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I have a problem regarding the power supply and my circuit.

As the picture shown, my power supply is connected to a voltage regulator, Low Dropout Voltage regulator (LDO) to be precise. The part number of LDO is MP9486A. I'm sorry that I'm not allowed to display the schematic but the schematic just a basic one, taking 90V and output 5V.

My question is, when I power the circuit, I see a negative current display on the LED of my power supply. I wonder why are the possibilities that the negative current come from.

enter image description here

Edit:

I think I should put the schematic as well. I'm allowed to put just part of it. enter image description here

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  • \$\begingroup\$ Is it always -0.01? Is the the least count on your ADC? \$\endgroup\$ – Bob Jacobsen Jun 28 at 3:55
  • \$\begingroup\$ There could be a slight offset in measuring current when you do not have a load. It is usually fine-tuned by multi-turn trim pots on the PCB that drives the display. I doubt an external DVM will show a negative voltage at Vout. \$\endgroup\$ – Sparky256 Jun 28 at 4:06
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    \$\begingroup\$ That model can't suck current, only supply it. I suspect a slight calibration error on the ADC. I think the tolerance on the current measurement is 1% +/- 0.02A check the manual. It can show a negative number to help in factory calibration. I used to work for TTi which is how Iknow. \$\endgroup\$ – Warren Hill Jun 28 at 5:39
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    \$\begingroup\$ I don't know how you could have taken a worse photo of your power supply. Learn how to crop images. \$\endgroup\$ – Elliot Alderson Jun 28 at 11:20
  • \$\begingroup\$ Hi everyone, thanks for the suggestion. BobJacobsen yes that's the least count of the ADC. Sparky256 I will take the DVM and measuring the offset as you suggested. Warren Hill I will take note on that calibration error. Elliot Alderson next time I will take a better photo for the power supply. Thanks for suggesting all the possible problem, really appreciate. \$\endgroup\$ – Chuang JY Jul 1 at 7:07
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It seems to me like you have both channels of your power supply connected in parallel. I notice that one channel shows a voltage of 59.40 volts and the other shows a value of 59.38 volts and, given that there is a slight discrepancy, being in parallel, there could be a real negative current flowing into the right hand channel fed from the left hand channel.

If you tweaked the right hand power supply channel to produce 59.41 volts I reckon you might see a negative current on the left hand channel.

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  • \$\begingroup\$ Ok Sir, I will log down this possibility on my log book. Next time going into the lab I will try the method that is suggested. Appreciate your advice. \$\endgroup\$ – Chuang JY Jul 1 at 7:08
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Polarity is always determined by where one chooses to connect the 0V reference which is also the return current node.

Thus any rise in negative input offset or rise in ground voltage can result in the current value at 0V becoming negative.

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There are two ways this can happen:

1) You have an actual negative current (which is easy to do at such low currents)

2) The power supply has an incorrect bias and is registering a slightly positive current as a negative one. In short, your seeing a measurement error

Get an accurate Digital Multi Meter (DMM) and put it in series with the power supply and see how correct it is.

I don't know if the supply pictured above is a cheap one. Be warned, I have found that some of the cheaper power supplies from China have start up transients that can exceed the voltage that the supply was set to, which can cause problems for circuits and components (like exceeding maximum ratings). I had a supply that was set to 12V, and for roughly 2ms it peaked to 19V, I don't use that supply anymore.

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