[mattmarks]

Good morning all - hope thisn't in the wrong thread.

I'm putting an Aim Dash in my spec boxster, but in order to it to register on the dash, AIM tells me that I need to put a "pull up" 2.7k ohm resistor as shown in this diagram below. The explanation per AIM is

Voltage or Resistance

Electrically all sensors can either be resistive (eg a temperature sensor) or voltage (a suspension potentiometer). The problem with fuel tank sender units is that they are normally resistive sensors, and AIM products require voltage signals to work. To convert a resistance sensor to a voltage, a pull up resistor is required, see figure 1 below

V signal = Vref * Rfuel

Rpullup + Rfuel

Equation 1 – voltage divider

This circuit is called a voltage divider, as the supplied voltage (Vref ) is divided between the two resistors (Rfuel – resistance of the fuel tank & Rpullup – resistance of the fixed pullup resistor). Rpullup is a fixed voltage, and Rfuel varies depending of the volume of fuel in the tank. The equation for the voltage output is shown above, in equation 1.

Therefore if two resistors are equal in value, the output voltage would be half the supply voltage. Similarly if the fuel tank resistor is half the value of the pullup, then the output voltage would be two thirds the supply voltage.


So two questions:

1) anyone know where I can get this resistor on a Saturday in NoVa? Amazon delivery is Tuesday.
2) Any additional knowledge on actually doing this?

Thanks!

[BillC]

What wattage resistor do you need? I think I have a bunch of random 1/4 watt resistors if you want to drive up this way -- let me know if you want to come, and I'll go dig them out.

[mattmarks]

Originally Posted by BillC

What wattage resistor do you need? I think I have a bunch of random 1/4 watt resistors if you want to drive up this way -- let me know if you want to come, and I'll go dig them out.

Thanks Bill, what would be great! However, i readily admit ignorance on the Wattage.

I found a converter that takes resistance and voltage to wattage and came up with the below. If I remember high school, amps x volts = watts, the math is right.

2700 ohms
5 volt circuit

Equals

0.00185185185 Amps
0.00925925926 Watts

Edit - is it ok to have a resistor with much higher wattage than needed? Looks like 1/4 watt is a couple orders of magnitude higher than these values.

Shoot me a PM and whatever time works for you and I'll swing by.

[BillC]

The watt rating is the amount of power a resistor can handle before letting the magic smoke out. You always want to make sure the resistor is rated for more than you need. So, 1/4 watt sounds like it will work for you.

And . . . I actually found the resistors! Amazingly enough, they were right where I thought they were.

PM inbound.

[Dr K]

Well...did Matt get a happy ending?