Voltage regulation with a Zener diode

[TJMack]

Let me preface this thread by slaying that I know only enough about electronics to be dangerous. I’m looking for an electrical engineer’s guidance.

I seem to have a lot of directional and headlight bulbs blow out, most recently both directional bulbs on one side at the same time. To me there seems to be an over-voltage problem, but I’m just guessing. I’m thinking that at times the generator/cut-out system puts out more voltage than the bulbs can handle.

I have three questions; (1) can a Zener diode be put into the system to limit the voltage to 6 volts? (2) if this is practical, how do I know what Zener diode and resistor to get? And (3) how do I wire it into the system?

I found a simple diagram on the internet that shows a basic Zener diode diagram that was done for 12 volts. I changed the 12 to 6 v. (see the attached picture) but have no idea if the resistor and diode would be the same.

[alexiskai]

Just on a practical level, you wouldn't want to limit the generator output to 6V, because then it wouldn't be able to charge the battery until the battery's output dropped below 6V, which would mean the battery was already severely depleted.

If you're correct that "at times the generator/cut-out system puts out more voltage than the bulbs can handle," the way to address that is by testing and adjusting the generator and/or cutout to produce the correct voltage. You can adjust the third brush and also check the cutout to ensure it's opening at ~6.5V.

Fordbarn regular ndnchf has some great videos on this subject.

[Kurt in NJ]

Check electrical connections between the generator and battery, the battery is the voltage regulator on a 3 brush system, a bad battery can also cause voltage problem

[rotorwrench]

The Zener diode is generally used on small generator applications like a motorcycle. These systems are limited on the system amperage output they can take so they work OK but they can get hot.

[stickshift]

This type of circuit is typically used on small circuit boards for components that are sensitive to over-voltage. For a car application, the components would need to be very large and able to dissipate lots of heat during operation. Typically lead-acid batteries are 2.2 volts per cell when fully charged. So with three cells on a 6 volt battery, it should be 6.6 volts when charged and higher when being charged. That's why older voltage regulators from the 50's and 60's used coils of wire and contact points to handle the amps involved. It can be done using solid state parts, but not as simple as the diagram because you want a wide range of voltage while being charged.

[Keith True]

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I think you need to dig around and find your real problem first.This is not a problem that requires a band aid on the A.I'm thinking loose connection somewhere,something that lets the battery be taken out of the circuit from time to time.You won't need a band aid fix after you find the real problem.

[stickshift]

Another thought... Batteries are voltage regulators themselves, so a loose cable or connection could cause voltage spikes from the generator that could cause damage. Reminds me of when I rode a Honda 90 to high school. Somebody stole my 6 volt battery so I rigged it to run off the magneto. I blew every light on the bike before I left the parking lot.

[Rob Doe]

FWIW, TJ, There is a known issue with the contacts in the sockets randomly, such as a sharp bump in the road, coming in direct contact with each other. Paul Shinn and Mark Maron have addressed the issue. When the major short circuit occurs, headlights???, its my rookie's understanding that the voltage level goes through the barn roof. The battery is the voltage regulator on the Model A.

As a teenager, out on a hot date to the drive-in, my Model A blew out every light bulb on the car that, was operating, when I hit a very hard bump crossing a bridge over the local rail yard! Had ta call my dad to the rescue...about midnight as I recall.

[AL in NY]

TJ, if you didn't find any loose connections that caused your bulbs to burn out, I would suggest you try using a generator band voltage regulator. James Peterson sells a really good one ([email protected]). This will probably solve your problems.

[jack backer]

Had a triumph trophy 250cc with a zener diode, at night you could light cigarets off it.

Voltage fluctuation affects all components, it shortens life.want a true fix? Do what ford did and run an alternator, rock solid consistent voltage at all rpm ranges.

[TJMack]

I figured it wouldn't be as easy as it looked in that diagram. From what pictures I saw of Zener diodes I didn't think they could handle the amperage draw of an automobile. Mention of the motorcycles reminded me of the 1969 Norton Manxman that I had. It too had a Zener diode voltage regulator that didn't work very well. "Lucas; the prince of darkness".

I'll have to do some checking. Thanks for the great input from everyone.

[Badpuppy]

The 1K resistor across about 1 volt will allow about 1 milliamp of current to pass. What is needed for regulation is a voltage regulator such as Jim Peterson's or Tom Wesenberg's internal regulators or the Fun Projects external type. It appears only Jim's is the last one standing.

[1stWoody]

Make sure you have good ground connections. I remember seeing an article on FordBarn about how to check with an ohm meter. A bad ground will cause the voltage to go high.

[TomInCologne]

Originally Posted by 1stWoody

Make sure you have good ground connections. I remember seeing an article on FordBarn about how to check with an ohm meter. A bad ground will cause the voltage to go high.

Isn't this the most important tip to follow? I have no experience with Model as or vintage cars, but i was so (pleasantly) surprised to see how my car changed after i cleaned all the ground connections and put in more ground connections between generator, starter and battery. The starter, which was slow and apathic before turned into a little whirlwind after extra ground connections were added.


The tip may be: before swapping anything, bring the ground connections in order...

[nkaminar]

Voltage regulation for a generator or alternator needs to be at the source rather than a clamping diode. For the generator this means controlling the field windings. Clamping the voltage with a diode will just mean that the generator or alternator will be pumping power into the diode which will overload the generator and overheat the diode or another electronic element that the diode is controlling.