How Can This Transistor Survive?

Discussion in 'The Projects Forum' started by Artbuc, Jun 8, 2014.

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  1. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Hi folks! This my first post. I am a retired chemical engineer who loves to DIY. Do not know too much about electricity although I did take Linear Circuit Theory as a technical elective and I can usually figure things out thinking about electricity as fluid flow. Quick project background: I have a Time Delay Module (TDM) on my garden tractor which instantaneously kills ignition and, after a second or so delay, kills the electric PTO clutch. If my butt lifts off the seat switch, current to the TDM is killed. Mine started to malfunction and I went to my tractor forum for help. Discovered another person (I'll call him Bill) had the same problem. Instead of spending $120 on a new TDM, he decided to build his own, which he has done successfully. I tried to do the same. Mine worked a few times and then the transistor fried. Why?

    Here is the TDM schematic from the tractor Service Manual:

    https://lh5.googleusercontent.com/-4CaKNQyx6ag/UF3lWcsp1xI/AAAAAAAAAy0/Ki1o79iPsPg/s562/%21cid_887DC29CB5F8481E86285FE760C88CDC%40WilliamVAIO.png

    Bill exposed his TDM circuit and discovered the Service Manual had R1 in the wrong position. He also identified the components. Here is his sketch:

    https://lh5.googleusercontent.com/-...q7Mi2jh4m78/s783/time+delay+interlock+(2).png

    Here is a picture of Bill's TDM:

    https://lh4.googleusercontent.com/-...AAzk/pmR4FbtQEj4/s631/IMG_20120922_170143.jpg

    I exposed the circuit on my TDM. It did not have D3. My transistor is a MPSA06 instead of a 2N3904. My R1 is 6.8K, not 7.5k ohms. I had already bought parts so I built mine with R1 =8.6k ohms, T1 = 2N3904, D2 = 1N4004, C1 = 10 microF electrolytic. I did not install D3 as shown in the Service Manual because my TDM did not have it and, from what I have read, it would not protect T1 anyway. Instead, I outran my headlights and installed D3 (1N4004) in parallel with the coil so it could act like a flywheel when the coil de-energizes.

    So, why is Bill's TDM working great and mine failed after a few cycles? The supply voltage is typically closer to 14vdc so I will use that number. Current to T1 base is 1.6mA. The relay coil resistance is 85 ohms so T1 collector is seeing 165mA. D3 should be protecting T1 by clamping transient voltage when T1 breaks ground connection through T1 emitter. In looking at the specs for a 2N3904, max collector current is only 200 mA and collector/base gain is less than 100 so I wonder if the currents killed T1 instead of the transient voltage spike?

    Hopefully you endured this long-winded story and can help me understand what went wrong and how to fix it. Thanks!
     
    Last edited: Jun 8, 2014
  2. crutschow

    Expert

    Mar 14, 2008
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    A higher current transistor would likely be better such as a 2N2222 but the MPSA06 should be okay. And, to fully saturate the transistor, the base current should be at least 1/10th of the collector current or 16.5ma. That means R1 should be about 580 ohms.

    You transistor probably burned because it was not fully turned on. The beta gain of 100 is for operation in the linear region, not as a saturated switch. You need to use a value of 10 when it is used as a switch. (Look at the data sheet for the operating conditions where it lists the saturation voltage).
     
  3. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Thank-you! I wonder why my TDM worked for over 20 years with the MPSA06 and 6.8K ohm resistor, ie base current = 1.6mA? Also, do you agree D3 serves no purpose as shown in the Service Manual and on Bill's diagram? What should I do to protect the transistor when it switches off the relay coil?
     
  4. #12

    Expert

    Nov 30, 2010
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    D3 would make sense as a zener diode, and I have never seen a 1N4000 series diode in a glass case, so it's probably a zener diode. And, yes, it keeps the relay coil from harming the transistor.
     
