Reading resistance of a 12volt coil

[Mad Professor]

Good Day All.

I am trying to workout the hardward and software to read the resistance of a 12volt coil.

The coil it's self is being controlled via a picaxe chip and a MOSFET.

one side of the coil is connected to the 12volt supply and the other side of the coil is switched ground via the MOSFET and picaxe.

As I am using the coil with a 12v supply how can I read the coil's resistance via the picaxe ADC pin's?

The resistance of the coils will be no less then 2ohms and no grater the 20ohms.

Thanks for your time.

Best Regards.

 

[Andrew Cowan]

Measure the voltage drop (ADC) and the current draw (shunt resistor).

Resistance =V/I.

Is it going to change lots?

A

 

[BeanieBots]

Well, that's a similar problem to measuring the RESISTANCE of a capacitor.
Is it REALLY the RESISTANCE that you want to measure or the IMPEDANCE which also takes into account the AC characteristics.

The DC resistance is as Andrew states.
Same as you would measure a resistor's resistance. Basic ohms law except you have to wait for a steady state current before taking the measurement. How long depends on the inductance but will be sub 1 second for most real life coils.

 

[moxhamj]

I'm wondering if this is a more complex question than it seems as there would be no need to measure the coil's resistance with a picaxe. Unless you really do want to just measure the DC resistance, in which case just put your multimeter on the resistance range and measure it as BB says. The coil's resistance will never change, so once you have measured it, write the number on the coil.

But if you want to measure the inductance, or the varying voltage on the coil, or something else, that is more complex.

 

[Mad Professor]

Thanks for your replies.

This is going to be something that I add to the next version of my Fuel Injector Test Bench project.

At this point in time I have to test each injector with a DVM, I would like just to be able to connect up all the injectors to my test bench and read the resistance of each injectors coil, and then run the selected tests.

So I have been testing the injectors unloaded resistance, I am sure most if not all of the injector test benches on the market test the resistance under load.

Thanks for your time.

 

[BeanieBots]

Except for a slight increase in DC resistance due to heating, it will not differ between loaded/unloaded.
What is the purpose of this 'resistance' check?
Is it to make sure it has been manufactured correctly?
What can go wrong with the manufacture? (number of turns?, wrong gauge wire?).
Or do you REALLY need to check the dynamic impedance to test for shorted turns?

What else are you testing? Activation speed? Surely that test will let you know if the dynamic impedance is correct?
What are the consequences of wrong "resistance"? Why does it matter?
Can you not test that characteristic? If you can't, then either you need to or it's not important.

Presumably your test rig has a fixed reliable drive voltage?
Then just measure the current drawn.
Personally, I'd use a low side current sense resistor with a differential amp (made from an LM324).
In your ohms law calcs, compensate for any driver transistor losses.
(these will be quite consistant and predictable once measured).

 

[Dippy]

Piece of cake Prof.

Want just 1 component plus shunt (aka Rsense)?
http://www.farnell.com/datasheets/5927.pdf

Want to keep costs down?
http://www.farnell.com/datasheets/10146.pdf
A quid plus carriage and VAT. Plus shunt

Power---shunt---injector---gnd.

2 ADC measurements. 10 minutes coding. 20 minutes posting here to say how brilliant it is and Bob's your jolly old Uncle.
Does it get any easier?

Oh, I forgot the extra post here to ask how to calculate the shunt value....

I've used both and also the similar TI INA169 (and derivatives). All work very nicely.

 

[flyingnunrt]

Not that I know anything about injectors or what you are trying to achive but
could you not have a pair of injectors in series and tap off the voltage between the two. the potential each side should be balanced unless one injector was bad assuming identical injectors etc.

 

[Dippy]

flying..: That would show relative differences, but not any absolute figures.
I'm not sure what that would achieve?

 

[inglewoodpete]

Any number of DC or static tests will not conclusively tell if the injector's coil is functioning correctly. As BB mentioned in post #6 above, the electrical test you really need to do is a dynamic impedance test or, better still, a shorted turns test.

 

[Mad Professor]

Dippy thanks for your reply.

The ZXCT1009 is stocked by RS (154-920) £0.59 each.

Do you happen to know to hand of any non surface mount units?

My current PCB already has 5w 0.1ohm resistors on the ground side of the coils for connecting to a scope when needed.

Heres a picture of the current PCB.

Thanks for your time.

 

[Dippy]

No, I don't know of non-smd parts sorry.

For prototyping I make little chip-to-DIP smd adaptors which I can plug into a circuit or breadboard.

If your resistors had been precision devices you could use simple op-amp gain circuit referenced to ground to amplify the I.R (Voltage) across those resistors.
I guess you could get a semi-reasonable value if you can measure those resistors accurately. I have no idea about their temp-co.

Sadly, for proper measurements you have to pay careful attention to the ground routes to ensure that the op-amp & PIC ground sides are at exactly the same voltage i.e. no signifciant currents passing in the ground line.
A not-so-good alternative is to have huge ground planes to minimise ground resistance.
(This becomes more important as shunt resistance is lowered, noting that some shunts are well under a milliOhm.)

You may be able to get some good ballpark results as a retrofit, but this should have been considered at design time (hindsight is 20:20 after all )

But good luck, I'm sure you can get a reasonable result with what you have as long as you don't expect <10% error.

 

[Mad Professor]

inglewoodpete: You&#8217;re right I will not be able to see how well the injectors coil is working just by a standard resistance test.

All I want to do is read each injector, and find out if the coil is a short circuit, open circuit, or what the resistance of the coil is.

All the injectors should be within 0.5ohm any grater then that and the injectors will be out of balance and possibly give the engine running problems.