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(Topic ID: 160242)

What causes coil/solenoid driver transistors to fail?


By yonkiman

4 years ago



Topic Stats

  • 7 posts
  • 7 Pinsiders participating
  • Latest reply 4 years ago by beelzeboob
  • Topic is favorited by 3 Pinsiders

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    #1 4 years ago

    Replacing burned-out TIP-102s and the damage done around them makes me want to focus more on prevention, which made me realize I'm not 100% sure I know all the reasons those suckers blow up. Here are my theories:

    1) Shorted coils (or a shorted flyback / "catch" diode across the coil). If the coil is shorted, the transistor will try to short the 70V supply to GND, and it's a race between the fuse and the transistor as to which one pops first. I didn't really think shorted coils was a thing (figured they'd fail open), but the "ramp down" coil on my Getaway schooled me on that. A shorted diode would cause the same thing.

    2) Open diode. Every coil needs a diode across it to limit the inductive kickback when the transistor turns off. Sometimes the diode is on the coil (it's most effective when mounted right at the coil), sometimes it's on a PCB. When the diode is working, it limits the max voltage the transistor sees to (coil power supply voltage + 0.7V). If the diode is open or missing, the transistor will see a kickback spike equal to (coil power supply voltage + 100s of volts) (basically the thing that will limit the voltage is the transistor's VCE breakdown voltage, and if it keeps seeing that spike, the transistor's gonna fail).

    3) A logic or software failure causing the transistor to be stuck on, so the coil is always energized (or a broken EOS switch/circuit that keeps the coil in the high power mode while someone's trapping a ball). This could generate a lot of heat (explaining some of the heat damage you see on PCBs) and eventually blow a fuse/coil/transistor/trace.

    4) Old age. Eventually the transistors just wear out and fail open (we hope) or closed (hello burned-up coil).

    So it seems like replacing your diodes every 5 years or so would be an inexpensive way to decrease your chances of driver board disaster.

    Thoughts? Am I missing any other causes of transistor death?

    #2 4 years ago

    Sounds like a pretty accurate list. Sometimes they just go and sometimes a locked on coil shortens the life.

    I would not regularly replace any diodes. But when you do replace them use 1N4004. Just stock up on these at 20/$1 they are beefier than the 1N4001 used before.

    #3 4 years ago

    Don't forget the diode on the PPB that corresponds with the blown transistor.

    #4 4 years ago
    Quoted from yonkiman:

    I'm not 100% sure I know all the reasons those suckers blow up.

    Actually I think you do...its just simply voltage

    #5 4 years ago

    The old rule-of-thumb in electronics always applies:

    "Any component can fail at any time for any reason".

    #6 4 years ago

    Heat failure from over current. There is a reason you see some transistors on a heat sink. I have seen transistors have solder balls squeezing out of the package case because it got so hot.

    #7 4 years ago

    Funny that you post this now. In restoring my AFM, I blew a fuse after I turned the machine back on. Coils had been completely removed, washed, and replaced...and I even made sure to wire the coils the same way they had been when they were in the machine. Problem is, some momo at some point had put a coil in with a diode, which AFM doesn't have. Since the position I put the coil back in had the wires reversed from where the coil was initially located (since I did a batch of 1200s and didn't bother to mark where they went back in...who does THAT???), the diode caused the transistor to blow out, as well as a fuse.

    So, yeah...if you buy a machine second hand, checking to see if your coils have diodes on them when they shouldn't is a prudent thing to do, especially if you'll be removing a bunch at once to shop or restore.

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