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Measuring Current on DC Power Supply

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maurosmartins:
Hello all,

I've been working on a small DC power supply controlled by two DAC (one for setting the voltage and the other for setting the current limit).
Voltage Range: [0-12]V
Current Range: [0-1]A

the output voltage is ok and measuring the current is also ok with load (for example 100 or 200mA). The problem happens when no current is flowing and I still have around 0,5V at the output of the differential amplifier (pin 8 highlighted on the schematic).



I think it has to due with ampop limitation to set 0V output because it is supplied with V-=0V and V+~=18V I'm using a portion of an LM324D to make the differential amplifier. Checking the datasheet I think that the maximum "Output Voltage − Low Limit" should be 20mV.

do you have any idea what can be happening?

I'm thinking on adding something like the lower left corner of following design to create a negative rail and supply the ampop:


looking forward your reply,
Best regards, Mauro.


HB9EVI:
two points:

- high side current sense is better done with a chip matching the purpose like the LTC6105
- the LM324 is no rail to tail opamp, so you cannot reach either of both rails; as a single supply opamp you can get close to GND but not close enough; since you use it on the high side, it's not going to work at all, since it cannot reach the positive rail close than maybe VDD - 2V

xavier60:
Where does the 15V come from? If it's well regulated, there would be less CMRR problems sensing current upstream of the MOSFET.

maurosmartins:
Hello, thank you for your reply,


--- Quote from: HB9EVI on June 12, 2019, 09:22:15 am ---two points:
- high side current sense is better done with a chip matching the purpose like the LTC6105
- the LM324 is no rail to tail opamp, so you cannot reach either of both rails; as a single supply opamp you can get close to GND but not close enough;
--- End quote ---

I agree with both, the first one was because I had no such IC and had LM324 in the part bin, the second I'm addressing it with a negative supply rail to supply the ampop as in the previous post. Nevertheless I was not expecting 0,5V this is much larger than what the datasheet states (20mV), or am I interpreting it wrong?

Additionally can you please explain a bit more on the following? it does work when current is flowing for example at 200mA it is supposed to wield 1V and it does.

--- Quote from: HB9EVI on June 12, 2019, 09:22:15 am ---since you use it on the high side, it's not going to work at all, since it cannot reach the positive rail close than maybe VDD - 2V

--- End quote ---


--- Quote from: xavier60 on June 12, 2019, 09:50:36 am ---Where does the 15V come from? If it's well regulated, there would be less CMRR problems sensing current upstream of the MOSFET.

--- End quote ---

@xavier60
it is more like 18V comming from the rectification / filtration of the secondary winding of the transformer.


Best regards, Mauro.

HB9EVI:
if you have enough headroom with the voltage, so 15V on opamp supply and 12v on the rail to measure, it can actually work, but still the LM324 is not the choice;

the bigger the current draw you have, the bigger the voltage drop on the resistor is - so CMRR of the opamp is less a problem; additionally comes the voltage offset which is more critical the smaller the difference between the two input voltages is.

discrete high side current sensing is possible with some tricks; for example you can bootstrap a RRIO opamp for low voltages like a MCP6001 together with a P-Fet, but still is working easier with a dedicated high side current sense chip

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