A diode in series with the positive supply lead will protect an electronic
device from reverse voltage. Perhaps this is how this particular device is
designed. It is even possible to use four diodes (called a bridge) to make
an electronic device polarity independent. I've seen a CB radio that was
designed this way.
Bob Donahue (Still Stuck in the '50s)
Email - firstname.lastname@example.org
Cars: 52 MGTD - #17639
71 MGB - #GHN5UB254361
Member: NEMGTR #11470
NAMGBR # 7-3336
Hoosier MGB Club
Olde Octagons of Indiana
----- Original Message -----
From: "JohnD" <email@example.com>
To: "MG" <firstname.lastname@example.org>; "Rick Fisk" <email@example.com>
Sent: Tuesday, August 28, 2007 9:45 AM
Subject: Transistors, diodes and fuel pumps
>A question for you electronic types: About 20 years ago, while siting at a
> bar in North Wildwood, NJ, a friend of mine did the transistor upgrade to
> the low pressure SU fuel pump from my TD. It has worked flawlessly and
> burned points have been a thing of the past. However, last weekend, I
> dropped in an Optima battery a friend had given me, and like a dummy, I
> connected it with a negative ground instead of the appropriate positive
> ground. Course, when I turned on the key, the fuel pump remained silent.
> banged on it, and did the usual stuff-- but the power was reversed through
> it for quite awhile before I came to my senses.
> Last night, I noticed my stupidity, turned the battery around, and the
> pump started right up!
> Why did the polarity-sensitive electronics not "fry" when the got juice
> the "wrong" direction? Should I assume pump failure is near in my future
> and swap pumps now?
> Many thanks for the education.
> Racing in the Past Lane!
/// unsubscribe/change address requests to firstname.lastname@example.org or try
/// Archives at http://www.team.net/archive/mg-t