  5. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Yes, I have watched the instructional videos and better understand biasing and saturation. I also see from various data sheets the basis for your 10x factor recommendation. However, I do not see how you calculated 580 ohms. I assume you subtract the saturation voltage across BE from the supply voltage and divide by 16.5 mA. Is this right?
     
  6. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Duh, brain cramp over. Gotcha, thanks.
     
  7. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Yep, looked at some pics of zener and they look exactly the same. Now, what voltage? I assume it would be higher than the relay supply voltage so it would not provide a ground path and prevent the relay from opening but lower than the max allowable voltage across the transistor? I would say a 20 volt zener would work fine, right?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You want a zener voltage that's comfortably below (say \leq75% of) the transistor rated breakdown voltage.
     
  9. #12

    Expert

    Nov 30, 2010
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    Yes. If the transistor is rated at more than 20 volts, a 20 volt zener seems right. You must consider that the charging voltage for a 12V battery can get as high as 15.5 volts, so you don't dare try a 14 volt zener on a 12 volt system.

    It is likely that the transistor is rated for 30 volts or more because that rating is common, cheap, and easy to find.
     
  10. NFA Fabrication

    Member

    Aug 12, 2012
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    I do a lot of 12V stuff, and have had to build similar stuff to what you are doing. This is probably what I would use off the top of my head (Obviously you would have to choose the timing capacitor [C1] value to suit your needs) to do what you are trying to do. I modified the diagram you posted above:

    [​IMG]

    I removed D1, as I didn't see it's purpose (Not fully saying it doesn't have one, I just don't see it). Possibly it was the output to the ignition, and the diode was to protect the electronics from a spike back from the ignition coil as it is an inductive load? And I have always added a protection diode when running relays or any other inductive load. I am no electronics genius, if I messed something up, I am sure someone will chime in.
     
    Last edited: Jun 11, 2014
  11. #12

    Expert

    Nov 30, 2010
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    Replacing D3 with a 1N4001 across the relay coil is, "the other way" to do it. Standard stuff. No problem.

    My only thought is to use a higher number than 1N4001, like the 1N4005 or 6 or 7 diode because they cost the same amount and have survival voltages in the hundreds. That is, if you're going to do a lot of these, buy the higher voltage diodes. $5 for a bag of 100 the last time I bought them. They are just more versatile.
     
  12. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    That is what I did. Radio Shack did not have anything higher than a 12 vdc zener. I had the 1N4004 I previously bought from Mouser for D3 so I used it as you suggested. Thank-you.
     
  13. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Thank-you all for responding to my posts and for providing this incredible forum. The videos and tutorials are outstanding. My Delay Module seems to be working great with the MPSA06 transistor.

    I did use a 550 ohm resistor as suggested to make sure Ib is at least 10% of Ic making it necessary to substantially increase the cap. I calculated it would take about 1.7 secs for the transistor to open with a 1000μF cap. I am not sure how much, if any, delay there is between the transistor opening and the relay opening, but the PTO does not disengage for 3 seconds. I must not be allowing for transients or some other real world effects. I can live with 3 seconds but if I get a rainy day I may drop back to a 500μF cap.

    Thanks again!
     
    Last edited: Jun 11, 2014
  14. Artbuc

    Thread Starter New Member

    Jun 8, 2014
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    Sorry, I did not see your post before the previous two. Yes, the input comes from a seat switch. D1 is there to protect against the ignition coil I assume because there is also a similar diode on the starting lug of the ignition switch. During normal operation (on seat with tractor in gear) the ignition module is supplied by D3 through the ignition switch. Do not have a clue why it needs to be 6A. I already had the MPSA06 in hand when I got the previous suggestion to use the 2N2222. Also, I already had 1N4004's. Re the 1k resistor, it would come close to, not meet the Ib/Ic > .1 guideline (if I am understanding the guideline correctly, lol).

    You may not be an electronics genius but you sure as heck know a lot more than I do!! Thanks.
     
    Last edited: Jun 11, 2014
  15. bertus

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    Apr 5, 2008
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    Hello,

    I am closing this thread as it violates AAC policy and/or safety issues.

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