RE: Building a Smart Electronic Load | Introduction (#1)

DAB — Thu, 02 Jan 2025 20:30:22 GMT

A very interesting project.

I like projects that try to reuse materials.

This one will keep you busy for a while trying to resolve the issues.

RE: Building a Smart Electronic Load | Introduction (#1)

michaelkellett — Wed, 01 Jan 2025 13:53:32 GMT

Try simulating your analogue stages.

There are several things that don't look good.

Your spec is incredibly ambitious - much faster than commercial ELs costing many £k.

The LM2904 has an open loop bandwidth of 0.7MHz and a slew rate of only 0.5V/us. The typical offset is 0.3mV (max 3mV) , equivalent to 0.3A (max 3A) output current offset error.

The 2904 won't take kindly to driving a big MOSFET gate with a 1R series resistor - increase it to at least 47R - but note that it is usually necessary to tune the series R according to the MOSFET gate capacitance and the feedback loop compensation.

Increase R9 to at least 1k and expect C1 to be of the order of 4.7nF. (30kHz bandwidth)

Never fit C14.

You have a differential amplifier in the feedback loop of U2A and this can be difficult to compensate - simulation will help.

Think about using a better op amp than the LM2904 (lower offsets, faster slew rate, greater bandwidth - OPA192 would suit).

Maybe increase the value of the current sense resistor to 0.01R - it will fry at 100A and dissipate 9W at 30A, other problems will be reduced.

Managing good Kelvin connections to a current sense resistor on a PCB requires great skill at 0.001R or an expensive 4 terminal sense resistor.

No (common) MOSFET can cope with 100A @ 200V - but with your circuit as it stands if you set it for 0.1A and connect it to a 200V source it will attempt to do that. (Not 0.1A but > 100A). This is because when the drain ciruit of the MOSFET is open and there is any current demand all then the feedback loop will increase the voltage on the MOSFET gate to about 12V. The LM2904 is slow and once current starts to flow it will take about 20us to switch the MOSFET off.

Many commercial loads (and almost all DIY designs) suffer to some extent from this problem. Most commercial loads won't actually self destruct if connected to a stiff high voltage source.

If you want fast pulses then protection gets very difficult. My usual approach is a M0SFET switch and a power resistor - and beware - even that can oscillate under some circumstances.

Hope some of this helps.

RE: Building a Smart Electronic Load | Introduction (#1)

dougw — Tue, 31 Dec 2024 17:44:27 GMT

Interesting build. An electronic load is on my list of things to build or acquire.