Hints and Tips - 6V Horn on 12V

From: David Stansbie
Sent: 07 October 1999
To: British-cars-pre-war
Subject: Zenner diodes

I have a 6 volt horn which I want to run on a 12 volt system on my MG J2. The resistance is 1 ohm. I could use a 1 ohm, 50 watt resistor but I've also heard that a 6 or 7 volt zenner diode might do the job. Has anyone tried this?

Also I'm still trying to source an Adamant steering box to replace the Marles Weller one that was standard fitting.

----------------------
David Stansbie


From: Bishop Peter
Sent: 07 October 1999
To: 'David Stansbie'; British-cars-pre-war
Subject: RE: Zenner diodes

I just run my 6v horn on 12v without modification - it works fine (extra loud!)

Pete Bishop
Swindon Uk


From: Dr G W Owen
Sent: 07 October 1999
To: Bishop Peter-QSWI217
CC: 'David Stansbie'; British-cars-pre-war
Subject: RE: Zenner diodes

When i did this on my Morgan, I simply wired a bulb in series of what I thought was the right wattage. Un-noticeable during the day it is amusing to light the underbonnet and night - which of course you shouldn't do-blowing the horn in the hours of darkness!

Geraint


From: Ian Grace
Sent: 07 October 1999
To: David Stansbie; British-cars-pre-war
Subject: Re: Zenner diodes

David,

Zeners are good for dropping a small voltage or in applications where the unit to be driven has a reasonably high resistance. In your case, you would use a 6V Zener and a drop-down resistor. But as the current going through the horn is 6 Amps, then the resistor would have to be 1 Ohm and 36 Watt rating (so you'd use a 50W), so you are no better off with the Zener in this case.

Of course, if you put the resistor down by your feet, it would keep the frostbite in your toes away...

Ian Grace
(B Sc, M Sc Elec Eng, Bath!)


From: Barry Lovelock
Sent: 07 October 1999
To: David Stansbie
CC: British-cars-pre-war
Subject: Re: Zenner diodes

I use a 12 volt horn on a 6 volt system and simply adjust it so it works, but in your case just use it and see what happens. I also use a 6 volt Klaxon on 12 volts, which of course has a 6 volt motor rather than the normal vibrating diaphram of a normal horn. It is very loud and much quicker to wind up than on 6 volts, but as the duty cycle is so low you are unlikely to do any damage unless you lean on it for too long.

Barry Lovelock.


From: David Cochrane
Sent: 07 October 1999
To: David.Stansbie
CC: british-cars-pre-war
Subject: Re: 6V horn on 12V

> I have a 6 volt horn which I want to run on a 12 volt
> system on my MG J2. The resistance is 1 ohm. I could
> use a 1 ohm, 50 watt resistor but I've also heard that a
> 6 or 7 volt zenner diode might do the job. Has anyone
> tried this?

As you are (hopefully) restrained in the use of your horn, you will probably find that you can just run it off 12 volts with no major modification. I assume that it is a make-and-break unit rather than a klaxon type. When you try it on 12V, you should hear a rather higher note than before, so adjust the contact gap setting screw inside to reduce the note. Make sure that it still starts up OK when you've done this. You can put a resistor in series if you insist, but I would have thought that 2 or 3 ohm would be more suitable, and around 20W (rather than 50W). A Zener diode is NOT suitable.

I always thought that Marles steering boxes had a good reputation for quality - perhaps the ones fitted to MGs weren't so good?

Regards,

David C.


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