CMOS "OR" IC keeps burning out

Discussion in 'The Projects Forum' started by camjerlams, Dec 15, 2014.

  1. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    Hi guys, I've just replaced the same 4071B IC for the third time.
    The output that keeps getting fried is the output of a 2 input OR gate.
    The inputs are bridged, so it's really just "on is on, off is off" and is fed from a nand gate which is holding up fine.

    See image: https://drive.google.com/file/d/0B24ZN7xs6vYKeS1HX2JxVGRDelE/view?usp=sharing (in my haste I left out the gate resistor 33ohms)

    So the output of the OR chip switches an NPN transistor which switches a small relay, which switches a large relay for a compressor.
    I've replaced the NPN even though it tested out because I had one, I've checked the relay coil and the clamping diode. IC's are running on, and working with 5V.

    The only other elements that I have changed are the clamping diode- it was a germanium diode which I replaced with a smaller germanium diode. Could this be not stopping back EMF supression? I didn't think it was critical to use the exact diode..

    I also replaced a ceramic 104 capacitor on this circuit with an MKT 104, again I didn't see that as critical..

    That's all I can say about this circuit, not much to it but for the life of me I can't figure out what is causing these 4071's to keep burning out on that same output leg.

    Is it worth changing back to larger germanium diodes? Anything else I could check for? Could the compressor itself be effecting anything?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You say GATE resistor. Where does that go? Gate of a mosfet (when BJT is showing)?

    If you really have a bjt transistor you need some current limiting - either at the base or at the emitter.
    Check the datasheet of the OR gate. You are exceeding the output current of the OR gate.

    Add a 4.7k to 10k ohm resistor to the output of the OR gate. If you need more current through the transistor, make it a Darlington.
     
    Last edited: Dec 15, 2014
  3. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    Sorry I'm not really familiar with terminology, it goes to the base i think, the "P" bit in NPN.
    The transistor is a 2n4401 and I was told by the electronics shop that the transistor wont draw more current than the IC can handle but I took his word for it.
     
  4. GopherT

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    Nov 23, 2012
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    Don't believe them. You need a resistor between the output pin on the OR gate and the base of your transistor.
     
    #12 likes this.
  5. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    OK, that's fine, there is a 33 ohm resistor there, that I left out of the diagram, it's there by design so should be ok, I will check to see if this value is good for the IC/trans combination.
    Thanks.
     
  6. GopherT

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    Nov 23, 2012
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    33 is too small. The voltage drop from base to emitter is 0.6 volts. The High level output off your
    OR gate is 4,6 volts. That is a 4 volt differential. Add a 33 ohm resistor. That gives 4/33 ohms = 121 mA!

    Look at the datasheet. Max amperage out of the OR gate should be about ONE mA. You need more resistance. 4700 to 10000 ohms.

    Also, do you intend for your collector to be connected to ground? As in your Drawing
     
    Last edited: Dec 16, 2014
  7. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    Wow.. Well that makes sense. I was looking into how to work out current demand for a transistor just now, thanks for this, I'll be repurchasing my IC's and a suitable resistor and hopefully getting this thing running again today!
    I'll post results
     
  8. GopherT

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    And the collector connected to ground...?
     
  9. camjerlams

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    Dec 23, 2012
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    What about the relay coil on the other side of the transistor? Would this also be part of the current calculation?
     
  10. GopherT

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    Nov 23, 2012
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    Yes, I thought it was on the collector when I first looked at it. Am I tired or did you reDraw it?

    It would be better to put the coil between the collector and positive voltage supply.
    Otherwise the voltage drop across your coil will push up your base voltage. It is a cleaner circuit design. And you still need the 4700 ohm resister at the base.
     
  11. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    The collector is connected to ground.
    Sorry, I'm the strung out one i think, the coil is indeed connected to 5V on the other side, and the transistor switches it OFF with 5V.
     
  12. #12

    Expert

    Nov 30, 2010
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    Using a transistor as a pure switch means you should only try to get about ten times as much current through the collector as you send to the base. Most 5 volt relays need a lot more than 10 ma. I think you will need another transistor to get the current gain high enough for the relay. My preference is to use a PNP as the collector load of the first transistor and the PNP fires the relay.
     
  13. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    OK, its on a busy PCB, I'm thinking I should look at getting a similar IC that can drive the relay coil for ease.

    I upped the base resistor to 6k8 and that dropped the voltage to the transistor to 0.8V and it didn't switch, at least my IC is still good though..

    Does anyone know how to find an IC just like the 4071B but with a higher current capacity on the outputs??

    Thanks Gopher and #12, I'm going crazy here.
     
  14. Brevor

    Active Member

    Apr 9, 2011
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    I don't think such a thing exists.

    If that means you can't change the circuit layout you could try to get a 2N5308, its a darlington transistor in a single transistor package. The pinout is not the same as a 2N4401 but you can cross the leads to get it into the pcb. I use this transistor for driving relays from logic level signals. For a 5 Volt logic level a 33K resistor on the base will work and it won't overload the output of the 4071.

    Another option would be to use a 2N7000 mosfet it would probably be easier to find than the 2N5308, also no base resistor would be needed.
     
    Last edited: Dec 16, 2014
  15. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    OK I've ordered this mosfet, thanks for the tip!
     
  16. ronv

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    Nov 12, 2008
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    Maybe you should redraw your schematic as it is wired - just to make sure. Then tell us the specs on the relay (coil current or resistance).
     
  17. Bernard

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    Aug 7, 2008
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    Output higher than 4071; 74HC32, VCC- 6 v, V ouy 5.2V , I out 5.2 mA; 74AC32PC, VCC 5.5 V, V out 4.6V , I out 24 mA.; Utsource, $ .57, 5-10 $ .40.
     
  18. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    https://drive.google.com/file/d/0B24ZN7xs6vYKNEFJeWtDeXVRVGM/view?usp=sharing
    https://drive.google.com/folderview?id=0B24ZN7xs6vYKLVhsajFXT1haYTg&usp=sharing

    OK I've drawn out more here, thanks for your interest.
    I don't understand how logic gates can work with nothing connected to the inputs, unless their job is to simply be on or off while the ICs are powered..

    Thanks for that, my difficulty now is figuring out how the transistors switch on and off.
    The relays switch bigger relays that switch compressors on. If a small relay is off, the compressor is on. So at the moment one compressor kicks in, the second stage compressor stays off (see permanently on relay). It's for a -80C freezer so I would have thought both would be on until the control temp is reached.
     
    Last edited: Dec 18, 2014
  19. GopherT

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    Nov 23, 2012
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    Ok, could you give an overview of your project, what you need done and what tools (power, sensors, ...) you have available. Then describe how those tools are getting the gaol done as you step use through your schematic(s).
     
  20. camjerlams

    Thread Starter Member

    Dec 23, 2012
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    OK, that's going to take me a while, I'll post when ready.
    My overall goal is to get this PCB back in original working condition and use the freezer again.
    I could parallel both compressor relays so they're either all on or nothing but I'd rather fix this thing properly.
    There are 4 other CMOS ICs, as you can see by the board it will take a while, for me at least, to trace things out and look up all the datasheets to see what's happening. I just traced back from the transistor gates until I got to the logic with no inputs and couldn't find schematics like that online, got scared and ran back to the forum.

    I'll keep interrogating this board and get some more info.

    Thanks.

    Also- I've tried getting a replacement PCB but was quoted $2600 so I'm just trying to work though this.
    If anyone knows where to look for this PCB Im all ears- the freezer is a Forma Biofreezer model 8458, it's considered lab equipment.
     
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