Slow Drain Capacitor

Thread Starter

NickInMN

Joined Jan 4, 2024
4
I have a vehicle that runs on a 12 volt system. The vehicle came with a volt meter gauge. I have verified that when the vehicle is running the gauge is seeing around 12.7 volts and 0 volts when off. However, the gauge shows well over 18 volts when the vehicle is running and the photo below is when the vehicle is off.

I have taken the gauge apart and adjusted the needle several times but it always ends up going to back to the incorrect valve. I even used some glue last time. What I think is happening is that when I shut the vehicle off the needle hits the stopping pin with enough force that it is spinning on the shaft and as a result it reads incorrectly.

Assuming that's true, is there some type of capacitor I could use to make it so the needle doesn't contact the stopping pin when the vehicle is turned off? I'm guessing I'd also want to put a diode in to prevent the capacitor trying to power all anything else on the circuit. If this sounds like it would work how would I determine what size capacitor to use?.

1704377563546.png
 

crutschow

Joined Mar 14, 2008
34,418
I have verified that when the vehicle is running the gauge is seeing around 12.7 volts
That voltage is abnormally low, and is closer to the battery voltage when the engine is off.
Typical voltages for a running engine with the alternator charging is around 14V.
What did you use to measure the voltage?

You could try adding a resistor ins series with the gauge to lower its reading.
You'd have to experiment with different resistor values to determine the correct value.
Start with 100Ω or so.
 

Thread Starter

NickInMN

Joined Jan 4, 2024
4
I don't want a resistor, that's not the problem I'm seeing. I have verified the voltage going to the gauge with the engine running and it is nowhere near what the gauge is showing. As I said, I've corrected this multiple times and eventually it ends up where it is now.

I think it is a mechanical problem of the needle sweeping to quickly, and moving on the shaft so that's the problem I'm trying to solve. Adding resistors to fool the gauge isn't a solution and wouldn't change the value when the vehicle is off.
 

kiroma

Joined Apr 30, 2014
66
It seems you're searching for a solution to a problem that is actually a defect of the gauge or another part. Maybe test with a new gauge.
 

MrChips

Joined Oct 2, 2009
30,802
Yes. I was going to suggest getting a new gauge.

When the motor is not running, the battery voltage should be about 12.6V.
With the motor running, the system charging voltage should be between 13.5 and 14.5V.
 

Thread Starter

NickInMN

Joined Jan 4, 2024
4
It seems you're searching for a solution to a problem that is actually a defect of the gauge or another part. Maybe test with a new gauge.
Yes. I was going to suggest getting a new gauge.

When the motor is not running, the battery voltage should be about 12.6V.
With the motor running, the system charging voltage should be between 13.5 and 14.5V.
Mechanically the gauge is fine, the problem is the needle slipping on the shaft. The voltage at the gauge with the engine off is 0, not 12. This circuit is an ACC circuit, in other words it only has power when the ignition is on.

I really just need to know how I can slow the needle down when power is cut to the gauge, and if a capacitor is the correct choice.
 
Last edited:

MrChips

Joined Oct 2, 2009
30,802
Mechanically the gauge is fine, the problem is the needle slipping on the shaft. The voltage at the gauge with the engine off is 0, not 12. This circuit is an ACC circuit, in other words it only has power when the ignition is on.

I really just need to know how I can slow the needle down when power is cut to the gauge, and if a capacitor is the correct choice.
Try 470μF 25V capacitor across the meter. Also connect a 100Ω resistor between the meter and the +12V connection.

1704386885262.png

Vin is your +12V input. Vout is your meter connection.
 

MrChips

Joined Oct 2, 2009
30,802
I will give this a try, thanks. Just out of curiosity, what role does the resistor play here?
The RC circuit shown is a low pass filter. It dampens the movement of the meter.

The value of R should be lower than 1/10 the resistance of the meter.
The product of R x C gives the time constant.
For example,
R = 100Ω
C = 0.000470 F
time constant = 0.047 seconds
 
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