AC Electronic Load Circuit

[EPAIII]

I was thinking about light bulbs, but they may be a bit hard to find now-a-days.

I once saw a test load for generators - BIG generators. It was a tank with salt water and a couple of electrodes. It could handle hundreds of KWs. I believe they adjusted the current with the distance between the electrodes and how much of them was submerged.

It seems to me that something for a few KWs could be done with a plastic bucket. Add a Volt meter and a current meter and you can do calibrations with it. Probably need to play around with the amount of salt in solution. Start with none and gradually add it until you get good readings.

I also seem to have a vague memory of dummy RF loads made in a bucket by ham operators. But perhaps they used a simple resistance coil in pure water. The water just cooled the coil so it could handle higher power levels.

Yes, if it measures the current using a Hall effect sensor, or current transformer, just wrap extra turns of thinner wire through it to fool it into thinking the current is higher. For example 10 turns will result in it reading 10 times the actual current.

Doesn't matter how it measures the current. Use external current transformer to induce enough current needed for the test loop.

Good point. A current transformer run in reverse would do the job.

[eric74]

The heat element is important but, right now I am interested take the best way to do each single stage of this project.

[fourfathom]

I also seem to have a vague memory of dummy RF loads made in a bucket by ham operators. But perhaps they used a simple resistance coil in pure water. The water just cooled the coil so it could handle higher power levels.

The traditional ham dummy load uses a power resistor in a 1-gallon metal paint cam, filled with mineral oil.  I've also seen a big coil of lossy coax cable as an RF dummy load, in a bucket of water (or cooled with compressed air).  That lossy coax method wouldn't work too well for 50/60 Hz applications.

Remember, that water is going to get hot.  If I did it right, 1 kW will heat 1 liter of water 80C (room temp to boiling) in about 5.6 minutes.

[eric74]

good tip! 

As I told, I am so curious about what the commercial devices use as heat element. eg. https://www.chromausa.com/product/programmable-ac-electronic-load-63800/
 

[shakalnokturn]

I mentioned washing machine heaters because that's what I use and I can find them cheap, a bucket of water usually gives me plenty of time at the power I need to test.
The first setup I had was thick nichrome wire drowned in brick-sized concrete blocks, those got hot too fast.

If long test times are a problem you'll have to consider overall dissipation, a large thermal mass is only convenient up to a certain point. Once size and cooling times start getting in the way you may as well go for something with forced air flow.

If you're on a budget there are other things that could be used such as hair dryers, toasters, fan heaters... That may work out cheaper than power resistors.

For current sense I'd use a shunt resistor (+opto) it makes the load easier to use on DC if needed.
The PWM filtering / snubbering will depend on the layout, the other aspect is filtering for control loop stability.
You may also want to measure input voltage (line losses) if a constant power mode is needed.

Another approach would be once more using a resistive load to set maximum power consumption and a triac with phase angle control, closed loop can be added. Definitely dirty and AC only.

[fourfathom]

And of course a PWM or Triac load modulation may cause the source under test to behave badly, or differently.

[dtmouton]

We use heater elements. The problem is that they have quite a lot of inductance.