Value of Resistor in 6 Volt Cut Out

[Jamie]

I have checked and I have a C35M dynamo fitted now. I have read that this needs a separate resistor (field?), but I am unclear whether this is in addition to the two in the cut-out or whether it is in addition to those.

Can anyone clarify for me, please, as I am getting confused.

Thank you.

Jamie.

[John Cornforth]

Hi again Jamie

To the best of my knowledge, the early C35A had the field resistor in a little casing on the dynamo, to suit early cut-outs with no resistor.
The later C35A used a resistor inside the cutout. The C35M (Late Ruby) also had the resistor inside the cutout.

Some cutouts had two resistors, which could be used for 3 different charge levels. This facility was not used by Austin, so both resistors are used in series. If you want to use a modern resistor, I would suggest a value of 2.7 Ohms and a rating of at least 3 Watts. A metal bodied (gold coloured) type is convenient, as it can be bolted to some convenient metalwork to act as a heatsink. These are usually rated at a higher wattage, but that is not a problem

[Jamie]

Hi again Jamie

To the best of my knowledge, the early C35A had the field resistor in a little casing on the dynamo, to suit early cut-outs with no resistor.
The later C35A used a resistor inside the cutout. The C35M (Late Ruby) also had the resistor inside the cutout.

Some cutouts had two resistors, which could be used for 3 different charge levels. This facility was not used by Austin, so both resistors are used in series. If you want to use a modern resistor, I would suggest a value of 2.7 Ohms and a rating of at least 3 Watts. A metal bodied (gold coloured) type is convenient, as it can be bolted to some convenient metalwork to act as a heatsink. These are usually rated at a higher wattage, but that is not a problem

Thank you, John.

May I just confirm that this 2.7 ohm resistor goes across the F and D terminals of the dynamo and is in addition to the 2.5 ohms and 0.5 ohms resistors in the cut?

Jamie

[John Cornforth]

Hi Jamie

No, you only need one resistor in the system, of about 3 ohms.

It doesn't matter whether this resistor is mounted within the cutout, on top of the dynamo or somewhere else. It always electrically connects between D (The output of the dynamo) and F (The input to the field coils). This supplies a low current via the resistor to the Field coils for Summer charge rate.

When you need Winter charge rate the resistor is shorted out by a switch so as to increase the field current and thereby increase the dynamo output current. This switch may be within the PLC2 Lighting/Charging switch, or may be a separate on/off switch if for some reason you are not using a PLC2.

[Jamie]

Hi Jamie

No, you only need one resistor in the system, of about 3 ohms.

It doesn't matter whether this resistor is mounted within the cutout, on top of the dynamo or somewhere else. It always electrically connects between D (The output of the dynamo) and F (The input to the field coils). This supplies a low current via the resistor to the Field coils for Summer charge rate.

When you need Winter charge rate the resistor is shorted out by a switch so as to increase the field current and thereby increase the dynamo output current. This switch may be within the PLC2 Lighting/Charging switch, or may be a separate on/off switch if for some reason you are not using a PLC2.

Thank you, John.

That is really helpful, clear and simple. I some nichrome wire, so shall see whether I have enough for the resistor. If not, I shall buy one. I shall be using just a normal switch as I have none of the original electrics or switchgear.

Regards,

Jamie

[Jamie]

Thank you for the help with this question. I have fitted a wire-wound 2.7 ohm resistor. It was 3 but I snapped the end fitting it. I retained the Schottky diode as the smart diode that I bought looks very delicate and has no way to fix it down. The Schottky is much more robust and has a screw lug attached.

Jamie.