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NPN transistor circuit behaviour

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glowlamp

New Member
Hello,
I'm using an NPN transistor as a simple current amplifier.
The circuit consists of a 5v step up converter ( from 1 or 2 AA batteries ) connected to a NPN transistor with a led load on the collector side.
I have a 100k pot on the transistor base in series with a 1k resistor which operates as a dimmer.

The circuit performs perfectly until the battery(s) approach end of life.

At that point there is a cutoff when the pot is turned to low resistance ( to allow max brightness ) and the led goes out. When turning the pot towards high resistance, the led comes back on again.

At a guess I think this is related to current draw / battery internal resistance increasing causing the voltage to drop below the base turn on voltage. It results in confusing reverse type behaviour for the dimmer.
Is there anyway to solve this simply - to make the led be as bright as it can be without cutting out ?

Any advice appreciated.
 

Grossel

Well-Known Member
I think that the cut off is caused by the step-up converter input reach too low voltage for the converter being able to operate. Then, since the current draw is cut, a normal battery behaviour is that voltage rises, until the converter can operate once again and you're able to turn on the led for some more seconds.
 

dr pepper

Well-Known Member
Most Helpful Member
Try shunting the 100k pot with a cap, say 100 - 500pf, this will lower the startup voltage and make start up less dependent on the pot setting.
 
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