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| Operational Amplifier driving MOSFET |
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| Zero999:
--- Quote from: Ian.M on April 28, 2018, 08:33:09 am ---Not me. You could switch to a comparator, but a LM393 isn't the best choice. Its a dual comparator with open collector outputs, and for driving a single MOSFET gate, you'd be better off with a single comparator with a push-pull output stage. However for such a slow circuit, (limited by the ORP12 and the load) there's no reason not to stick with an OPAMP. --- End quote --- It was me and I wouldn't recommend using a comparator to drive a MOSFET. It's probably slightly better, than the LM358, but not much. In this case, I think the MOSFET is unnecessary. The LED can be driven from the LM358 or LM393. The spare op-amp/comparator could be used to drive, the LED, which would prevent the voltage drop from interfering with the comparator circuit. I'd recommend the LM311, which can drive up to 50mA out, with a very low saturation voltage. |
| Ian.M:
Errr the O.P's also driving a 12V 20W Halogen bulb, which will have a cold resistance of under one ohm!. With a >16A turn-on surge, there's no realistic way of getting rid of the MOSFET. Your buffered LM358 circuit looks nice for isolating the MOSFET gate drive from the hysterisis feedback. Drive the LED off the comparator stage, sourcing current so it doesn't affect the Vol, and the MOSFET gate off the buffer stage. |
| mike_mike:
--- Quote from: Ian.M on April 29, 2018, 12:09:44 am ---Drive the LED off the comparator stage, sourcing current so it doesn't affect the Vol, and the MOSFET gate off the buffer stage. --- End quote --- Can you please be more explicit ? I did not understood this. 1. Are there any modifications that I have to do ? 2. Or it is ok to use the circuit as it was in the last schematic uploaded by me ? |
| Ian.M:
It should work as-is, but Hero999 has proposed a small improvement (top schematic in reply#30) If you want to adopt his improvement, as you need to drive both a LED and a MOSFET, move the connection to R4 for his output LED from pin 7 to 1 to drive it off the first (comparator) OPAMP in the chip, and drive the MOSFET gate resistor from pin 7 of the second (buffer) OPAMP. |
| mike_mike:
Thanks for the reply. I tested the circuit and it is working as I expected (I tested the last schematic that I uploaded in post 26). It is OK now to make the PCB @Ian.M ? |
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