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See the circuit with the opamp. Replace the resistor and zener in series with a potentiometer. That will make it possible to control the voltage over emitter resistors and therefore also the current through collector.hi hero999,
can you tell me how to make this single transistor constant current source?
See the circuit with the opamp. Replace the resistor and zener in series with a potentiometer. That will make it possible to control the voltage over emitter resistors and therefore also the current through collector.
is op amp necessary?
what about this circuit? it doesnt use op amp
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That's close but not quite there.
The current through the LOAD will be regulated, not the LED.
Try this.
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The diodes used in my circuit are not critical, the 1N4148 will do.
The circuit you posted won't be any better than an LED with a resistor connected in series because the transistor is in series with the LOAD, not the LED. Read the Wikipedia article and you'll see why.
hmm... is this good enough?
I'm using multisim.What simulator are you using?
Have you measured the current through the LED when the pot is at different settings?
My only concern was that it might not turn off fully.
This circuit should fix that.
View attachment 41514
The voltage across the LED should vary, but not by much.
The current through the LED should vary and the intensity will vary.
I suggest you build the circuit in real life rather than blindly trusting a simulator.