Dodgy transistors???

I just assembled a circuit which has a transistor to switch the coil on a relay (driven from logic level of microcontroller). The transistor is a general purpose TO92 package, 0.5 A switching type (like a 2N2222 or similar).

I had a problem with it that I've seen on odd occasions in the past. Every now and then I seem to get a leaky transistor that doesn't shut off properly. (I always have a diode in parallel with the relay coil to shunt the 'back emf' when the coil is switched off - to avoid damaging my transistors).

I've also seen this problem in other switching applications (not relays).

Am I getting my transistors too hot when I'm soldering, or is there something I should be looking out for to avoid this problem?

Cheers Glenn

Reply to
Glenn
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What cct are you using ?

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Cheers ......... Rheilly P

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Reply to
Rheilly Phoull

Hi, First look at the power rating of the coil. Driving transisot should be able to handle that. As I know 2N2222 is not a good driver. Other fact is that 2N2222 is not capable of handling 0.5A. Look at the base voltage comming to yout transistor. It should be at zero when off since it seems that you are using a NPN transistor. Use a small capactior like 0.01 uF across the coil as well.

Pubudu

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pubuducg

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