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Re: Is a relay needed in a fog light installation

To: "Barrie Robinson" <>
Subject: Re: Is a relay needed in a fog light installation
From: "Bob D." <>
Date: Sun, 29 Dec 2002 14:14:59 -0000
I think the idea is to add a separate wire of sufficient size for the fog
lamps. I personally would add a line to the battery (where the heavy battery
cable first meets a terminal) with it's own inline fuse. The circuit
controlling the relay would only need to provide the miniscule amount of
current the relay coil requires. A nice arrangement might be to power the
relay with the head light circuit though a switch. That way the fog lamps
could be turned off if not required, and would automatically turn off when
the headlights are turned off.

Bob Donahue (Still stuck in the '50s)
Email -
52 MGTD - NEMGTR #11470
71 MGB - NAMGBR #7-3336

----- Original Message -----
From: "Barrie Robinson" <>
To: "Bullwinkle" <>; "mgs" <>
Sent: Sunday, December 29, 2002 7:44 PM
Subject: Re: Is a relay needed in a fog light installation

> Folks at world,
> Christmas cheer has dulled my senses - but surely a relay will not
> compensate for wiring intended for only one lamp - and allow two current
> sucking lamps to be put on a wire intended for one only one.  Doubling
> amperage on the wire may just run it a bit hot.
> While mumbling on....I have to confess that I am puzzled over the use of a
> relay.  It does not change the voltage, it does not change the draw of the
> electrical device, it does not make wires carry more current
> (devices).  What it does do is allow a power circuit drawing big amps to
> controlled by a low amp circuit (switch).  Obviously this saves on thick
> wire and heavy duty switches but this is negated by the cost of a relay.
> to the drop in voltage, the use of slightly thicker wire would help the PD
> which surely does not drop all that much - or does it?  Never worked it
> or measured it, but I suspect the resistance of connecting wire is very
> small.  I am missing something as manufacturers would not use relays
> especially as they are comparatively expensive.  So can anyone explain in
> engineering terms what their advantage(s) is(are) please?
> At 02:26 PM 12/22/2002 -0600, Bullwinkle wrote:
> >Jay
> >
> >On the MGA and earlier, the original wiring for the fog lamp
> >was intended for only one fog lamp.  If two lamps are fitted
> >then a relay is needed as the wiring is not heavy enough for
> >two.  The MGA parts book lists the relay as for "USA only."
> >My guess is that at this time it was permissable in England,
> >and maybe Europe, to run with only one "head" lamp.  Thus in
> >fog conditions the heads were shut off and only the fog
> >used.  In the US, probably most states had laws requiring
> >two "heads."  Thus if one were to shut of the low beams, you
> >needed two fogs to comply with state regulations.
> >
> >There is and added advantage to using a relay in that the
> >wiring may be shorter depending on the battery location.
> >Thus there may be less "line loss."
> >
> >The wiring of the relay may be done in such a manner that
> >the fogs only operate when the heads are on low beam only,
> >work only with no heads, etc.
> >
> >My driving lamp is wired into a two position switch.  In one
> >position the lamp is aways on.  The other position the lamp
> >is only on when the high beams are up.  Thus putting the
> >heads on low beam causes the driving lamp to go off.
> >
> >Blake
> Regards
> Barrie
> Barrie Robinson -

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