High voltage reading driving a analog car display

Projects General Guidance
1

I have an electric car and want to read the voltage and drive the on board fuel gauge on the car. The voltage of the car is 144v dc. I think the fuel gauge is around 70ohms but will have to measure to make sure. I am looking at using an opto (4n25?) for safety but know little about them. I am wanting to read voltage and then use a pwm signal to drive the display using an arduino. Any schematic would be useful. Thanks Randy

2

The fuel gauge responds proportionally to voltage? It's an electro-mechanical gauge?

I don't think you need PWM or a microcontroller... A [u]voltage divider[/u] (2 resistors) can knock-down 0-144V to 0-12V or 0-14V. If the meter's resistance is constant, the meter can act as the bottom half of the voltage divider and you only need one resistor.* A 750 Ohm resistor in series with 70 Ohms will give you about 12V from 144V.

I am looking at using an opto (4n25?) for safety but know little about them.

Opto-isolators are not linear (analog) devices. They are digital.

70 Ohms seems low for a "meter", but if it's from a normal (liquid) fuel gauge it may have a 70 Ohm parallel resistor inside to work with the fuel-level sensor. The Arduino can't directly drive a 70 Ohm load (it needs a minimum load of about twice that) and it can't directly drive put-out 12V for a 12V gauge. So if you decided to use PWM, you'd need a transistor or MOSFET.

  • (The meter's resistance will "interact" with bottom resistor in the voltage divider. If you're going to use 2 resistors, look-up the formula for two resistors in parallel.
3

Is there a good way to isolate the 144v from the 12v supply voltage?

4

DVDdoug:
Opto-isolators are not linear (analog) devices. They are digital.

They are as digital as any transistor: they all have a linear range which can be useful for analog circuits. It is pretty rare to see an opto used this way. There are some optos which are optimised for this mode of operation.

5

I think it's not that simple.
Fuel tanks go from 100% to 0%.
A battery migh be flat at 85% of the initial voltage.
You need a "smart" circuit to display battery charge.
Leo..