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400V/10A-100W mini dynamic electronic load

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T3sl4co1l:
For offset: put a small divider from VREF to +IN, which is to say, a large value resistor from U2-2 to U1B-5. Zero doesn't have to be zero, and indeed you might not want it to be, for exactly reasons like this. :-+

If it's not oscillation of course, drift is most likely tempco of R13 or insufficient Kelvin connection thereto.  R10/R14 are differential inputs so wire them to R13 separately from the main current path, or use a 4-terminal resistor.  U2 and RV1 drift, and Q2 and D1 leakage, are also contributors.  In particular, LEDs are also photodiodes, which is to say, D1 leakage depends on ambient light.

Tim

ElectronSurf:

--- Quote from: T3sl4co1l on March 22, 2023, 01:44:21 pm ---For offset: put a small divider from VREF to +IN, which is to say, a large value resistor from U2-2 to U1B-5. Zero doesn't have to be zero, and indeed you might not want it to be, for exactly reasons like this. :-+

If it's not oscillation of course, drift is most likely tempco of R13 or insufficient Kelvin connection thereto.  R10/R14 are differential inputs so wire them to R13 separately from the main current path, or use a 4-terminal resistor.  U2 and RV1 drift, and Q2 and D1 leakage, are also contributors.  In particular, LEDs are also photodiodes, which is to say, D1 leakage depends on ambient light.

Tim

--- End quote ---

The shunt tempco is terrible, I did tried to have separate paths for differential inputs though:



Thanks for the info, I don't think I can make it better than what it is because of the limited budget and equipment.

ElectronSurf:
I did a few changes to the circuit to have faster response;

- There's no LEDs in the circuit anymore.
- The op amp (A) was clipping to the negative rail when no supply was connected, by adding a diode I reduced that to -0.7V.
- Op amp (B) also use to swing all the way from positive rail to negative rail, now the voltage swing equals zener voltage 5V and -0.7V.
- Also instead of pulling reference voltage down, now it's rising the inverting input of op amp (A).
- Found a 5W non-inductive 20mΩ resistor in my stash so Increased the shunt value to 20mΩ. it has better temperature coefficient.
- Increased voltage reference to 2.5V, LM336.
- Increased integrator RC value, it has smoother response now.
- Decreased input voltage sense voltage divider value to have more current flowing, it was too sensitive before that.



I'm also working on a fan controller circuit, after building this project I realized that the fan shouldn't work always at full speed. by adjusting the fan speed I can keep the heatsink at almost stable temperature which helps with stability and drift.

ElectronSurf:
The prototype that I made works but have two main problems;

1- Current sink doesn't go to absolute 0A
2- LT1014 is slow

To solve first problem I want to use OP07 to buffer the voltage reference and use the offset null feature to zero the offset.

Want to replace LT1014 with NE5532 which have higher slew rate but It's not a precision op amp, so I had to increase the shunt value to 100mΩ and this force me to reduce the current sink to 5A max to keep the shunt power dissipation lower than 5W.

This is the design I end up with:



Also I want to change the oscillator op amp to TL074 and add fan control feature:



Please let me know if you see any error(s) or have any improvement ideas.

blackdog:
Hi,

The speed of your current loop depends on your opamp, the MOSFet used and your compensation.
The LT1014 is not a fast opamp but your compensation with the 10nF capacitor might be a bit too much.

A buffer behind the current loop opamp usually helps, so the opamp sees a much lower load and you have more current to drive the Gate properly.

Keep in mind, that the buffer should be wideband enough.
I still have a small collection of LT1010 buffers lying around that I then use for that kind of application.

Not everyone has a collection of IC buffers in stock, but for experimentation you can also put the two opamps of the NE5532 in parallel.
Those then come inside your DC loop of the opamp.
Because of this, you will not suffer from offset errors of the NE5532, resulting in at least 60mA of peak current for the Gate of your Mosfet.
You will have to experiment with the 10 Ohm resistor, and the combination of 3K3 and the 10nF capacitor.

Make sure you pay attention to the build-up, especially if the MOSFet is reasonably fast.
You may need a Snubber from the Drain to the ground point of your 0.1 Ohm resistor.

Happy experimenting!  ;)

Kind regards,
Bram

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