David Massey wrote:
>
> Only Superman or Steve Austin (Lee Majors not the wrestler) could turn the
> motor by hand fast enough to generate that kind of back EMF. Besides the
> diodes are not even in the same circuit as the motors. The diodes are in
> the circuit with the coils of the relays that run the motors. What is
> causing the diodes to fail (unless it is an intermittant short to ground)
> is the voltage spike from the relay coil (which is inductive) when the
> limit switch opens.
My problem in deciphering the circuit is that there is no switch diagram
on the schematic I have, and assumed such because I wasn't sure of the
connections in the switch, and because of the warning in the manual not
to turn the motors by hand without first disconnecting them.
But, I agree with the above--that's more likely. It's a lapse of the
mind--I'm used to contemporary relays with a built-in suppression diode,
which we use, exclusively, at work. Forgot that the relays in this thing
do not have them, and yes, that would be a more likely cause.
And, yes, a short to ground _before_ the relay coil could also take out
the diode quite easily.
> Nonetheless, when 400 volt diodes are as cheap as anything you can buy, why
> use 50?
True enough, and Messrs. Lucas probably have the answer, lame as it is.
<smile>
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)
|