can I let BFY50 oscillate?

Discussion in 'The Projects Forum' started by michaeljdixon, Oct 8, 2010.

  1. michaeljdixon

    Thread Starter New Member

    Oct 8, 2010
    2
    0
    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
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Can you post a schematic?

    The BFY50 is a RF medium power switching transistor.

    Bertus
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    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.
     
  4. michaeljdixon

    Thread Starter New Member

    Oct 8, 2010
    2
    0
    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
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    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.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You might want to use something like the circuit Roman Black used for his OCXO heater instead. It keeps the temperature very constant. No need for a comparator.

    Link:
    http://www.romanblack.com/xoven.htm
     
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