Author Topic: Turn-off of constant current source using the reference voltage  (Read 1673 times)

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Online brumbarchrisTopic starter

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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?
 

Offline awallin

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Re: Turn-off of constant current source using the reference voltage
« Reply #1 on: May 02, 2019, 03:02:59 pm »
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..
 

Online brumbarchrisTopic starter

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Re: Turn-off of constant current source using the reference voltage
« Reply #2 on: May 02, 2019, 04:45:43 pm »
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
 

Offline MarkF

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Re: Turn-off of constant current source using the reference voltage
« Reply #3 on: May 02, 2019, 05:39:04 pm »
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.

   
 
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Online brumbarchrisTopic starter

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Re: Turn-off of constant current source using the reference voltage
« Reply #4 on: May 03, 2019, 09:11:06 am »
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
 


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