Sunday, February 12, 2017

Follow the Bouncy Needle

Since I'm on a roll with the electrical wizardry, I thought I'd tackle another little project: smoothing out the needle on my fuel gauge.

The fuel gauge is pretty simple.  A sender with a float on an arm fits in the tank.  As it moves it changes the resistance of a coil inside the sender.  The gauge is powered to 12v and is grounded.  It picks up the difference in voltage based on the position of the sender's varistor and moves a needle, where no resistance is 'full' and max resistance is 'empty'.

 

This works very well... too well, in fact.  The gauge is so responsive to the sender's position that the needle bounces a lot while you drive, as the sender moves up and down while going over bumps or around corners.  It 'twitches'.

The solution is to add a capacitor in parallel with the sender that can act as a voltage buffer.  The capacitor charges when the sender moves to full and discharges when it moves towards empty.  The drag during the charge state (because the capacitor is taking some of the volts from the sender until it's full) and the extra volts the capacitor supplies during the discharge state smooths out the voltage changes enough to let the sender bounce up and down a little without affecting the voltage at the gauge.


Now, the needle still moves but it doesn't jump around if I jiggle the sender.

I tried a few different combinations, but found a 220uF, 16v non-polarized capacitor worked best.  It was easy enough to solder in place and it fits between the gauge terminals, so I'll still be able to install the gauge in the dashboard.  And, I made sure my sender worked well, too.


2 comments:

  1. Add a dab of hot glue / epoxy to the terminals or glue whole cap down --- breaking the terminals is a common fault in vibration tests - may save you a disassemble later

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