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Using DC electronic load for 50Hz AC Sinewave

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alm:
If your power requirements are modest (dozens to maybe 200W), there are power resistance decades, like the Clarostat 240C and those made by Time Electronics, that are a more convenient to use than a bunch of power resistors and clip leads. I believe there are also versions with relays that can be automated. You do need to pay attention to how much power they can handle depending. Usually each decade is rated for a certain current, so make sure you stay within that. For example for the Clarostat the 1 Ohm decade can handle 5 A, so at 1 Ohm it can handle at most 5 A^2 * 1 Ohm = 25W, while with the first decade set to 9, it could handle 5 A^2 * 9 Ohm = 225W.

I did a quick test with a Kikusui PLZ-300W load which implements constant resistance using an analog feedback loop (see manual with specs and theory of operations here) set to 180 Ohm, a standard bridge rectifier and a HP 33120A function generator set to 50 Hz sine 20Vp-p (with 50 Ohm output impedance). I wouldn't want to use it for AC testing. Yellow is the AC voltage going into the bridge rectifier, blue is the current going into the bridge rectifier measured with a current probe (Tek P6302 + AM503B, vertical scale on scope is correct), gray is the voltage from the bridge rectifier going into the load (same scale as yellow), and white is the current from the bridge rectifier flowing into the load (same scale as blue). So clearly the load is quite slow in responding to the abrupt voltage changes even at low current levels, like the overshoot after the zero crossing:


The bridge rectifier itself is not a problem as long as your voltage is high enough, and could be improved somewhat if you'd use Schottky diodes instead of standard rectifier diodes. Here is the same test showing the AC voltage and current when replacing the load by a 150 Ohm resistor:

unknownparticle:
Very coincidentally I was going to try this as a load for testing audio amps, think I'm going to test it out on a relatively low powered single board power amp first to see what happens ???  I have an old school DC  electronic load by ACDC Electronics, a real beast from the '70's that can handle upto 750 watts, and also a Kikusui 300 watt unit which is much more recent. Should be interesting.

alm:
I'm a little bit surprised it's performing so badly, because the PLZ-300W manual (page 16, fig 2-3-4) shows a flat impedance up to 1 kHz.

unknownparticle:
Interesting. I have the PLZ-303W, I'll check the specs on that.

peter_mcc:

--- Quote from: alm on November 08, 2022, 03:00:51 pm ---I did a quick test with a Kikusui PLZ-300W load
--- End quote ---

Thanks for doing that - unfortunately it is as I expected and didn't work  :'(

I've come up with another solution using a fixed resistor bank and a programmable AC power supply which I have.

Thanks everyone for your input!

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