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| What does this transistor do? |
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| cybermaus:
So this is a controller for a old MIG welder. The 555 circuit on the right, together with the two darlingtons, is providing a 1KHz PWM control for the speed of the welding wire feed. The 555 circuit on the left is for pulsed welding, so a pulse of 2 to 0.5Hz, so it switches on-and off the welder (and the PWM) I cannot for the life of me figure out what the function of the top transistor is supposed to be. It looks like a standard half H bridge, but if you look better, it does not fit, because there is no need to feed +30V into both sides of the motor. Under ideal conditions, it turns on when the bottom turns off, like a H bridge, so not a problem. But what is it supposed to do? Some snubber for the motor? The diode already does this. It does not drive anything? Well, in fact, it drives a short when the bottom transistor also turns on. It is a problem, because everything is not ideal, the reset signal of the 555 takes some time to propagate, and during the transistor swapover it briefly shorts. This eventually burns out the the transistors. If a simply leave out the top transistor, everything simply works. Which is BTW also how I found the problem: After decades of working well, the welder stopped, and I discovered both transistors burned. I replaced them both, and it all worked a few days, and then burned again So again, if I leave the top transistor off, it all works. And now I suspect that is how it all worked all those decades. Top transistor was probably broken all this time, I should only have fixed only the bottom one But what is the top transistor even supposed to do ?!? See attachment Also I have been playing with falstad simulator. https://tinyurl.com/2f8poj3o |
| Andy Watson:
--- Quote from: cybermaus on December 17, 2022, 11:19:58 pm ---I cannot for the life of me figure out what the function of the top transistor is supposed to be. --- End quote --- I think it is supposed to halt or reduce the wire feed by stopping the motor in proportion to the "pulse off" period. You wouldn't want to be feeding wire into a cooling melt puddle. It's a somewhat brutal method of stopping the motor! --- Quote ---It is a problem, because everything is not ideal, the reset signal of the 555 takes some time to propagate, and during the transistor swapover it briefly shorts. This eventually burns out the the transistors. --- End quote --- Indeed - brutal :) Have you considered that there may be some other component failure that is allowing too much overlap of conduction between the two transistors? Are V21 and/or V31 functioning correctly? --- Quote ---If a simply leave out the top transistor, everything simply works. Which is BTW also how I found the problem: After decades of working well, the welder stopped, and I discovered both transistors burned. I replaced them both, and it all worked a few days, and then burned again So again, if I leave the top transistor off, it all works. And now I suspect that is how it all worked all those decades. Top transistor was probably broken all this time, I should only have fixed only the bottom one --- End quote --- Perhaps it's a design error! Perhaps the top transistor blows before it leaves the factory and most users just compensate by turning the wire-feed down a notch? |
| magic:
--- Quote from: cybermaus on December 17, 2022, 11:19:58 pm ---It looks like a standard half H bridge, but if you look better, it does not fit, because there is no need to feed +30V into both sides of the motor. --- End quote --- I wrote a different answer initially as if the PNP were driven by PWM rather than the pulse controller, this is deleted now |O Andy is right; short circuiting the two ends of a motor stops it. |
| cybermaus:
--- Quote from: Andy Watson on December 18, 2022, 12:00:00 am ---I think it is supposed to halt or reduce the wire feed by stopping the motor in proportion to the "pulse off" period. You wouldn't want to be feeding wire into a cooling melt puddle. It's a somewhat brutal method of stopping the motor! --- End quote --- You mean like an active brake right. Dropping V+ on both side with some oemph behind it to stop it by force rather then by freewheeling? Does that even work? Is such an concept normal? Never seen that done before, but this is not my area of expertise. --- Quote ---Indeed - brutal :) Have you considered that there may be some other component failure that is allowing too much overlap of conduction between the two transistors? Are V21 and/or V31 functioning correctly? --- End quote --- Considered? Yes. Figured out which one? No. The 555 (actually, they used a 7855 clone) seems to work well. --- Quote ---Perhaps it's a design error! Perhaps the top transistor blows before it leaves the factory and most users just compensate by turning the wire-feed down a notch? --- End quote --- Tempting thought But no. I seen variants schematics, all using the same. So it would be a very widespread mistake. |
| magic:
--- Quote from: cybermaus on December 18, 2022, 11:17:49 am ---You mean like an active brake right. Dropping V+ on both side with some oemph behind it to stop it by force rather then by freewheeling? Does that even work? Is such an concept normal? Never seen that done before, but this is not my area of expertise. --- End quote --- It's not about applying V+ because the motor doesn't care about common mode voltage. It simply is about shorting the two sides together. Try to short the motor with a wire (with everything powered off) and turn it by hand, it should become much harder. The motor should also be difficult to turn in reverse direction, because it's shorted by V20. |
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