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Help With Circuit

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Hi guys,
Sorry for the confusion. Steve do you have the new circuit with correction. Thanks for the help with this.
Thanks in advance
Raj
Look at the corrected one posted by audioguru
 

Audioguru

Sep 24, 2016
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Your first schematic has the load connected to -5V so the output voltage is positive-going. It needs a P-channel Mosfet.
The last schematic has the load connected to ground and a negative-going output voltage impossibly comes from magic. You could connect the load to +5V and use an N-channel Mosfet.
Which one do you want?
 

(*steve*)

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Which one do you want?

As suggested previously, read the other thread. He wanted a constant current source peering powering a load on the -5v rail.
 

Rajinder

Jan 30, 2016
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Hi guys,
I need a negative constant current source.
I can't see the corrected circuit. I presume I have to use a p channel for this.
What will the circuit look like?

Why is my current circuit working?
I can't understand, it's starting to get confusing.
Thanks in advance
Raj
 

Rajinder

Jan 30, 2016
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Hi guys.
Just seen the new circuit. Are there any limitations if I need to alter the sense resistor for different values?
Could you explain the calculations for op current.
Thanks in advance
Raj
 

Rajinder

Jan 30, 2016
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C
Your first schematic has the load connected to -5V so the output voltage is positive-going. It needs a P-channel Mosfet.
The last schematic has the load connected to ground and a negative-going output voltage impossibly comes from magic. You could connect the load to +5V and use an N-channel Mosfet.
Which one do you want?
N I change the n channel circuit to give me a constant negative current?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Unless your requirements have changed, use a p channel mosfet.
 

Rajinder

Jan 30, 2016
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OK will do.
This will give me a negative current source and the calculations you showed earlier will still apply? Are there any limitations to the value of sense resistor that I can use?
I'm sorry about all the questions but I was finding it difficult to understand the operation of the circuit.
Best regards,
Raj
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Since it is the same circuit, yes the same rules apply.

The limitation on the maximum size of the sense resistor will be due to the input impedance of the op amp, noise, and leakage in the mosfet.

The minimum value of the sense resistor is dependant on the current and power ratings of the mosfet, the ability to turn the mosfet on hard enough, circuit resistance, offer voltage of the op amp, etc etc.

What current do you require?

One important factor is the magnitude of the negative voltage presented to the op amp. This subtracts from the voltage available to overcome Vgs(th). You may want to keep it under 1V, but you need to study the datasheet for the mosfet (whatever you choose)
 

Rajinder

Jan 30, 2016
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Hi Steve,
Many thanks for your help.
We are talking microamperes, typically from 1-100uA.
But it needs to be a negative current. You have helped me immensely, I am really thankful.
Best regards,
Raj
 

(*steve*)

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1 to 100uA should be fine. Use a resistor which drops 1V at 100uA... 10k as your sense resistor.

A logic level mosfet capable of a couple of hundred mA should have leakage way below 1uA
 

Rajinder

Jan 30, 2016
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Hi Steve,
Some further help is required. What would be the easiest way to modify this circuit to meet the following requirements:
1. To have a positive constant current
2. To make this current source floating I.e. no ground connection on output side


1. Use n channel FETwith connection reversed

2. My thoughts were some sort of galvanic isolation, or current transformer or photovoltaic circuit. I am not sure.
Best regards,
Raj
 
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