constant current source design

[Jay_Diddy_B]

Hi Tom45 and the group,

The Vbe of the transistor has a temperature coefficient of -2mV per degree C.

The current will rise by about 2mV / Re, where Re is the emitter resistor.

Regards,

Jay_Diddy_B

[Arjunan M R]

Since you don't need much accuracy, why not just use a simple 1 transistor current sink.

The circuit below uses a 2N3904 and 3 resistors. Testing shows that it works down to 0.2 volts.

Current was about 1 ma at 0.2 volts, up to about 1.2 ma at 25 volts.

If you don't have 5 volts available, just recompute the base bias resistor divider for whatever voltage you do have.

NO I'm not using this circuit.Since I have to make the BOM as low as possible I will go with the other.
I'm not going to buy three different types of resistors.
In the other I only have to buy one type of resitor extra.49.9  \$\Omega\$.I already need 4K7.

Thanks for reply.

[Arjunan M R]

I don't need accuracy but the current has to constant as possible. Because I'm using an 18 bit ADC to measure current . I don't want to zero it out whenever i change the voltage.