Electronics > Beginners

Why isn't this circuit working?

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I attempted to build a constant current source out of transistors, similar to this one
I build this circuit it doesn't work, I troubleshoot it, it still doesn't work, I simulate the circuit on pc, it works. Grrr...
Is there anything wrong with my wiring?
(EDIT: The image is inverted, sorry about that, seems to be some error when I uploaded the image here)

I don't see any obvious problems.

Have you checked voltages?  The voltage at the base w/ respect to ground should be ~1.4V.  The voltage across the emitter resistor should be ~700mV.

What's the part number on the transistor?  Did you swap emitter and collector, perhaps?  Is it an NPN type?  Have you checked the transistor with your meter's diode check feature?

What voltage is your source?

What is the to value of the resistance going from Vcc to the base?  I see you are using 3 resistors there.

What is the resistance of your pot?  Can your constant current source stay in compliance for all possible values?

What's the value of the emitter resistor?  Are all the resistance values what you expect them to be?

Additionally check connectivity of the prototype board. They play up sometimes.

Fixed! The problem ended up to being a faulty diode. I measured the voltage across the RE and it was at 11.30v which was definatley looking suspicious, afterwards I changed the resistors, still same voltage so then I measure the first diode and it shows the correct ,but when I measure the second one and it shows no drop. Thanks for helping me out! :D

Why would R=0.7/I?  Won't there be more than 0.7V across the resistor because of the diodes?


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