help for transistor selection to operate relay

[Briez]

I have a 30A 12v relay to drive a single phase instrument. So I am using 18v transformer to provide relay coil voltage through transistor bc337. But my problem is that after some days I noticed that my transistor becomes short and relay operates without firing transistor. Actually I don't know what is bc337-25 collector voltage range. So help me to solve this problem.

Posted via Topify on Android

 

[D.A.(Tony)Stewart]

If you did not put a protection diode in reverse polarity across the coil, the transistor will fail from the large voltage spike on the coil inductances when the transistor is opened.

 

[crutschow]

Why are you using a 18V transformer to drive a 12V relay coil? That will overheat the relay.

 

[Briez]

Thanks for reply. . But due to I have to operate my circuit in low voltage, I mean I want to drive my circuit at 120v ac. So I have designed my transformer as per that requirement. ..

Please give me solution...

Posted via Topify on Android

 

[THINKER_KAM_MAN]

just a suggestion


put a zener diode in series with the repay supply to
drop it to the correct relay voltage or if you want you can use a
12V zener diode /resistor in stead to provide the 12V relay supply

 

[Audioguru]

A transformer produces AC. A transistor and most relay coils use DC which is completely different. Did you really try to use 18VAC?

What is the rated coil voltage and current of your relay? Is it 12VDC? Then why didn't you use 12VDC??
Why didn't you look at the datasheet for the BC337 which says its maximum voltage is 45VDC and its maximum allowed continuous current is 500mA?
The datasheet for this transistor and for most other little transistors shows that they saturate pretty well and make a good switch when the base current is 1/10th the collector current. How much is your base current?

I agree that a diode is required across the relay coil to stop hundreds of volts from the inductance when it is disconnected.

 

[Briez]

I am operating my relay by signaling BC337 base by resistor is 1k. I am applying uC output pin to 1K resistor then base of BC337.

Posted via Topify on Android

 

[Audioguru]

I am operating my relay by signaling BC337 base by resistor is 1k. I am applying uC output pin to 1K resistor then base of BC337.

Then the base current is about 4.8V - 0.7V)/1k ohms= 4.1mA and the relay coil should draw no more than 4.1mA x 10= 41mA.

But you forgot to tell us the current used by the relay coil or tell us its voltage and resistance.
We also do not know how and where you connected the transistor and relay coil together. Is the relay coil connected between the collector of the transistor and +12V?

 

[Briez]

My relay details is like,

Nominal voltage: 18vdc
Current drawn: 52mA
max. Operating voltage : 13.5
min release voltage: 1.8v
coil resistance: 360ohm.


and connection is relay coil connected to +12 and transistors collector.

Posted via Topify on Android

 

[Audioguru]

My relay details is like,

Nominal voltage: 18vdc
Current drawn: 52mA
max. Operating voltage : 13.5
min release voltage: 1.8v
coil resistance: 360ohm.

and connection is relay coil connected to +12 and transistors collector.

If the relay is 18V then its MINIMUM operating voltage is 13.5V or 15V so it will not work from 12V. Its electromagnet is not strong enough with only 12V.

Your transistor became shorted because your circuit is missing a diode to stop the hundreds of volts produced by the inductance of the relay coil when it turns off.