Hi, This is my first post here. I'm working on rewiring my old car and am stumped when it comes to deciding what Zener diode I need.

I am putting it between the alternator and coil to prevent reverse flow of electricity when the key is switched off. Without this diode the system will stay running.

So I'm dealing with anywhere from 10-14 volts and just need to stop the flow from going backwards. What Zener diode should I get? What would the specs on it be? IE: watts and resistance?

I'm a novice when it comes to this stuff so go easy on me. Thanks for any help you can offer.

-Adam

 

Why a zener diode? Power diodes prevent reverse currents, but zeners actually allow it, dropping their rated voltage in the process.

Whilst I know diodes, cars aren't my strong point.

 

I'm not sure that a zener diode is required... What other diode would work for only allowing a one way current with the voltage being around 14v max?

With the power diode is there any risk of frying it? The power will try to loop back around in the system, thus keeping my car running when I tell it to stop.

 

Any power diode, the important rating in this case is current. How many amps do you think it would conduct when it is carring current?

 

Though it is low resolution... You can see the diagram here that I am dealing with.. If you look in the upper left corner at the alternator, from the plug #1 goes through a "resistor diode" before going to the coil resistor. I didn't wire in a diode when I first started the car and because of it I had to pull the coil wire to turn it off because the system will keep itself energized with the key off.

I ended up disconnecting all of my accessories thinking that they were back-flowing the current to the coil with the key off, no success. So I reverted to the internet and admitted confusion and asked for help. Turns out the diode is pretty important.

I must have started and stopped (pulled the main coil plug wire) 10 times. Shocking the heck out of myself every time with three to five hits of 40,000v. I didn't need anymore of that...

 

Amps that it will be carrying.. Hmm.. I'm completely guessing here but I think my alternator is only rated to 63 amps or so and I think they mostly flow through another wire, but I could be wrong. As long as I can stop the reverse flow, and allow forward flow, I'll be happy.

 

The good news is I think we have enough information to solve your problem. The bad news is I'm not the man to do it. We do have car buffs on this site however, so you should get what you need soon.

 

I really appreciate your help - I'm already picking up a lot of information I've always wondered about from browsing this site..

I'm thinking this one might work.. It will block ALL backwards current until the PIV and then fry, right? So as long as I get one big enough to sustain forward current, it should work.

 

Almost any power diode will cut off just fine, the diode you're looking at fries if it is conducting more than 3 Amps. The good news is that is a fairly small diode, there are bigger and better, and you can parallel them to distribute the load.

 

Nice. Again. I appreciate all of your help.

If anyone else out there has any input, I can use all the help I can get to figure out the right one.

On a sidenote, I just had to call the police for some sort of domestic disturbance outside my house.. Some people yelling at each other like maniacs.. Sounds like a girl/guy..

 

I've got a good digital multimeter - I think I'll hook it up tomorrow and see how many amps are going through it. It should be a good bit of info to have.

 

I believe this is what you're looking for. Sounds like you converted from generator, to alternator. I've marked up your drawing in red to show change. That wire feeds the alternator to excite the field, allowing it to produce power. When the key is cut off it back feeds the ignition system allowing it to stay running.There is a "one wire" alternator made to do what you want, but this is a lot cheaper. I use a 6 amp diode, available just about anywhere
with the cathode (banded end) facing the alternator. In the drawing you show what looks like a "resistance diode???" it would go in that location.

Just to clarify: it is NOT a 50 volt ZENER just a standard diode.

 

That diagram is poorly drawn, too convoluted for a very simple system.

What says "Resistor diode" should be a plain pilot lamp, the one on the dash that shows 'not charging' or 'Alt' See no reason to be a diode. Can be a resistor too, but will tell no tale.

The alternator has tho terminals besides the output terminal : one is the reference, that reads the voltage of the system at the point you want it to be monitored in order for the internal voltage regulator to output 13.8V.

Notice it loops in the schematic being connected to the alternator output. On a GM alternator from the 70'-80's, that is the terminal that is NOT the closest to the alternator output marked 'Bat'

The other terminal with the pilot light enables the alternator to generate, can be connected to the coil + with the bulb in series.

If the ignition switch is bad or misconnected, it will give trouble also. Check the carburetor return spring not fully closing, main reason to 'overrun'

Do not believe everything posted on the web. If I find a better schematic I will post the link.
Miguel

 

I'm working on rewiring my old car and am stumped when it comes to deciding what Zener diode I need.-Adam

What year, make etc. ?
Does rewiring mean replacing wires because of cracked/corroded insulation or installing new wires that have burned up or been cut out/butchered by some one else?

 

If your working on a car and need the diagrams check with alldata and see if they have your model.

 

I agree the diagram is poorly drawn but it is the most complete and usable one I could find. I put the diode in the system and it worked. The key turned the system off.

I'm going to do some more tinkering with it this morning and hopefully end up with a functional system.

As for the make and model. It is a 1963 AMC Rambler Classic 550. (originally an inline 6 using a 12v generator) Now running a small block Chevy.

Old pic - http://picasaweb.google.com/aralbright/Bodywork/photo?authkey=_-QulcLwqYQ#4978388264844066834

 

Coolness to the extreme. I like older cars.

 

When i was muuch younger we put a '67 z28 302 in a 64 Rambler American station wagon. We used the old Hurst front motor mount(one size fits all). We had a 350 turbo and '65 Mustang rear end. The Mustang rear bolted right in and allowed us to keep the same bolt pattern all around. We had to relocate the battery in to the spare tire well in the back of the car. I believe the drive shaft was from a '55 chev. You'll need to watch the front end, trunions aren't/weren't very durable. Do like we did and have fun!!!