EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: brumbarchris on May 02, 2019, 02:38:04 pm
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Hi all,
I am currently studying this constant current source with opamp and P channel MOSFET. there is something I do not understand and that I would like to ask. The way I see it, the opamp will alwas drive the gate of the MOSFET so as to keep the negative input to the same voltage as the positive input. I would therefore expect that if I increase the reference voltage to 5V, the gate voltage will rise high enough to completely turn OFF the mosfet and to ensure the negative input also reaches 5V. I do not see this behaviour in simulation, though. If I rise the VREF voltage, the VGAT only increase so much, and the Vin voltage is clamped somewhere at 4.15V. So I guess the question is: why doesn't the opamp block the MOSFET when VREF=5V?
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try a higher supply-voltage for the op-amp. the output will go more or less close to the positive supply rail but not beyond..
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My intended goal was to turn off the mosfet by connecting the positive input of the opamp to the supply voltage. And I only have/want a single supply voltage in this circuit.
Cristian
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The LT1006 opamp output can only go to (V+) - 1.2V. Which will probably not completely turn off the MOSFET.
Are you just trying to switch the output on/off or control the current?
For just switching, this circuit for a solar panel charger may help.
(https://www.eevblog.com/forum/beginners/turn-off-of-constant-current-source-using-the-reference-voltage/?action=dlattach;attach=723663;image)
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Yep, it was the limited output capability of the opamp to swing to the upper rail. Changing the opamp solved it, thanks.
I now ran in different problems, but I will post in the Projects and Technical stuff forum section, as it is not "beginners" stuff.
Regards,
Cristian