Dynamo Field Resistor

[David Stepney]

My RP saloon has a C35M Lucas dynamo fitted, but retains its cutout box designed for the earlier C35A instrument. As a result, I have no summer charge setting that works.

Being a complete electrical dunce when it comes to these things, I consulted my 1940 (eighth) edition of the 'Motor' Electrical Manual which appears to suggest that there should be a resistor between the D and F terminals on the dynamo which is shorted out by the lighting switch on the dashboard to give winter charging.

1. Am I right in this assumption?
2. If so, does anyone know how many ohms the resistor should be?

Given that the dynamo is currently (no pun intended!) on my bench awaiting its new brushes, I would quite like to put it back and get both settings working.

Also, I assume that, as presently set up, there is no field fuse in the circuit as the earlier instrument had the resistor and field fuse under a cover on top of the dynamo case. Where should the fuse go in the field circuit, please?

[Reckless Rat]

About 5 or 6 ohms should be enough. I made a "Heath Robinson" one for my RP which (earlier) had a 35M dyno. Just wind some resistance wire around a pencil and fasten in between the two terminals. Leave it in the wash from the fan to keep it cool. Trial and error to get the right level but it's a cheap enough fix. Resistance wire from Maplins or the Bay of E.

[Biddlecombe]

Regret Bruce, Maplins has gone bust in UK

[Reckless Rat]

Thanks for the update. I didn't know. Resistance wire IS available on the Bay though. It's where I got mine. I think I bought about 12". Cheap as them things you make from pommes de terre.

[David Stepney]

Thank you Reckless Rat. I have ordered some Kanthal 26 AWG resistance wire from ebay, which has a resistance of approx 11 ohms per metre. Any idea where the field fuse should go in the circuit?

[Reckless Rat]

Early RP wiring diagram

Later Ruby wiring diagram (depends on your cut out)

http://www.da7c.co.uk/technical_torque_a...ing_v3.pdf

[jansens]

That Ruby wiring diagram is excellent, thank you. Will be a great help wiring up my special.

Simon

[Bob Culver]

I have never fully figured the purpose of the fuse on later cars  as the Cornforth diag. It seems to only protect the resistance, so does not seem very vital. It can be located anywhere where heat from tolerable. It would be at risk if the dynamo output became isolated from the battery whilst on Summer.Curiously the fuse does not protect the field winding on Winter.

[David Stepney]

Merci mille fois, mon brave!

The Ruby wiring diagram tells me everything I want to know. I think that my car would have had a dynamo with the little box on top that contained the field resistor and fuse originally but a later pattern dynamo has been substituted.

[Reckless Rat]

De rien mon pôte. My RP was like yours until the C35M chucked off its mortal coil. I've replaced it with a C35A which as you say has the little box on the top. Assuming you're still 6v on three brushes, set max output at no more than 8 amps, and summer at about 1 amp. You might have to fiddle about with the length of the resistor to reach a happy medium.

B