Why does this not work

106

02:23:26

I tried to build a constant current source for driving some 1w leds using one opamp and one transistor. On breadboard this circuit works fine but here it shows alway an error. The voltage at the emitter's resistor is measured and compared with the reference voltage at the left led. you always get the same voltage at the emitter and thus the same current through the leds.

published 10 years ago

WTFCircuit

10 years ago

Check this: http://everycircuit.com/circuit/4634995076366336

indianachones

10 years ago

Add a 47 ohm at the NPN base

dale0788

10 years ago

Has it something to do with the gain of the amp? If i reduce the gain it seems to work somehow..

rich11292000

10 years ago

Put a switch from op amp to base

avid0g

7 years ago

The transistor is driven into saturation. I placed a 2200 ohm resistor in series with the base junction and increased the OpAmp gain to 500 in order to make this clear. You can solve this by reducing the reference voltage using a voltage divider.

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