Constant current with varying source voltage

[gatzschm]

Hi,

I want to design a circuit that shall draw either 0A or 500uA from a voltage source that is varying from 30-50V, when a digital control signal is either 0V or 3,3V respectively. The circuit shall operate up to +200 degrees Celsius. It is important that the current drawn from the voltage source does not vary much with the varying 30-50V voltage, say maximum 5% variation within the operating temperature range from 25deg.C to 2000deg.C.

Can you help me with this?

Thanks!

regards,
Robert G.

 

[siddarth.ghaste]

Hi
Check with this attached diagram....

Its hard to get accurate constant current source, because transistor VBE changes with tempearature. for every one degree celsius change in temperature VBE change is 2.5mV.

- - - Updated - - -

- - - Updated - - -

output current is 0.7/R2....

 

[gatzschm]

Thanks for your suggestion!

Where would I connect my control signal to switch the current on/off? The problem with the circuit you suggested is that it will draw a current all the time? I want to be able to draw no current at all.. either 0A or 500uA which shall be controlled with a 3,3V signal.

regards,
Robert