stand out, like the multi-part Youtube series from John Sculley:

Then the contributions from Jay_Diddy_B on the EEVBLOG.

I also studied the tear-down and schematics of the Array 37XX Dynamic loads done by another frequent user Kerry Wong, that can be found here:

Here is another one:

As I mentioned earlier, I could only find one DIY design that features a true AC/DC capability. That project from Rainer Schuster can be found here: (there are two versions of diagrams available, you really have to search for the latest one)

Here is a Youtube video of the instrument with an elaborate explanation of the AC mode of operation and background theory.