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Measuring Current on DC Power Supply

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xavier60:
The TL051 is a good choice. Suitable op-amps need to have a common mode input range that includes the + rail.
If a BJT is used, a diode or pull-up resistor is likely to be needed for it to be turned off fully.
 A diode and resistor could be added to the series pass MOSFET to allow the op-amp to mostly discharge the output capacitor, making a preload circuit less needed.
The loops should be properly compensated to allow a much smaller output capacitor to be used.

larsdenmark:
What about using an LMV324A as replacement for the LM324?

Is this newer opamp any good for this? It is rail to rail and much cheaper than the LT1014.

David Hess:

--- Quote from: larsdenmark on June 13, 2019, 09:06:16 am ---What about using an LMV324A as replacement for the LM324?

Is this newer opamp any good for this? It is rail to rail and much cheaper than the LT1014.
--- End quote ---

If its 5 volt supply limit is acceptable, then the LMV324A can be used.  But there are lots of similar low cost CMOS parts with at least a 15 volt supply voltage.

The LT1014 is expensive but supports the full 30 volt supply range of the original LM324.

maurosmartins:
Hello all, thank you very much for your replies!



--- Quote from: David Hess on June 12, 2019, 05:04:24 pm ---
--- Quote from: xavier60 on June 12, 2019, 02:50:09 pm ---
--- Quote from: maurosmartins on June 12, 2019, 10:01:53 am ---Nevertheless I was not expecting 0,5V this is much larger than what the datasheet states (20mV), or am I interpreting it wrong?
--- End quote ---
The datasheet graph for negative output saturation voltage versus current shows the problem.  The LM324/LM358 can only actively pull down to about 0.5 volts with a PNP emitter follower.  The rest of the current is provided by a 50 microamp current sink down to 20 millivolts or so.

--- End quote ---
Can you please post the mentioned graph I was unable to find this information.

in the meanwhile I've included the following to my circuit:



and supplied the LM324 with the -5V from the zener voltage, the no load voltage came down to about 50mV.


--- Quote from: xavier60 on June 12, 2019, 11:02:53 pm ---The TL051 is a good choice. Suitable op-amps need to have a common mode input range that includes the + rail.
If a BJT is used, a diode or pull-up resistor is likely to be needed for it to be turned off fully.
 A diode and resistor could be added to the series pass MOSFET to allow the op-amp to mostly discharge the output capacitor, making a preload circuit less needed.
The loops should be properly compensated to allow a much smaller output capacitor to be used.

--- End quote ---
The advantage of this circuit is that it doesn't rely on matching resistors, correct? additionaly can you please explain better why the TL081 is a better choice then the LM324? Is it because of the ability null out the output?

on a different topic I've also started to test the current limiting feature:


placing the LED2 in that location was a bad idea and was removed as it changes the output voltage, additionally there is something wrong with the circuit I still haven't figure out what it is, if I increase the load something starts buzzing possibly due to oscillating of Q2 turning on and off.

I'll continue to investigate.

Again thanks for the replies,
best regards, Mauro.



--- End quote ---

David Hess:

--- Quote from: maurosmartins on June 16, 2019, 05:57:23 pm ---
--- Quote from: David Hess on June 12, 2019, 05:04:24 pm ---
--- Quote from: maurosmartins on June 12, 2019, 10:01:53 am ---Nevertheless I was not expecting 0,5V this is much larger than what the datasheet states (20mV), or am I interpreting it wrong?
--- End quote ---

The datasheet graph for negative output saturation voltage versus current shows the problem.  The LM324/LM358 can only actively pull down to about 0.5 volts with a PNP emitter follower.  The rest of the current is provided by a 50 microamp current sink down to 20 millivolts or so.
--- End quote ---

Can you please post the mentioned graph I was unable to find this information.
--- End quote ---

Said graph is included below.

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