Re: low power coils with diodes

On Mon, 5 Dec 2005 11:55:40 -0800, Randy P. <pinball@xxxxxxxxxxxxxxxxxx> wrote:
> So for the two coils that were not firing, an examination of the transistors
> showed that they'd been (incompetently) replaced before. So I swapped both
> of the TIP102s and *also* the associated 2N4403s.
>
> Now instead of the coils not firing at all, once they are triggered to fire
> once, the next time they fire they stay locked on forever. I.e. the right
> slingshot fired and retracted once, and the next time it fired it stayed
> locked on. Similarly, Red's mouth eject fired and retracted once, and the
> next time it fired it stayed locked on.

You're missing the diodes that need to be in the circuit to prevent this from
happening. Your new TIP102s are now blown and will need to be replaced again.

> What's going on here?

The TIP102 acts as a switch, completing the circuit. This turns on the coil.
The coil, while on, is a big electromagnet, turning electrical energy in to
magnetic energy. This part worked fine for you.

When the switch turns off, however, there's quite a lot of energy stored in
that magnetic field. Energy is not destroyed, it has to go somewhere. As the
magnetic field collapses, it turns back in to electrical energy. This
creates a high voltage electrical spike, much higher than the rated voltage
of the TIP102 that experiences it. The spike then fries the TIP102, shorting
it internally.

The next time the TIP102 is activated, it locks on, and blows up.

The diode is added to this circuit to prevent this from happening. It
acts to absorb and block the spike from hitting the TIP102.

--
| David Gersic http://www.zaccaria-pinball.com |
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