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constant current source circuit

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Kick

Full Member level 6
Hiii all,

For constant current source circuit,can I use any type of op-amp? or in other words all op-amp will support constant current application? In my project I want to give constant current of 1.5mA to a pressure sensor. Thanks in advance.

Hi,

you are experienced. So yu should know that it all depends on voltages, current and the used circuit.
So show us the circuit that you have in mind, and give the values additionally.

***
You know you could build a current source with OPAMP, or with additional BJT or FET. It could be high side, or low side. And the sensor maybe needs 1V to work or 15V or anything else...
The sensor could be simple resistive, capacitive or a four resistor strain gauge....

Klaus

You could make a very simple circuit with a transistor and a resistor. Depending of the current direction you will implement NPN or PNP, you should give us information about the sensor.

If it was a "high side" current source, using a medium gain (say, 300 minimum) PNP transistor such as BC558C (check datasheet for more information but I think its minimum hFE is about 490) and a resistor you could bring it to life.

IC=1.5mA

IC=IB.hFE

IE~IB+IC <=> IB < 100IC (with such a high hFE, IB should be about 490 times lower than IC, so don't worry).

IE~(VBB-VEB)/RE --> With the same condition said before. VBB is the voltage at the base.

Being RE the resistor you connect between the positive rail "VCC" and the transistor emitter terminal.

As IE is almost equal to IC, then supose a value for RE (say 4k7) and calculate the VBB voltage (at the base) you need for the 1.5mA current.

The higher the hFE the lower the IB will be. I recommend you to use a preset to vary the VBB voltage. Don't use a value higher than (hFE/10) times the RE because you may create a voltage divider and your calculation wouln't be that easy.

Hope being useful.

How accurate and stable does the constant current need to be?
What is the pressure sensor load resistance?

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