Re: Weird transistor failures



Tom R wrote:

> I have had a number of failures of simple transistor LED drives which I
> really can't figure out why:
>
> I am using a standard bc182 driven from 1 k directly from a PIC. The emitter
> is grounded, the collector has two white leds (connected in parallel) , a
> red LED in series and a 240 Ohm resistor. The drawn collecter current is
> around 25mA.
> The voltage is 12v dc and everything works perfectly for months
> until...eventually (and not always) the transistor begins to leak such that
> the LED's become just visible in the dark. The transistor will still turn on
> fully but not fully off.

There's nothing wrong with your circuit - although you're using a pretty high
base current.. I'd tend to use 10k in the base connection in such an
application.

Where did you get the BC182s from ? Don't see them around much these days -
dodgy stock ?


Graham

.

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