Educate me on diodes

[alloy]

I have a digital dash in my 55 chevy. Friday I was able to go to a cruise in and drove home in the dark. It's the first time I used the headlights.

The only real problem I had was with my high beam indicator. It's on with both the high and low beams when the engine is running. Not running the indicator light works perfectly. So it's not the led in the dash, it's something with the wiring.I contacted American autowire and they suggested putting a diode in the feed wire to the indicator. I'm not sure if that will work, but I have to start somewhere. The wiring is correctly installed.

The only thing I know about a diode is it's a one way electrical valve. I've searched and basically found that explanation and it's as far as it goes. AAW didn't specify a diode to use and I've looked up 12v diodes and found zener diodes, axial diodes, triac, tunnel, and on and on forever.

I just need a diode to prevent electrical back flow. So what diode will I need for this application?

 

[pdentrem]

Usually the high beam indicator lamp would see 12v from the floor headlamp switch. On the wire that leaves the switch to the high beam lamps, not the wire that supplies 12v to the switch. It seems that wire is seeing 12v with key in run and it shouldn’t.
Did AAW supply a wiring diagram that we can see?
Pierre

 

[mksj]

Any standard type of diode like a 1N4004 will work, the current is on the order of a few mA if going to the LED indicator lamp. I often use them in my VFD builds when there is a back feed situation where DC control power feeds back through the wiring from another source. As noted they are directional so the + anode is from the power source + and the cathode is in the direction toward the negative/ground in a DC circuit. If you PM me your address, I can drop some in the mail to you.

Without a wiring diagram, it is hard to say what is going on and if that will solve the problem.

 

[great white]

Mksj has got it.

But a wiring diagram would be needed as the probable intent of your advisor is to stop currnt from getting to the high beam led when high beams are not selected. you need to suss out where the voltage is coming from before being able to make a diode work.

If thats the case, I’m a little surprised your cluster wasn’t designed with one already in the circuit.

A bulb will light ip no matter what polarity is fed to it. An led is a “one way” voltage device. Meaning, if you try to reverse feed voltage through it, it does not light up. so in order for your high beam led to work, it must be feed proper polarity, even if from the wrong source.

That would indicate to me that a diode might not work because you’d be blocking the proper voltage whether from the correct source or not. This is where the need for a wiring diagram comes in. You need to trace it out to see where there might be some “cross talk” between ciruits…thats the point where you would want to try a diode to block current/voltage from the led.

 

[rabler]

The other thing a diode does, beyond only allowing current to flow one way, is that it will drop the voltage by a small amount, around 1/2 volt, but depends on what type of diode (.3 - .7 is typical). That may be enough to keep noise on that signal line from triggering the input. The ignition system can generate a lot of noise, obviously only when the engine is running. One of the first radio designs was essentially an arc across a gap, same as a spark plug. If the digital high beam indicator is a high impedance input transistor it wouldn't take much noise to trigger it.

In any case, this sounds like a bit of a hack to fix a problem that should have been addressed in the original high beam indicator circuit design.

If you try the diode, put it as close as possible to the instrument cluster input as you can.

 

[GeneT45]

Finally, just to clarify: you *don't* want a zener diode, triac, or tunneling diode (those all have special features you don't want) and "axial" just refers to the package type - meaning one lead comes out of each end of what is usually a cylinder.

GsT

 

[alloy]

The wiring to the headlights is very simple. 12v fused power to the dimmer switch,single low side wire to the headlights. High side has 2 wires, one to the high beam, other one to the indicator light.

It works correctly with the engine off, so its wired the right way. It's some kind of feedback somewhere, I guess you would say it's bleeding current somewhere.

 

[great white]

Well, that would seem to me to narrow it down to an ACC power source somehow bleeding back through to the high beam led.

I’d probably start with a dmm at the led wire and walk it back until I found something that was putting 12v on the high beam led when in ACC and dead when off. Thats only the starting point though….

 

[alloy]

I've been thinking about this and it really doesn't make sense.. The front end wiring (lights) is hidden and ran through flexible plastic conduit underneath the fender to prevent rocks and crap from the tires hitting the wiring, and then crosses over to the right side under the radiator support cross brace. There is no 12v wiring coming from the alternator near the front light wiring other than one lead that powers the fuse block..

I understand that a diode is a one way valve so to speak. If put near the indicator it wold stop power flowing back from the indicator down the wire to the fuse box. I can't see where that would do any good, the feedback is going to the indicator led not from the led.

So in my mind something is happening with the engine running, there is approx 13.5 volts, about 1.5 volts higher when not running.

The main power lead from the alternator runs over to a mega fuse block, then the harness is connected to the fuse block with one 10 gauge wire.. I just can't think of where the power is bleeding over. All the wires that are hot with the engine off are hot with the engine running. There is just 1.5 volts more running than not running.

 

[cathead]

Could it be that the alternator is generating some voltage spikes( I call it hash) that generates
enough EMF(electromotive force) to cause this condition?