Easy peasy

[Randall]

I have a Harley with a three wire sensor.

red - 12v
black - gnd
white - 5v square wave signal return.

I want to make sure it never sees more than 12v. I have a clamping zener/resistor in mind to shunt anything coming down the line over 12v to ground. From what I have read (have not confirmed), sensor draws about 10 mA.

Firstly, good idea? Bad idea? If it is an acceptable idea, choosing values is hanging me up a little. I had a 12v 1N4742 1.3 watt zener that I cooked because I am doing something wrong.

So if this will work OK< what wattage zener and resistor ohms should I use?

The circuit I am looking at:

Line feed 12v > 12v zener > resistor > gnd. Circuit load connected to junction of 12v feed and zener. My thinking being, 12v supply chugs along going it's merry way onto the circuit load until something higher comes along and starts to turn on the zener, which dumps the extra voltage to ground thru the resistor, lowering the voltage and starting to turn off the zener.

Is this right?

[x-pro]

which dumps the extra voltage

Where is the excess voltage coming from if the source is giving 12V ?

[x-pro]

Instead of Rn, your sensor is turned on.

[x-pro]

Line feed 12v > 12v zener > resistor > gnd. Circuit load connected to junction of 12v feed and zener. My thinking being, 12v supply chugs along going it's merry way onto the circuit load until something higher comes along and starts to turn on the zener, which dumps the extra voltage to ground thru the resistor, lowering the voltage and starting to turn off the zener.

Please draw a schematic of this circuit according to your description, if you are familiar with drawing circuits.
If not, we would like to see a photo of these parts and connections.

[Randall]

High voltage from spikes or surges in the 12v motorcycle generating system. rotor, stator, regulator/rectifier

Trying to protect a 12v sensor that is failing electrically.

[x-pro]

Then it's probably better to put an additional similar part before the circuit above B72210-S 110-K101 (S10K11) , if the mains voltage does not exceed DC14V, and a 50-100mA fuse in front of it.
The stabilizer can be omitted.
If an emergency situation occurs, the fuse must blow and possibly the varistor itself. The cost is cheaper than that of a sensor.
How to connect the varistor correctly:

[Randall]

The above circuit is too involved for my application. I need simple and very small, like a couple of components spliced into a wiring harness type of thing. That's why I am fooling around with just a zener and a resistor. I don't even know if I have a problem there. But a sensor keeps needing to be replaced, so I'd like to put in some type of 12v clamp to catch any spikes that MAY be damaging the sensor.

Is this practical? I don't need precision, this isn't high end sensitive electronics I am concerned about. It's a Hall Effect sensor screwed into the transmission of a motorcycle, sending pulses to a speedometer. And it is a real PITA to change out.

My sensor is $75 and becoming harder to source.

[x-pro]

If the sensor burns out frequently, the motorcycle's electrical system needs to be repaired. The fault may be in the battery or, more likely, in the alternator.
For example, faulty rectifier diodes inside the alternator.

You're well aware of the problem yourself.
You can also simplify the circuit by eliminating the fuse and varistor. They simply protect the circuit more reliably against overvoltage.

[g1]

Is there already a voltage regulator giving that 12V?
Normal charging circuit will allow around 14V to charge the battery. You don't want to be clamping that line. Could end up damaging the alternator/charging coil.

[Chuck H]

An ordinary zener straight across the +/- sensor terminals should have worked fine. Unless the sensor is elevated somehow and it's negative terminal isn't actually grounded. In which case there would be LESS than 12V across the sensor as a load. How were you implementing the zener?

[Chuck H]

Is there already a voltage regulator giving that 12V?
Normal charging circuit will allow around 14V to charge the battery. You don't want to be clamping that line. Could end up damaging the alternator/charging coil.

I thought about this too and just hoped Randall had already seen to any of it before posting.?. So my post above strictly applies to "a sensor that needs 12V" and isn't part of another mechanism.

[g1]

An ordinary zener straight across the +/- sensor terminals should have worked fine. Unless the sensor is elevated somehow and it's negative terminal isn't actually grounded. In which case there would be LESS than 12V across the sensor as a load. How were you implementing the zener?

Also, if across the 12V line, cathode goes to +12V, anode to ground.

[Randall]

I have not tried anything on the bike, just bread boarding. There is a rectifier/regulator unit. I have no reason to currently doubt it.

I am not wanting to regulate the whole 12v system, just the sensor. It is on the Acc circuit with all the lights and such, not the Run circuit. But, let's not get too much into motorcycle troubleshooting. I have outlets for that.

I don't need to do this, so if there is no quick and inexpensive way to reasonably accomplish it, I can give up on the idea.

[Chuck H]

But a sensor keeps needing to be replaced,..
And it is a real PITA to change out.
My sensor is $75 and becoming harder to source.

Then

I don't need to do this, so if there is no quick and inexpensive way to reasonably accomplish it, I can give up on the idea.

Am I getting a mixed message?

It might help to know what EXACTLY this sensor is sensing. It might be possible to use a capacitor in parallel to a zener if you're getting things like coil spikes on this line. The same reason "condensers" are used in old ignition systems. It may even be detrimental to the "sensing" function to regulate this unit. Whether it's prone to failure or not if you were to somehow delete what it's supposed to be sensing with spike mitigation and regulation it would no longer work anyway.?.

[Randall]

OK. I need to replace the sensor. I don't need to install a 12v clamp for a spike problem I may not even have. And I'm liking the idea less all the time.

I didn't want to go there, but the sensor is embedded in the transmission and reads gear teeth magnetically. It converts them to 5v square wave pulses and sends them to the speedometer. 12v in, signal out and ground. That's it. I was thinking that spikes in the 12v might possibly be contributing to the premature death of this sensor, so why not try to add some protection?

Now I see it is a bigger and more involved idea than I had initially thought. I was thinking in terms of just the sensor, but as it was pointed out, it's really the whole 12v Acc leg with all that is connected.

Too big and too much. I'm out.

Thanks for helping me along with this.