1/2 Charge Resistor - Advice Please.

[John P]

Hi,

My Ulsteroid has a C35M dynamo and what I believe is a CF3 cut-out.

Neither the dynamo, cut out or switch panel incorporate a ½ charge resistor so at present I only get any charge in the winter position.

I’ve seen various suggestions of winding an open coil of resistance wire and attaching it between the dynamo terminals but it’s all a bit vague and having an exposed coil sound a bit vulnerable especially if it's not wound on a core.

Is there a proprietary wire wound resistor I could use and what sort of resistance / current capacity would I need?

Any suggestions would be much appreciated.

John.

[Ian McGowan]

one of these resistors off eBay 3ohms 25W £3-49 ea.
Hide behind dash - making sure that the terminals are insulated - and connect between "F" and "D" terminals. On winter this will be shorted out to give full output from the dynamo.

https://www.ebay.co.uk/itm/25W-Golden-Al...%3A2334524

[John Cornforth]

I second the advice for a metalclad 3 ohm resistor.  To achieve its full 25 Watt power rating it is designed to be bolted to a flat heatsink.  For the summer charge application I'd expect the dissipation to be only a few watts, so you should be OK just mounting it in air.  From a mechanical point of view probably still best to bolt at least one end down to something firm. The metal cases are electrically insulated from the resistor.

[Steve Jones]

I've been using these since the mid 1990s with no problems at all. One of the bolts for the Patent Plate makes an ideal fixing behind the dashboard.

Steve

[David Stepney]

Following advice on this forum given previously, I fitted a 100W 2 Ohm resistor, bolted to the bulkhead behind the Dash. £2.97 from fleabay. It cuts the charging rate exactly in half.

[DaveT]

Very useful information, my 1935 Ruby only charges on the winter setting if I switch to summer then there is no charge at all. So I've just spent £3.49, and I'll follow your advice and fit it between F and D.
Thanks 
Dave

[John Mason]

I use a little Non to expensive electric box of tricks made by Ron Bailey a member of the PWA7C with instruction from him slight modifications are made to the wiring of the dynamo and the 3rd brush is removed. This allows for better charging of the battery with no requirement for Summer/Winter switching. I have had this for the last 6 years without any problems. The other benefit is that it charges faster and is kinder to the battery as it shuts off charge completely when the battery is full of charge

John Mason

[David Stepney]

I recall from my youth being told that a cold start using the starter took the dynamo between 5 and 10 minutes to bring the battery to full charge.

Bearing this in mind. when I first start the car, I use the full charge setting for ten minutes and then turn the switch to the half charge setting for the rest of the journey. After hot start, I switch to full charge for between 3 and 5 minutes and then go back to half charge.

I haven't had any battery problems so far.

[Bob Culver]

The power dissipated is the volts dropped squared divided by the resistance ; so 10 watts should be far more than adequate. Someone here quite recently used a bulb so if you have lots of half dud ones could experiment.
The original fuse in the later cars is a mystery. It seems to only protect the resistor.
Dave T the resitor is not visible and my not be electrically connected . John Cornforths circuit diagram might be useful.

[John P]

Hi,

I bought one of the 3 ohm resistors as suggested and have secured it behind the dash using one of the Patent Plate fixings.

I've soldered some tails onto the resistor and now need to connect it to the switch plate.

However, I'm not at all sure where it should be connected as my switch panel is marked as on the wiring diagram below.

   

Any clarification would be appreciated.

John.