Cutout issues (again)

[JoeRowing]

As part of my efforts to get a working dynamo I've been poring over the electrics today and I've discovered an interesting thing - as soon as I turn the ignition switch on the cutout closes and it stays stuck closed after the ignition is off unless I flick it open or disconnect the battery. This happens even without the dynamo attached (I was doing diagnostics).

Presumably this explains why the dynamo is in such a mess, it must have had a few years of the battery burning out the coils I guess?

I can't for the life of me work out what's happening. I'm trying to work out if it's too high a current draw on ignition or mis-wired at the cutout.

[Ivor Hawkins]

Sounds like a job for mr Cornforth!

[David Stepney]

The cutout bobbin has two coils wound into it: A fine high resistance voltage coil that senses the voltage rise from the dynamo as it begins to charge, pulling the points together, and a thick low resistance current coil that takes the charge to the electrical system and thence to the battery. It sound as if one of them is going straight to earth.

One can get cutouts repaired, but I don't know where. Perhaps another forum member will know. Otherwise, I should look out for a replacement, or get one of the diode replacement cutouts that are available..

[JoeRowing]

thanks - i'll keep my eye out for a replacement.

[bob46320]

Joe, I would suspect your wiring before I put a 'new' cutout into service. Its easy enough to disconnect the cutout and dynamo from all the existing wiring and "jury rig" a new circuit from scratch. Have a look at the Cornwall Austin 7 site for a simple test circuit.
Bob

[Howard Wright]

Hi Joe

If you can’t get it working these are a good replacement.

https://www.themagnetoguys.co.uk/smart-diode-cut-out


Cheers

Howard

[JoeRowing]

Thanks all. Today's job is to pull it apart then!

[JoeRowing]

Tested the cutout and the fine wiring is open circuit.

[John Cornforth]

Hi Joe

I have been on holiday in deepest Wales where mountains and mobile internet don't mix !

An open circuit finely wound "Voltage/Shunt" coil is clearly not going to help matters, but I am still a little surprised at the behaviour you describe. With the cutout initially open, there should be no current flow through the thickly wound "Current/Series" coil either. With no change in magnetic field from either coil, one wouldn't expect any movement of the cutout armature.

As the two windings are wound one on top of the other (Thin one on the inside), it's not impossible there is a short circuit from one to the other due to chafed enamel insulation. The most thorough test is with all wiring disconnected from the cutout. The voltage coil from +D (Yellow Dynamo Output) to E (Earth) should measure about 36 ohms. The current coil from +D to A should be open circuit with the points open, and near short circuit with the points held closed. Do make sure that your meter probes are sharp and pressed hard enough to any contact to pierce through tarnish. If the cutout passes these tests, the finger of suspicion swings more towards the wiring, not forgetting the short connection from the Earth terminal to the car's body. Another item to check is the ignition warning lamp, which should ideally be in working order and definitely not short-circuited.

If there is a fault and you need to get the cutout rewound, here is the detailed information:

1. Shunt winding: 5000 turns 36SWG (About 8 thou), 140 turns per layer, 36 layers. Start and finish at bottom of bobbin. Wind anticlockwise as viewed from Armature. Start and finish to be connected to flexible tails and identified. (Start = Earth, Finish = “D”)
2. Inter-winding insulation: thin PVC or similar
3. Series winding: 50 turns, 16SWG, 17 turns per layer, 3 layers. Start at bottom leaving 10cm tail to connect to “A”; finish at top and connect to armature contact. Wind anticlockwise as viewed from Armature.

The shunt winding has a resistance of 36 +/- 3 Ohms, which means that at 6V approximately 160mA flows in the winding. Thus it produces 0.16 x 5000 = 800 Ampere-Turns of magnetic attraction. In order to counteract this, the force in the series winding must be equal but opposite. As the winding has 100 times fewer turns, this would occur at 100 times more current, about -16 Amps. The reason the cut-out operates at a lower reverse current, as seen on the ammeter, of somewhere between 5 and 10 Amps is that the spring on the armature acts to assist the unlocking action; hence the “drop out” current is significantly lower than the theoretical 16A.

[Ray White]

It may not be relevant but when I had my cut out restored a few years ago I discovered that the 'helper' spring was weak to the point of being useless. I experimented with different springs until I found one that worked. I have had no cut out problems since.

Incidentally, (and I am not recommending it) but before I had my CF1 cut out restored, as a short term measure my auto electrician father in Law fitted what is sometimes called a "hot wire" regulator. A heavy insulated wire - ideally nichrome - connects the battery to the dynamo via the ignition switch. The charging and ignition circuits should be separate so this is not exactly textbook! What happens is that the wire heats up as current passes one way or the other. This has the effect of increasing resistance thus slowing the current. Again experimentation is needed but it is an old trick that goes back to when Adam was a lad. It was fitted to my car as a 'stop gap' ... that continued to work well for several years!

One word of warning. Never leave the ignition switched on for long without the engine running... and be sure to disconnect the battery if charging up is needed.

Ideally sort out the problem correctly rather than take any risks.

Ray.