Re: Star Wars Power Supply

On Tue, 17 Aug 1999, John Robertson wrote:

> True that switchers can introduce their own noise, and there certainly are
> unreliable brands out there, but I find the switchers have the one nice
> feature of no catastrophic failures. A linear supply can short out the pass
> transistor and thus put the unregulated 10.3VDC across the game board. I
> have seen this on a Williams pinball game, a Pinbot (I know, I know, but,
> hey, the shots are vectors, aren't they?) where the MPU board was
> completely destroyed, chips with their tops literally BLOWN off.
> Other games that use multiple voltages if one voltage fails, then the RAM
> can burn up. So that's why, perhaps for rarest of games, I would recommend
> seriously thinking about installing a switcher.

        Most linears DON'T have this "feature." One notable one that does
(like you mentioned) is the Williams power supply +12V line. Oh wait,
you're talking about pinballs...I'm not sure what the Williams pinball PS
looks like, but the vid. power supply does have that "feature" on the +12V
line.
  
> Now, one other good solution might be to get a SCR "Crowbar" circuit and
> install it just after the pass transistor. Gottlieb pinballs (vector shots
> again!) use this, and I have seen the SCR do it's job a few times over the
> years, and protected the logic boards from regulator failures.

        It's even simpler than that. Just put a Zener (one that is say 1V
higher than the nominal output voltage) across the output. They sell
"crowbar" zeners now (which I think are actually just fast zeners) which
look like they are specifically designed for overvoltage protection.

Joe
Received on Tue Aug 17 11:25:16 1999