can I let BFY50 oscillate?

Thread Starter

michaeljdixon

Joined Oct 8, 2010
2
Hi guys
In a simple thermal control circuit, I have the output from LM339 to switch my heater on or off. Goes through 2k to the base of the BFY50, with 5k pull up to 5V, and 1k pull down to 0v. The heater is a 47 ohm resistor and it controls a treat with the collector voltage oscillating between 1-2.5 V every couple of seconds, ie never hard one way or the other. It doesn't get hot. Can anyone tell me, is it ok to use it this way or will it go pop in time? (I've always thought of trannies as switches)
Thanks in advance
Mike
 

wayneh

Joined Sep 9, 2010
17,496
Hi guys
In a simple thermal control circuit, I have the output from LM339 to switch my heater on or off. Goes through 2k to the base of the BFY50, with 5k pull up to 5V, and 1k pull down to 0v. The heater is a 47 ohm resistor and it controls a treat with the collector voltage oscillating between 1-2.5 V every couple of seconds, ie never hard one way or the other. It doesn't get hot. Can anyone tell me, is it ok to use it this way or will it go pop in time? (I've always thought of trannies as switches)
Thanks in advance
Mike
Should be just fine. If it's not hot, it should last "forever". Not sure you need a pull down resistor - the LM339 does the pulling down.
 

Thread Starter

michaeljdixon

Joined Oct 8, 2010
2
thank you. The pull down is there to reduce the hysteresis; without it, and even with no +ve feedback around the LM, the switching band is about 10C. The pull down reduces this to about 2C, which is just what I want.
Mike
 

retched

Joined Dec 5, 2009
5,207
michaeljdixon said:
(I've always thought of trannies as switches)
I always though of them as...

Nevermind. ;)

OK. How long has the IC been oscillating when you have checked is temperature?

I would check it over time to see if it is ramping.

You may come back in 10 hours and find a scorch mark where there was once a BFY50.
 
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