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2N3904 NPN dead twice. — Parallax Forums

2N3904 NPN dead twice.

T ChapT Chap Posts: 4,223
in General Discussion
I have a sensor with push pull output, active low. Marked SensorOut. On the board I accidentally left out a base resistor to the 3904 NPN. I found a convenient point to cut a insert a through hole 1K on the board, and it is soldered from a trace off the sensor to the pad/pin of the base at the 3904. Twice this 3904 has died. I replaced it a few weeks ago, I know the board has been on for many days spread out over several weeks, but yesterday I turned it back on to watch it again and today the NPN is shorted from Base> Emitter with 82ohms reading. My only thoughts are the heat from soldering direct to the pin, but 2 weeks for it to die the second time around?

The circuit basically just allows an NPN buffered output and an LED driver on the low side, inverting the active low out to active high for LED ON. Now the LED is stuck ON.

Any thoughts on this?
Screen shot 2016-06-11 at 7.08.08 PM.png
1294 x 617 - 103K

Comments

  • Hal AlbachHal Albach Posts: 747
    Could you try a higher base resistor? Most data sheets for the 3904 show a 2.5K to 10K for switching circuits.
  • Peter JakackiPeter Jakacki Posts: 10,193
    Double check the resistor in-circuit and 1k is a bit low but should be okay although R4 at 49.9k looks weird and is way too high.
  • T ChapT Chap Posts: 4,223
    It is a 1k to the base, 49k is high but means very low current. Makes no sense how this is shorting out from base to emitter twice in a row. The base is high at 3v3.
  • kwinnkwinn Posts: 8,697
    T Chap wrote: »
    It is a 1k to the base, 49k is high but means very low current. Makes no sense how this is shorting out from base to emitter twice in a row. The base is high at 3v3.

    Double check the connections and resistances from Vcc to collector and GND to emitter on the transistor. I suspect it has nothing to do with the 1K base resistor but with a 49K resistor on the collector you could try a higher value base resistor.
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2016-06-12 06:42
    If you have a 1k resistor to the base but a 49k in the collector that is most back to front and any power you save in the collector is totally lost in the base. It is a BJT after all, not a MOSFET.

    Now when you say that the base emitter is "shorted" you do mean that you have checked this in both directions haven't you? The base will look shorted because it is effectively a diode, so use the diode checker to check base emitter connections. 700mV give or take 100mV in one direction and open in the other.
  • T ChapT Chap Posts: 4,223
    The base>emitter was shorted, checked with power off after the device was on for 24 hours. In ohms mode, it was 82ohms compared to the other 3904 next to it was megohms. The LED was stuck on with power applied. Now, I just went out to check as you suggested in diode check in both directions, and now the 3904 is working again and is no longer shorted. Very strange that 24 hours on, it fails and reads failed for a number of minutes while I was probing it. Now 24 hours off and it is working. This is not an illusion on my part, the device was stuck shorted and now it is not. I will replace it again and try to use less heat applying the resistor, and use 10k vs 1k, and 10k on collector to 3v3.
  • Peter JakackiPeter Jakacki Posts: 10,193
    Ohms check on semis is only valid if you check in both directions but diode check is the quick easy way. Another thing to watch out for in some circuits is residual power so if something looks faulty with the meter then also do a volts check as that will mess up your readings otherwise. It is extremely unlikely for the 3904 to fail in the manner you described. Look for other reasons if the LED is stuck on such as NPNOUT shorting to ground. Short the base-emitter of TR2 and make sure that the LED turns off as it should otherwise your LED activation is external.
  • T ChapT Chap Posts: 4,223
    When it was failing with power on, the push pull output of the sensor was toggling low for active. The base was sitting at GND and never moved. I agree this part should not fail and I have installed tons of the them, that's why it is strange in this instance with this new circuit board. But nothing changed since I turned it off, it just sat on the bench. With the base sitting at GND essentially and power unplugged the only residual is a few small caps, nothing that would explain this strange shorting effect. I think I have to contribute it to hand soldering the 1k onto the base pin and putting the part in a degraded state.
  • fengfeng Posts: 39
    Your schematic shows a supply voltage of 3.3V to the sensor.
    I suppose that the VCC voltage is 5V or more.
    You ENable the sensor with 5V and that is more than the supply voltage.
    Do you have a link to the original schematic?



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