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MG MGB Technical - Amps and Volts

I need a little help from the electrical experts for amps and volts

Car 68 MGB

Converted the dash to a 1967 metal dash. Still using the original rocker switch for the hazards. I finally got around to trying to change it to a toggle switch.

I got a copy of the changes from I believe it was Dan Masters and started looking for a toggle switch that matches the 67 switches.

I found a switch at Radio Shack that looks very close to the original switches. It is a Double Pole Double Throw On On switch without a center position.

In the data from Dan he says the switch should be rated at 10 amps. The switch says it is rated at 6 amps at 125 VAC. Will this switch be strong enough to run the hazard flashers on B with its 12 volt power? Or will it heat up and be a potential source of an electrical problem or even worse a fire?

Thanks in advance

Cris
Cris DeYoung

Cris - The 6 amp switch should be adequate, just. I would feel better about it if it were the 10 amps that Dan suggests. The voltage rating is not important, that just telly you that it will switch 125 volts without any arcing. I don't know where you are in California, but you should be able to find a real electronics store somewhere around you (Radio Shack is not one). If you can't find anything, try this site: http://www.allelectronics.com/index.html. They have a good selection of switches and have a lower minimum than the larger mail order places. Good luck - Dave
David DuBois

The power drawn by the turn lights when in hazard mode is 84 watts.

A 10 amp 12V switch will carry 120 watts

A 6 amp 125V switch will carry 750 watts

Direct current has a higher mean power over time and I would halve the AC rated switches safe power capacity which still leaves 375 watts - more than enough to operate the hazards.
Chris at Octarine Services


If power used by by the hazard circuit is 84 Watts, then the current on a 12V circuit will be 85/12 = 7 Amps.

This is greater that the AC rating of 6 Amps on the switch that you have found.

The DC rating of this same switch will be somewhat less than 6A. Chris suggests a factor of 50%, so you are left with a DC capacity of just 3Amps.

However, the switch is double pole then both poles can be wired in parallel, which will give you a capacity of 6 Amps.

One of the enemies of switches is heating caused by current overload. The hazard circuit current is intermittent, perhaps off and on equally, so in practice the heating effect of the current is halved, so using the switch will probably be ok.

A workaround would be to use a relay (current capacity usually about 30A), or a 10A DC switch if you could find one.

Cheers
Ian F


Ian Fraser

Hi all.

There are two issues concerning the current rating of a switch, these being the current that it can carry continuously without damage and the current that it can switch without damage. I doubt if the continuous current capability is significant in this case.

Mains AC current goes 'through zero' 100 or 120 times a second (depending on where you are) which reduces the duration of any arc, so the ac switching rating will be significantly higher than the dc rating as Ian says.
An arc can occur at quite low voltages (certainly as low as 12V, especially if switching an inductive load like a relay or a fuel pump coil). Arcing is the most likely cause of eventual switch failure, though simple mechanical failure seems to be significant in some aftermarket switches.

Using two contacts on the same switch does not effectively double the contact rating as the two contacts are very unlikely to open and close at exactly the same time, this is further complicated by contact bounce.

While the voltage that is being switched does have a small influence on the current that can safely be switched it is not as simple as the power formula suggested above.

Any incandescant light bulb (ie the ordinary filament type) has a lower resistance when cold so will draw a higher-than-nominal current at switch on.

To summarise, I wouldn't use the 6A switch.

Don
Don

Thanks again for all of the information.

I liked this switch because it closely matched the original switches on the dash. I found other switches that hand more capacity but they had the round metal handle.

I will spend a little while longer looking for the right style of switch.

Cirs
Cris DeYoung

Cris, if you can't find a suitable switch you like, use the radio shack switch to control a relay that is rated for the proper current.
John H

This thread was discussed between 29/01/2006 and 30/01/2006

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