Dodgy transistors???

[Glenn]

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

 

[Rheilly Phoull]

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

What cct are you using ?

 

[[email protected]]

What cct are you using ?

--

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

Where theres a will, I want to be in it.

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