Electronics > Projects, Designs, and Technical Stuff
minimum load circuit for lab psu
nemail2:
OK so first I tried 56k for R1 and 1.5k for R2 and R3. There was still voltage fed into the PSU and I was seeing about 60mV instead of 9-10mV. Certainly better but still not where I'd like it to be.
So I went up to 100k with R1 and down to 47 Ohm for R3 (keeping R2 at 1.5k). Output voltage fell to 20mV instead of 9-10mV and according to the 121GW meter no current was flowing via R1 into the PSU (before there was like a few dozens of µA flowing into the PSU). So I'm not sure whether the PSU isn't going down to 10mV because it is virtually unloaded or if the current mirror is still upping the voltage on the PSU output. Also, the current was increasing up to 2.5mA as I was increasing the output voltage of the PSU up to 16V. With the original values, the current stayed stable (at some 0.6mA).
I'm not sure how to overcome this. Is it even possible to make a circuit which works down to like 10 mV and stays stable all the way up to 16 V? I mean it would be OK if the circuit does not sink much current at very low voltages, it seems that my PSU is happy with very little amounts of current to stabilize in low output voltage regions.
The circuit which I was using in the previous revision (see attachement) with the zener diode basically did the business (after I added a series resistor to the zener it stopped heating up like crazy) but it is quite unstable so I'm having a hard time calibrating that additional current out in software. That thing varies in input voltage and with temperature but at least it brings my PSU down to the desired 9-10mV with ease...
I'd also be willing to buy some fancy chippy which can do the business for me, even for quite a price if it does a reasonable job. But I'm afraid, as often, this is nothing you can solve by simply throwing money on it...
Thanks for any advice!
duak:
Three things to think about:
- a MOSFET current mirror,
- add a negative supply - it just needs a volt or two and a few milliamps.
- an incandescant light bulb - consider them to be PTC thermistors and have a wide range of resistance.. Try something like 24 V at 20 mA or even a small line operated one.
nemail2:
--- Quote from: duak on July 10, 2019, 03:01:13 am ---Three things to think about:
- a MOSFET current mirror,
- add a negative supply - it just needs a volt or two and a few milliamps.
- an incandescant light bulb - consider them to be PTC thermistors and have a wide range of resistance.. Try something like 24 V at 20 mA or even a small line operated one.
--- End quote ---
regarding the MOSFET current mirror i found this: http://tera.yonsei.ac.kr/class/2013_2_1/lecture/Lect_22%20MOSFET%20Current%20Mirror%20and%20Active%20Load.pdf
unfortunately this is way beyond my knowledge. I'll churn through it but I highly doubt that I'll get something useful out of it...
negative supply: where would i put that? are we talking about the npn current mirror?
incandescant light bulb: sounds like a 50's solution :-D and I can't imagine how that would work (again because of lacking knowledge) :-D
xavier60:
The residual voltage should simply be due to R1's current flowing through R3 and it's voltage also appearing at the Collector.
First check what the real minimum regulated voltage is by checking what the CV op-amp is doing at minimum voltage setting.
My bench supply wants to regulate to 0V so the CV op-amp reacts by swinging its output hard low.
My other bench supply( 20A ) uses a MOSFET so there is no residual voltage applied to the output. When I set the regulation to 0V, I can accurately measure the 47 ohms across the output terminals.
nemail2:
--- Quote from: xavier60 on July 10, 2019, 03:30:51 am ---The residual voltage should simply be due to R1's current flowing through R3 and it's voltage also appearing at the Collector.
First check what the real minimum regulated voltage is by checking what the CV op-amp is doing at minimum voltage setting.
My bench supply wants to regulate to 0V so the CV op-amp reacts by swinging its output hard low.
My other bench supply( 20A ) uses a MOSFET so there is no residual voltage applied to the output. When I set the regulation to 0V, I can accurately measure the 47 ohms across the output terminals.
--- End quote ---
The CV op-amp is outputting 6mV when the PSU is set to 0mV. Just measured that.
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