Electronics > Projects, Designs, and Technical Stuff
Programmable Electronic Load, 0-5A
Kleinstein:
The circuit to generate the 2 inputs is also a little odd, there should be a simpler way with just 2 OP instead of 3.
There is a limited use for the negative half of the voltage range. The load circuit is made to work with positive voltages only.
So I guess it is OK to just skip the negative voltages and keep the circuit simple and this way more stable.
The OP for the offset makes some sense, but here may be a simpler solution: Use an OP to invert the 2 V reference and just add some part of the inverted voltage to the divider. This version is less sensitive to the OPs quality. However the OP for the current regulation is way more important anyway, as the voltage level is smaller by something like a factor of 10.
So there are only 2 critical OPs in the circuit: the one to read back the total current and the regulator OP. The others (e.g. reference shift and voltage read back) could be lower grade ones.
To combine separate controls for several FETs and a single master regulator could work in a way that the OPs for single FETs do the fast regulation and the master from the common precision shunt only does a rather small, slow correction to the set points for the single regulators.
For the slow corrections one could get away with the already amplified signal and thus only 1 performance critical OP.
So one could have a combination like an OP27, ADA4528 or OPA209 for the shunt amplification and than RC4558, TLE2021 or similar for each MOSFET and the master corrections from after the amplification.
Another point to care about is saturation of the load - if the external voltage is too low to allow the set current to flow, the regulator with slew up the gate voltage quite high and would be slow to react if later connected. This could be a problem if the voltage source is connected with the load already on. This could be unintentionally by a poor contact. After saturation there could be high current spike - possibly even damaging or blowing the fuse.
One way to protect against this would be to disable the load, if the voltage is too low (e.g. < 0.5 V). Alternative one could turn down the current on low voltage. This could kind of set a minimum resistance (should be higher than shunt + 2*R_on) - so the regulation should never go into saturation.
Micke:
Related matter ;)
DISTRELEC sells IXTQ42N25P - MOSFET N, 250 V 42 A 300 W TO-3P for 3.91 Euro.
https://www.distrelec.biz/en/mosfet-250-42-300-to-3p-ixys-ixtq42n25p/p/17111086?queryFromSuggest=true
Good thing is that it has specified DC SOA in the datasheet :-+
I am building a precision current sink 0-2A for calibration purposes with this MOSFET, chopper amp LTC1152 and 1R VISHAY VHP-4 current shunt, results from prototyping good so far, 6:th digit in current fluctuate a little :)
JeanLeMotan:
--- Quote from: Micke on March 10, 2019, 09:07:26 am ---Related matter ;)
DISTRELEC sells IXTQ42N25P - MOSFET N, 250 V 42 A 300 W TO-3P for 3.91 Euro.
https://www.distrelec.biz/en/mosfet-250-42-300-to-3p-ixys-ixtq42n25p/p/17111086?queryFromSuggest=true
Good thing is that it has specified DC SOA in the datasheet :-+
I am building a precision current sink 0-2A for calibration purposes with this MOSFET, chopper amp LTC1152 and 1R VISHAY VHP-4 current shunt, results from prototyping good so far, 6:th digit in current fluctuate a little :)
--- End quote ---
This is great!
Ordered 4 of them.
Thanks a lot for the hint.
JeanLeMotan:
--- Quote from: Kleinstein on March 09, 2019, 09:29:10 pm ---The circuit to generate the 2 inputs is also a little odd, there should be a simpler way with just 2 OP instead of 3.
There is a limited use for the negative half of the voltage range. The load circuit is made to work with positive voltages only.
So I guess it is OK to just skip the negative voltages and keep the circuit simple and this way more stable.
--- End quote ---
You're right. My initial use-case was to show the negative voltage in red so the user knows the load is connected in reverse. But that is not needed really.
I changed the schematic to use one op-amp plus over voltage and reverse polarity protection for the ADC with a zener.
--- Quote from: Kleinstein on March 09, 2019, 09:29:10 pm ---The OP for the offset makes some sense, but here may be a simpler solution: Use an OP to invert the 2 V reference and just add some part of the inverted voltage to the divider. This version is less sensitive to the OPs quality. However the OP for the current regulation is way more important anyway, as the voltage level is smaller by something like a factor of 10.
--- End quote ---
Good idea. Done.
--- Quote from: Kleinstein on March 09, 2019, 09:29:10 pm ---Another point to care about is saturation of the load - if the external voltage is too low to allow the set current to flow, the regulator with slew up the gate voltage quite high and would be slow to react if later connected. This could be a problem if the voltage source is connected with the load already on. This could be unintentionally by a poor contact. After saturation there could be high current spike - possibly even damaging or blowing the fuse.
One way to protect against this would be to disable the load, if the voltage is too low (e.g. < 0.5 V). Alternative one could turn down the current on low voltage. This could kind of set a minimum resistance (should be higher than shunt + 2*R_on) - so the regulation should never go into saturation.
--- End quote ---
Makes sense. I will add this feature in software. Not sure if I can drop the current on low voltage as it's the opamp (U9.1) that increases the gate voltage due to the target voltage drop not being met. So no matter how low I will set the target voltage (so target current), the opamp will keep the gate wide open.
This might be an issue with 4-wire measurements as the voltage connection might be solid but the current one (the load) might be broken. I will add permanent sensing of load voltage as well, even when using 4-wire measurement so that I can know the load voltage at all times (hope it makes sense).
I changed the schematic and created the PCB. Tomorrow I'm ordering this.
https://easyeda.com/jeanleflambeur/electronic-load
Thanks for the hints.
Kleinstein:
The protection against saturation and thus the gate voltage to slew up all the way is something that should react relatively fast, e.g. within a few 10 µs, depending on the speed of the OP part.
For the protection is should be the voltage at the load, not a possible 4 wire connection for an extra voltage measurement.
The limitation of the effective resistance of the load, could be some kind of clamp to make sure the set voltage is never high than something like 1/3 or 1/5 of the voltage at the load. So if the voltage is to low one should reduce the set voltage with a kind of precision "diode".
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