RE: reversing a 12v line - number of relays?

["Paul" <groups@xxxxxxx>]

Should have known there'd be a DJ reference in there somewhere ;)
Paul.

-----Original Message-----
From: Keith Doxey [mailto:ukha@xxxxxxx]
Sent: 03 June 2003 09:25
To: ukha_d@xxxxxxx
Subject: RE: [ukha_d] reversing a 12v line - number of relays?

No.

As both terminals are at the same potential there is nothing to drive the
motor.

Any motor becomes a generator when you remove the power from it. The
armature continues to spin in the magnetic field and genratates a small
electrical current. The relays being in the same position effectively short
circuit this current thereby increasing the load on the
"generator". This
means it needs more mechanical energy to turn the armature. as the only
mechanical energy is the momentum contained in the whirring armature this
energy is rapidly used up and hte armature becomes stationary very quickly.

In your situation this is a positive advantage because once your projector
lift has reached the desired position you want it to stop as quickly as
possible.

If you have ever seen a Direct Drive Turntable such as a Technics SL1200
you
will have seen electronic braking in action. If you press the stop button,
electronic braking is applied momentarily and the turntable stops dead but
if you just turn off the power to the deck if freewheels for ages before
coming to rest.


Keith

www.diyha.co.uk
www.kat5.tv


> -----Original Message-----
> From: xslash@xxxxxxx [mailto:xslash@xxxxxxx]
> Sent: 03 June 2003 08:39
> To: ukha_d@xxxxxxx
> Subject: RE: [ukha_d] reversing a 12v line - number of relays?
>
>
> are there any damaging effects by shorting the motor? (as in
> supplying +12v to
> both terminals of the motor?)