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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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