Fuel Pump Current Waveform without an Amp Clamp.

After watching a couple of recent SD videos show the use of an amp clamp to look at the current waveforms on fuel pump and other motors it occurred to me that by going back to old style technology by connectiong the oscilloscope across a "shunt" ressistor because of Ohm's law the scope would show the current wave. So I did a quick test current looping a fuel pump relay and putting a low value shunt ressistor in the circuit. Simple and cheap if you don't already have an Amp Clamp.

Smart!

So how does the scaling work on the voltage trace? Is it possible to measure the overall current draw of the pump, or will this only show the commutator segments?

Thank my high school physics teacher not me -- great teacher taught a lot of practical stuff.
The trick is getting a shunt resistor that hs low enough ressistance not to reduce the current flowing by any significant amount but still give a big enough voltage drop for the scope to read accurately and is a nice round number to make the maths easy. In theory the input resistance of the oscilloscope comes into the equation but in practice the imput resistance/impedance of a scope or DMM is so high it can be ignored.

A good value for the shunt resistor is 0.01 Ohms

The volts drop across the shunt resistor is just V=IR so I=V/R

If for example the scope or voltmeter shows 100mV across the 0.01 ohm shunt resistor then

I=V/R = 0.100/0.01 = 10 amps current.

Which fits in the probe setting for amp clamps on many oscilloscopes.

One thing to watch is the channel ground wire will be floating around battery voltage so care has to be taken not to short it more so if the scope dosen't have the grounds for each channel isolated from each other.

It should work for other circuits, I tried it on the blower fan on my daily driver it showed nicely it was PWM controlled -- which I wasn'teven aware of! I see no reason why it shouldn't work on ignition coils and injectors.

Any chance you can post some pictures of the setup?

I'm excited to try this. Sounds like a workable method for anyone that might own a scope but not a low amp probe.

It's probably important that the .01 ohm resistor is rated to carry the current of whatever circuit you've got it hooked to? :huh:

Ok I'll sort out a parts list tomorrow I used a 50 Watt Resistor.
The formula for the power power dissapated in the resistor is I squared R
A resistor with 10 Amps flowing through a 0.01 ohm resistor has to dissipate 1 Watt.

This was the resistor I used.



This type is probably easier to use.

I like the heat sink! Thanks for the pictures, sir.

Glad you mentioned that second type of resistor, 'cause I'm pretty sure that's the type my local Radio Shack is gonna have in stock. :lol:

A couple of developments on this, I had a free hour last night and ran a couple of tests.
It occured to me that if only the current wave form was needed just hooking the oscilloscope leads up to show the voltage drop across the fuse would work so I tried it and it worked fine.

The second is problem with the original setup getting the current measurement scaled correctly Ohms law seems to be letting me down as to read current directly I need to set the scope's probe setting to 40 amp per Volt (25mV/Amp). My scope has this custom probe option I know most automotive scope don't.
I think the value of the resistor supplied might be wrong, the markings look correct as I have been over my Ohms law caculation a dozen times and checking a very low resistance with multimeter won't work.

Andy.MacFadyen wrote: A couple of developments on this, I had a free hour last night and ran a couple of tests.
It occured to me that if only the current wave form was needed just hooking the oscilloscope leads up to show the voltage drop across the fuse would work so I tried it and it worked fine.

I'm glad you mentioned this! 'Cause I was thinking about the waveforms I've seen on the scope while voltage dropping starter cables. There's always some kind of drop, and it usually looks exactly like a starter current waveform. Good thinking, sir.

Using a multimeter as a shunt resistor also works - tried it on a couple of meters but the Owon B35T had the correct internal resistance to make the maths easy