RE: Building an electronic load for the lab

DAB — Sat, 16 Nov 2024 19:15:00 GMT

Fun post, I always like home brew projects.

RE: Building an electronic load for the lab

michaelkellett — Sat, 16 Nov 2024 10:23:35 GMT

There are a couple of problems with your design, which you may have noticed already.

The 'tuning' scheme you have adopted reduces the feedback at low frequencies as well as high. You will get much better low frequency and DC control (and easier tuning) if you compensate the feedback loop by adding a small capacitor between the op am output and its negative input. Adjusting the value of the capacitor and the base resistor will usually allow you to tune the design by experiment and get satisfactory results. You can use power MOSFETs or even IGBTs for the power device but the Darlington transistors will be OK.

The other thing you will find is that when there is no load connected the feedback loop will wind up the op-amp output to the power rail. If you then connect a load it will take the op-amp some time to get things under control during which a very large current will flow through the load. This is a common issue even with quite expensive commercial designs of electronic load. Having an off switch that biases the op-amp output low is a good plan - although you can still have problems if the load itself switches on and off.

Remember is that the TIP122 can't handle 65W (what ST say in the data sheet) in real life ! But you won't be able to keep the tab on the transistor at 25C.

With 40V across it it can only cope with about 0.75A (at 25C) = 30W, at 80V the maximum current is less than 200mA.

extract from ST data sheet

The difference in the cooling of the transistors might be a purely mechanical thing (if one is a little bent or burred).

Nice blog - thanks.

RE: Building an electronic load for the lab

dougw — Fri, 15 Nov 2024 18:10:04 GMT

Nice job using available parts and time. An electronic load is also somewhere on my bucket list.