Discerning AC from DC

Other Hardware General Electronics
1

Hello,

i'm trying to figure out if one of my motor ignition lines is DC or AC. My standard run of the mill multimeter gives weird readings on both scales. I'm not really sure what is going on.

What would be the most conclusive test that i can run to figure it out...with just this multimeter? (have no oscilloscope)
Should i use a capacitor in series? Use a diode?

Thing is that the voltage on the line can probably reach almost 300V, wheres i'm not sure i have such a diode (or a non-electrolytic capacitor for that matter).

Thanks

2

It's hard to know how your meter is going to react if it's on the "wrong" setting. I just plugged my Fluke meter to the AC line with it set to DC, and it reads about zero. (The readings are a few millivolts with the readings jumping around.)

When set to AC, my meter reads zero from a 9V battery.

i'm trying to figure out if one of my motor ignition lines is DC or AC.

What kind of system is this? Is the motor speed-controlled or just on-off. Are there any markings on the motor?

[u]PWM[/u] which has a DC component and an AC component. Or, maybe it's a stepper motor?

You can make a [u]High-pass RC filter[/u] and it will block all DC. If you get zero-volts, you know for sure it's pure DC. If you get a reading that doesn't prove it's AC... It could PWM (pulsed DC).

3

This is a gasoline engine?

If so, the spark voltage will be around zero most of the time, with a brief excursion to a high DC voltage. The voltage will be DC, in that the time-average voltage is different from zero, and it will be AC, too, in that the time-average voltage isn't constant.

It's not clear that your meter will measure either component accurately, because the voltage waveform is quite chaotic, and the meter design may presume that the voltage is strictly AC or DC, or that the AC component is strictly sinusoidal and close to 50 or 60 Hz.

This page describes what happens during a spark, and includes a link to a graph of the spark voltage over a single cycle: Spark Plug Wire Test. You can see schematics, and schematic-ish drawings, of a traditional ignition system and an electronic system in the Wikipedia article, "Ignition system," here: Ignition system - Wikipedia, which may illuminate their operation.

It may be that your voltmeter isn't the right instrument for this measurement.

4

Hi, were does this wire run from and to, if it gets to over 300V then it is probably the primary side of the ignition coil.
The voltage there is a combination of high voltage oscillations and DC.
See attached diagram, it is only instructional but shows you the basic waveform.
What do you want to do with the signal off this wire?

I would be very careful if it is around HT spark plug wiring as the voltage and energy levels are dangerously high.

Tom........ :slight_smile:

7900307Picture0_00000035543.jpg

5

Some multimeters measure average absolute value rather than rms voltage on AC scale,
most have a bandwidth limit (can be as low at a few hundred Hz, ie OK for power but not
too great on audio). Also I doubt most multimeters block DC when set to an AC range so
the DC offset will confuse the reading.

Basically only trust the reading for low frequencies, sinusoidal with no DC offset unless
you know for sure what its capabilities are.

6

It's a motorbike gasoline engine yes. And the ignition box is somewhat of a mystery to me. Normally you have AC and DC versions, but this one has got me baffled. I measuring the line that goes to the high voltage spark coil.

Thanks Tom, it's very insightful. It's probably why my multimeter is going haywire.

As doug said a high-pass filter might do the trick. Otherwise i'll just have to track down someone with an oscilloscope.

Thanks for the help.

7

Hi, why do you want to look at that wire, what do you want to do with the voltage waveform?

Tom..... :slight_smile:

8

For now i just want to check the ignition system if it's working properly as well as learn what type do i have.

9

Hi, unless you can get a part number off the control system, an oscilloscope may be the answer.
Unless you know someone with a older type electronic engine analyser.
Have you googled to see if info on the bike will tell you anything?

Tom....... :slight_smile:
A picture of the coil and the control box might help.

10

What bike is it?
Might able to find a diagram of the wiring and CCD unit.

11

I tried searching but the CDI unit is custom made and the workings are proprietary, so no diagram. The unit is also epoxy sealed. Opening would destroy it.

12

CDi's produce lethal output voltages, be aware... much more energy than magnetos.

13

I'll be careful, thanks for the info.