Multimeter Peak Voltage Reading

[Hondaboi1729]

Hi,

I need to find the peak voltage of the ac current produced by the sensor connected to the alternator on my motorcycle. The workshop manual recommended a peak voltage adapter which goes on a multimeter.
I don't want to buy this expensive thing.
I was wondering since V_p=V_rms*sqrt (2). I can just use the reading on the multimeter and multiply it by sqrt (2). However when you are measuring the AC voltage with a multimeter does it display the RMS?;The readings aren't going to fluctuate rapidly as the voltage of the AC changes is it?

Secondly I also thought if the above didn't work I could just use the max function on the multimeter to work it out? That way at the end the peak voltage will be displayed. Is this a good idea?

Thanks

 

[davenn]

However when you are measuring the AC voltage with a multimeter does it display the RMS?;

most multimeters read RMS unless otherwise stated

The readings aren't going to fluctuate rapidly as the voltage of the AC changes is it?

no

 

[davenn]

Secondly I also thought if the above didn't work I could just use the max function on the multimeter to work it out? That way at the end the peak voltage will be displayed. Is this a good idea?

That's not going to give you a peak reading, it's only going to give you max fluctuations in the RMS value

 

[Svein]

Use an oscilloscope. Way back when.. I was in electronics development and I used a 'scope for everything.

 

[GrahamN-UK]

most multimeters read RMS unless otherwise stated

They will be calibrated to display RMS values for a sine wave input. Only more expensive multimeters with a true RMS feature will actually give RMS readings for a non-sine wave input.

You can only get the peak value by multiplying the RMS value by sqrt(2) if you have a sine wave signal. As the alternator instructions specify using a peak-reading probe we can assume you don't have a sine wave signal and you can't multiply a multimeter reading by root 2 to get the peak value.

As already stated the best way to read the peak value is an oscilloscope. If it's a reading you will need to keep making you could cheaply build your own peak probe with a signal diode, a capacitor and, if needed, a resistor. If I did that I'd still want to use a scope the first time to check the probe reading.

 

[davenn]

They will be calibrated to display RMS values for a sine wave input. Only more expensive multimeters with a true RMS feature will actually give RMS readings for a non-sine wave input.

yes that is correct

 

[XZ923]

When you say the sensor attached to the alternator I'm assuming you're referring to the crank position sensor (some service manuals refer to is as the pickup coil, signal generator, and numerous other names)? If so you can't conclusively rely on simply multiplying your meter's Vac reading by sqrt(2) due to the actual output of the coil. It's a pulsing Hall Effect sensor that connects to the semiconductor switch inside your Ignition Control Module (on older CDIs it's connected to an SCR gate; this may be different on modern vehicles with computerized ECUs). If you put an oscilloscope across it you'll see it's not a perfect sine wave.

As far as cost, modern major manufacturers offer them at fairly reasonable prices. You can also find numerous articles online about making your own. When it comes to measuring the crank sensor you definitely need to be taking a peak measurement otherwise you're shooting in the dark. The meter's "normal" Vac reading, which is basically the meter's best attempt at RMS unless you have one of the expensive true-RMS meters, can be indicative but not conclusive.

 

[jim hardy]

Since you're doing this test should i assume something isn't working ?Does the manual direct you to make this measurement "open circuit", that is with the wire that goes to voltage regulator unplugged from the alternator stator ?

If so
Connect a diode in series with your meter and set it for DC.
Then measure alternator output at several RPM's writing down RPM and volts.
Your alternator will charge the meter's input capacitance to peak voltage several times each revolution
and that's what you meter will report.
It's not exact
but since you're probably going to get around fifty volts a volt or so error won't matter much.
If you plot volts vs RPM on a piece of graph paper you should get an almost straight line.

BUT
If this is an in-place measurement with regulator and battery connected and working properly
you'll get a lower voltage, probably just a little more than battery voltage. Be aware the diode will lower your reading by a fraction of a volt.
If you get fifty volts probably the regulator has failed or there's a fuse blown someplace.Lastly
Read battery voltage . It should increase when engine is running,
that's a "poor man's quick check" . If it settles around 14 volts then everything looks okay...
Real poor men who don't have a voltmeter just observe brightness of at the headlight at night , out of the city where our eyes can detect the subtle change in brightness .. (or darkness if it's a Lucas .)old jim

 

[XZ923]

It would really help if we knew what bike you were working on

 

[jim hardy]

@Hondaboi1729

are you still out there ?