Easy? / Possible? way to trigger 12 V relay with 6 V?

[motolectric]

I have a friend who is working with a classic car restorer. They are using relay based kits I suggested they build to upgrade the headlights. They would like to also add those to some older cars with 6 volt systems. They are running into problems with the size of the old 6 volt relays and the headshells on the cars with the outboard lamps.

The newer style micro relay would make things easier but they need at least 8 V to trigger.

Is there a small (small, cheap & easy to get) transformer or something that could be used to step up the voltage?

Thanks for any tips/advice.

M./

 

[Sceadwian]

No, you'd have to get a DC to DC converter.

 

[crutschow]

If the relay will stay pulled in at 6V you might be able to use a capacitor bootstrap circuit to provide a 12V pulse from the 6V battery to pull in the relay. Basically you charge a large capacitor to 6V and then connect the capacitor in series with the relay coil (+ side to the coil) using MOSFET switches. You then apply 6V to the minus side of the capacitor which generates 12V across the relay coil (6V from the capacitor plus 6V from the supply) to pull it in. As soon as that happens you connect 6V directly to the coil (which could come from a spare relay contact) to keep the relay pulled in.

 

[Boncuk]

Hi,

Carl's suggestion works reliably only if the relay contactors don't have to supply full rated power.

When holding the relay closed at 6V the pressure on the contactors is decreased compared with full pressure at nominal relay coil current.

Boncuk

 

[MrAl]

Hi Boncuk,


Really? Since when is the force curve for the armature a linear function of the applied voltage?
In most relays (maybe all) as the armature pulls in the force increases until it pulls in all the way,
and there it forms a complete magnetic circuit with low reluctance which keeps it all the way pulled
in with lower current than before when the armature was not that close to the core.
That's the way most relays work, and that means a lower voltage will keep it pulled in, with just
the same force on the contacts themselves.
Sure, it may require more than 6v to keep pulled in, but if it does than it will open completely.
It will open completely because if it opens just a little then the current requirement to hold it there
goes up again, which means it opens even more, and that causes another increase in the
current requirement, and it opens more, until fully open or near so.

I guess there is always the possibility though that the cap is not given enough time to charge up
and so the relay wont operate that one time, when it would be assumed that it would.
I think it at least deserves some testing.

If there is a 12v rail somewhere of course a transistor could be used to turn on the new relay.

Alternately perhaps the relay can be replaced with a lower voltage coil type.

 

[VideoGuy]

I think your easiest bet is to replace the relay with a 6v one. While the capacitor would likely work, I would be concerned that in an automobile, it could be problematic. Let's say you drive over a bump and that opens the relay mechanism because 6v is not quite strong enough to hold it closed firmly.

 

[colin55]

12v RELAY ON 6V SUPPLY
This circuit allows a 12v relay to operate on a 6v or 9v supply. Most 12v relays need about 12v to "pull-in" but will "hold" on about 6v. The 220u charges via the 2k2 and bottom diode. When an input above 1.5v is applied to the input of the circuit, both transistors are turned ON and the 5v across the electrolytic causes the negative end of the electro to go below the 0v rail by about 4.5v and this puts about 10v across the relay.

 

[Boncuk]

This circuit uses one more transistor and switching on is done instantaneously, off with a little delay of approximately 100ms.

All those circuits depend a lot on the coil resistance.

This example won't work if the supply voltage is just 1/2 of the relay voltage at a coil resistance of 600 Ohm.

Before putting in that many parts I'd consider using a relay matched for the supply voltage anyway.

Boncuk

 

[MrAl]

Hi again,


Of course another way is to just use a second relay with a 6v coil to drive the large capacitor and 12v relay.
Really needs to be tested carefully though any way it is built.

 

[MrDEB]

why not use a 5 v relay? or maybe an scr instead of the relay??
An SCR is maybe the easiest way to do this.

 

[Diver300]

What size relay do you want?

I once rewound a standard automotive relay (one of the ones that is about a 1 inch cube) to be current operated rather than voltage operated. It took some time, as I counted the turns that I took off, and there were about 2000.

You could try the following:-
Unwind a 12 V relay until about half the turns are removed.
Join up what is left to the terminals.
Replace the turns that you took off.
Connect the replaced turns in parallel with the original half, making sure that the turns go the same way round.