MTFCA Forum

I’ve been checking out a few miscellaneous cutouts that have accumulated over the years, and was surprised to find a diode in one of them.

I think this one is actually set up for positive ground… I can’t see a polarity indicator on the diode, but current flows when the input terminal is connected to 6-vt Neg and the output is connected to 6-vt Pos. Conversely, the test light is dark when input is 6-vt Pos and output is 6-vt Neg.

So, which wires are crossed… the ones in the cutout or the ones in my brain?

You want the diode to show continuity from the input side (gen terminal) to the output side, (ammeter wire terminal) not the reverse. You want current to flow from the generator terminal through the diode, then through any load, then to ground. Current must not flow from the output (front) terminal back through the cutout to the generator.

Think of the generator as a water pump and the cutout as a check valve.

Thanks for your replies, Pat, and I understand the basic goal: allow current to flow from the generator to battery (assuming sufficient voltage differential) but not the reverse.

My roadblock is in interpreting the test results. Intuitively, I assumed (there's that word) that when the generator terminal is connected to positive and the battery terminal to negative, this mimics the installed situation, yet my test light does not illuminate. Hence, my concern that this diode cutout is set up for a positive ground car.

The graphic below is from Electronics for Dummies, and to this layman, seems to support my guess that this particular cut-out is actually wired for a positive-ground car.

And that is my question: is this cut-out wired for a positive or negative ground car?

I would suggest looking at the website for the Nu-Rex company.....www.nurex.com. ....They produce both positive cut outs for the Model A, and negative cut outs for Model T 's. I'm sure by contacting the company they could accurately give answers to your questions...... Some have complained that the price they charge is too much, but I am assured that you have the best quality dial cutout on the market to purchase from Nu-Rex. !!

diode, not dial cutout.....

Chris,

The diagram is correct. You might want to see if there's a part number on the diode - if so you could look that up online and it would answer your question directly.

Otherwise if your generator is negative ground (as in a Model T) it will be outputting +VE on its terminal and you'd want the cutout to operate when that's applied to its input.

However bear in mind that while the Model T was negative earth the Model A was positive earth, and it can use a similar cutout. In that instance you'd want the cutout to operate when -VE is applied to its input terminal. If this is what you have it's likely from a Model A.

Just to add a little; it's not difficult to change the polarity of a generator equipped vehicle, so if it was the only cutout you had, and your vehicle was the opposite polarity, it'd only be the work of a few minutes to change. As long as you didn't have any electronic equipment attached it wouldn't make any difference to the running of the machine. It could even be good for your coil points, but that's another story...

Again, my thanks to all who replied, and especially Luke.

My intuition was apparently correct, and this cutout suits a Model A or positive-ground Model T. Good to know!

It will work for either, just reverse the terminals. As long as current goes from the generator to the battery when the generator is turning, and none goes back into it when it is stopped, life is good! A diode is simply an electronic device that only lets current go through in one direction. No moving parts to go bad, as simple as it gets! As long as you don’t exceed the rated current in amps, it’s pretty hard to kill.

Are there markings on the back of the diode? Also, in looking at various cutouts on the internet at ones with round mounting holes in flat tabs were for vintage tractors ie positive grounds

Bill is quite right of course, as long as physically reversing the [solid state] cutout is possible, and you don't get it wrong (let's not apply power to the generator!).