I'm looking for help building a circuit to protect an analog voltage gauge with a range from 0 to 16V. It's for an antique car so it needs to withstand a few amps. I know how to put things together but have no design experience. At first I thought I could use a zener diode with a 16V reverse breakdown voltage so when the voltage hit that level, the current would bypass the gauge. But that would make the gauge needle drop to zero right? I saw another discussion that might hold the answer here but, to be honest, I'm not smart enough to understand it. Any suggestions on how to do this?

 

No. A zener diode alone will not work. If the voltage exceeds the zener voltage, the zener diode will die a quick death followed by the gauge. You need to put a resistor ahead of the zener diode and the gauge. The gauge will still read the zener voltage, not zero.

 

How long time the over voltage is expected? A 1ms, 1s, 10s?
What is the reason of overvoltage?

If the overvoltage persists a long time there is an option to use a switching regulator in front of circuitry (load) that will maintain the constant voltage (effectively).

 

I'm looking for help building a circuit to protect an analog voltage gauge with a range from 0 to 16V. It's for an antique car so it needs to withstand a few amps. I know how to put things together but have no design experience. At first I thought I could use a zener diode with a 16V reverse breakdown voltage so when the voltage hit that level, the current would bypass the gauge. But that would make the gauge needle drop to zero right? I saw another discussion that might hold the answer here but, to be honest, I'm not smart enough to understand it. Any suggestions on how to do this?

A lot depends on what this voltage gauge is measuring the voltage of. In particular, whether the thing it is connected to has some means of limiting the current it can possibly deliver. You mention "a few amps". What makes it "a few amps" and not "a few hundred amps"?

 

Is it a hot-wire gauge? If so they are almost indistructable, they will not be bothered by 16V if they are only intended for 14.4V

 

If my battery voltage was over 16V I'd want to know the reason why! Charging a battery at over 16V can reduce the life of the battery so I'd want to know about it. I'd measure the resistance of the voltmeter and consder scaling it by adding a series resistor. and calibrating. Absolute accuracy is less important than monitoring the voltage which might be around 12.5V when the ignition is switched on, drop a bit when the starter motor is drawing current and rise to around 14.5V when the car is running.

With a traditional dynamo, it's voltage can go too high so the battery should be charged via a voltage regulator which actually works as a primitive electromechanical PWM device. Firstly, I'd check if your regulator is working properly. I believe you can buy electronic replacements if it's not repairable. Modern alternators self regulate.

Your meter is probably a moving coil meter which actually measures current. It could be hot wire, which has a time lag so you are not seeing the instantaneous value, or possibly even moving iron but they are mostly for high current meters. You are not looking at high current, just voltage difference. The moving coil meter actually measures current and is very sensitive, so to measure voltage there will be a resistor in series with it. It could, for example, have a 10mA full scale deflection so in order to measure 16V full scale it'd have a 1K6 resistor in series with it. Adding 400R is series would make it roughly 20V full scale. The actual resistance of the coil is small - can usually be ignored. So measure the resistance and increase with a proportionate series resistor to re-scale.

Just thought, instead of a resistor you could add a zener diode in series with the meter. For example, a 5.6V zener would modify a 0V to 16V meter to 5.6V to 21.6V. Note that you are not needing to think about high current, the actual current drawn will be small. To get better linearity you could add a resistor in parallel with the meter to increase the current drawn through the zener to where it’s well past the knee.

 

To clarify, in response to the questions, the truck has a dynamo and a voltage regulator. I wanted to monitor the current and voltage but there was only one hole in the dash where an ammeter was located so I replaced it with one designed for small aircraft that has volts and amps in the same gauge (2 needles). The gauge is connected to the starter solenoid and has a shunt for the ammeter. I double checked and, when reving the engine, the gauge shows 30 amps and pegs the voltage needle. (Yes, I was wrong when saying "a few" amps). I know I could just get rid of this gauge and get an ordinary volts- or amps-only one with an appropriate range, but I really like this one and would like to find a way to avoid destroying it.

 

To clarify, in response to the questions, the truck has a dynamo and a voltage regulator. I wanted to monitor the current and voltage but there was only one hole in the dash where an ammeter was located so I replaced it with one designed for small aircraft that has volts and amps in the same gauge (2 needles). The gauge is connected to the starter solenoid and has a shunt for the ammeter. I double checked and, when reving the engine, the gauge shows 30 amps and pegs the voltage needle. (Yes, I was wrong when saying "a few" amps). I know I could just get rid of this gauge and get an ordinary volts- or amps-only one with an appropriate range, but I really like this one and would like to find a way to avoid destroying it.

Put a shunt across the ammeter and recalibrate the ammeter scale.

 

Perhaps you could clarify “the gauge is connected to the starter solenoid”? Are you saying that the ammeter is connected in series with the starter solenoid and the voltmeter is connected across it? This doesn’t sound right - once the engine is running there would be no current going through the solenoid. Maybe you could sketch a schematic? What is the nominal battery voltage?

If both the ammeter and volt meter are connected across the solenoid the current you are measuring when revving the engine is just the current flowing through the shunt and should read low when the starter solenoid is engaged

 

No. A zener diode alone will not work. If the voltage exceeds the zener voltage, the zener diode will die a quick death followed by the gauge. You need to put a resistor ahead of the zener diode and the gauge. The gauge will still read the zener voltage, not zero.

What happens depends entirely on the source resistance, the zener diode current rating, and the size of the heat sink. There is/was a motorcycle company that used a shunt zener diode to regulate the output of the permanent magnet alternator. When it worked things did get rather hot. When it quit working the lights burned out from over voltage. Some crazy tech guy added a linear voltage regulator so that the lights would not burn out. The idea did not spread very far, I don't think. But it worked.
If I use a zener that can handle 12 amps at 18 volts with an adequate heat sink, the the ten amp alternator can survive and the lights wont burn out . BUT it is not the small zener diode most folks have seen.

 

Most lead-acid battery voltage gauges read up to 15V.
16V may have the needle on the end stop, but that isn’t going to kill it, but it is telling you something: sort out the battery charging circuit, before it kills your battery. That’s the whole point of having a battery voltage gauge.

The best way of electronically limiting its movement would be to find the value of internal resistance, remove that resistor and replace it by two in series each of half the value. Then place a 7.5V zener from the junction to ground.

 

To protect that guage's voltmeter you can use a series resistor and a zener diode. Of course it is likely that a series resistor will drop the meter voltage reading some.
A better scheme would be to add a better voltage regulator to the "dynamo" system. The simple fix for the over-voltage is to move the voltmeter connection towards the battery end of the charging connection. I doubt that the battery voltage ever gets that high.
In addition, stop and think what that 30 amps of charge current is doing to the ten-amp wiring, and the dynamo brushes. You really need to correct the regulator settings.