Hi folks.
This is on the tails of a circuit I made to adapt an old fuel gauge to match a new style tank sender - which works in reverse. The old gauge uses two coils of 100 ohms each and a 150 ohm resistor to 10v. The old style sender pulls down the centre point to move the needle between full and empty.
My circuit reverses the action so that full reads full and empty reads empty by operating a N-Fet in the linear region as a current sink. This seems to work fairly well.
My problem comes when turning the ignition off. When the gauge is static at a reading of the tank and the supply is turned off, the voltage supply to the gauge and also the electronics begins to fall. Somewhere along the curve to zero the Fet ceases to conduct - at which point the gauge needle pegs to the Full position as it sees an open circuit which equals full. I am trying to stop this behaviour.
I've tried adding a P-Fet controlled by an op-amp as a schmitt trigger to spot when the supply falls and conduct to bring the needle down, but in the Spice model I still see a phantom peak as the op-amp gives up.
I need to find a way of pegging the control to the gauge down to zero, or removing the 10v supply to the gauge when it starts to fall below about 8v. Basically it doesn't have to work below 8v supply.
Thank you for your time and suggestions.
This is on the tails of a circuit I made to adapt an old fuel gauge to match a new style tank sender - which works in reverse. The old gauge uses two coils of 100 ohms each and a 150 ohm resistor to 10v. The old style sender pulls down the centre point to move the needle between full and empty.
My circuit reverses the action so that full reads full and empty reads empty by operating a N-Fet in the linear region as a current sink. This seems to work fairly well.
My problem comes when turning the ignition off. When the gauge is static at a reading of the tank and the supply is turned off, the voltage supply to the gauge and also the electronics begins to fall. Somewhere along the curve to zero the Fet ceases to conduct - at which point the gauge needle pegs to the Full position as it sees an open circuit which equals full. I am trying to stop this behaviour.
I've tried adding a P-Fet controlled by an op-amp as a schmitt trigger to spot when the supply falls and conduct to bring the needle down, but in the Spice model I still see a phantom peak as the op-amp gives up.
I need to find a way of pegging the control to the gauge down to zero, or removing the 10v supply to the gauge when it starts to fall below about 8v. Basically it doesn't have to work below 8v supply.
Thank you for your time and suggestions.