MoTeC Global Forum

Hi,

We noticed a slight current reading error on PDM15. For instance, according to the log pulled from ADL3 our radiator fan draws a steady 5A but directly measuring the current with a multimeter connected in series with the fan shows a steady draw of 6A -- tried with other meters and had same result.

Was wondering if anyone can shine a light whether it is possible to calibrate (if possible) the readings?

Thank you.
Eric S.

Eric - Assuming this is related to the other post, then you are on the right track.

You probably need a scope to measure the draw over the instantaneous interval (if one can assume that an instant has width??).
The current should start high and decay down to the 5-6 Amps in a f(x) = 1/x fashion.
So you need to see what happens when X --> 0.0 .
Whether that affects the voltage at the battery or at the ECU depends on where and how the wires run.
But you probably want the ECU on its own wire to the battery and the Fan on its own wire to the battery.
Or some capacitors to store current, or some resistors to limit current.

Probably having a drawing would help, but measuring current indicates that you are heading in a good direction.

by adding a meter in line with the load you are probably causing the extra load due to longer wire run, connection, wire gauge losses etc, unless you are using a clamp style meter...

Allan wrote:by adding a meter in line with the load you are probably causing the extra load due to longer wire run, connection, wire gauge losses etc, unless you are using a clamp style meter...

I don't think so, the load would decrease if the VOM added impedence.

When I make the wire length infinate or reduce the diameter to zero, the VOM's reading of the Amperes also goes to zero.

(if one was forcing current through then the voltage would go up, but we are usually using a a fixed 12V (or 14-15V), and the current is a function of the voltage and the impedence.)

eric_s wrote:Hi,

We noticed a slight current reading error on PDM15. For instance, according to the log pulled from ADL3 our radiator fan draws a steady 5A but directly measuring the current with a multimeter connected in series with the fan shows a steady draw of 6A -- tried with other meters and had same result.

Was wondering if anyone can shine a light whether it is possible to calibrate (if possible) the readings?

Thank you.
Eric S.

There are at least three ways to 'measure' the currrent... (that I can think of).
One would be to measure the impedence of the load and compute the current draw.
This would be prone to error if one did not know the voltage.
e.g. a 1-Ohm load would have 12-A at 12V and 14A at 14V.
So I could envision that is the current was computed using 12V and the stsyem is running 14.X-V then you could have 20% higher current from the 20% higher voltage.

So without some input as to how the current is measured, then I would suggest putting a know resistor on an output, and then try using a big dimmer switch on the front end and vary the voltage and observing the PDM's stated current with the measured and comuted current.
If it was varying then one could compute the actual current as:
Current(actual) = Current(PDM)*(Voltage/12V)

But this is all just a guess...