Converting cut out to 12v

[12jslater]

Before Anything is said I know 6v is fine however this a special so originality is off little concern, what value resistor do I use from the cutout to earth? I believe it’s around 30ohms but what would the wattage be? 

Thank you!

[John Cornforth]

Hi

The shunt coil for a CFR2 should have a resistance of 33 to 39 ohms.  Other 6V cutouts will be similar.
I suggest you measure yours with an ohmmeter between the +D and Earth terminals.  
Add an equal value resistor between the cutout earth and the car earth and it will now pull-in at  13 +/- 0.4 volts rather than 6.5 +/- 0.2 volts.
You might need a couple of resistors in series to get the right value, e.g. a 33 ohm plus a 3.3 ohm to get 36.3  ohms.
The higher the value, the higher the pull-in voltage
The resistor will dissipate just over a watt, so a wirewound type rated at 2 watts or more will suit.

[andrew34ruby]

Or dispense with the cutout and use a diode to prevent the battery from draining through the dynamo. Purpose designed diodes are available or just use a couple of diodes in parallel, using diodes from an old alternator rectifier. Like this...
https://www.ebay.co.uk/itm/256080235299?...NdEALw_wcB

[PaulS]

The attached may be helpful.
I bought a repro Lucas 12v RB106 regulator and cut out, I disabled the regulator as it is not needed.

[12jslater]

If I wanted to connect one of those alternator regulators how would I go about it? I’m not familiar with them?

[andrew34ruby]

If I wanted to connect one of those alternator regulators how would I go about it? I’m not familiar with them?

I simply snipped a couple of separate diodes out of that pack, and joined them in parallel.

[PaulS]

Current through a 30ohm resistor I=V/R, 12/30 = 0.4 Amps max through the resistor.
Power =  IV, 0.4 x12v = 4.8Watts, therefore a 10W wire wound resistor or a metal clad one mounted to the chassis should be good.

The resistor may already be included with the 12v cut-out.

[John Cornforth]

Hi PaulS

In the application discussed, the added 30 ohm resistor only has 6 volts (nominal) across it, the other 6 being dropped in the shunt coil. So that's 0.2 Amps and 1.2 watts dissipation in the resistor. If you assume 7 volts then the dissipation is 1.6 Watts.

I'm pretty sure that 12 volt versions of the cutout use a higher resistance shunt coil, presumably 120 ohms with twice as many turns of finer wire so that it works with 0.1 Amp.