alternator testing with Oscilloscope.

[Canadian1968]

Trying to test my alternator with an oscilloscope. Posted a picture of my reading. I'm still learning to use this thing . So bare with me

Does this test show an AC ripple of 55mv PKP? Is that acceptable?

I also tested with a simple multimeter first. Set to AC I got a reading of .1v

 

[Paul_B]

Personally I wouldn’t be concerned with 55mV peak to peak of ac ripple especially if your car is keeping the battery charged and you have good voltage when the engine running without problems.

Looking at your oscilloscope picture, I’m seeing the vertical grid is at 100mV (100mV between grid lines vertically) which makes 55mV peak to peak seem correct. I don’t know if your oscilloscope can go to a lower voltage range such as 20mV to better see the signal you are getting.

For your voltmeter where you measured 0.1 volts of AC, I would expect it to be similar to the oscilloscope. Now how they get the AC measurement can vary in terms of if they are a true RMS measurement or not. Some meters can do interesting measurements with some waveforms since they are usually thought to measure more sinusoidal waveforms. I have also seen some AC voltage meters not really measure below 0.1 volts so it could be round 55mV up to 0.1 volts.

There is a lot of potential for electrical noise in your measurements in an engine bay due to things like ignition systems and electric fans/motors if present. Your oscilloscope cables can easily pickup electrical noise and make things appear worse than they are.

As a note, I have only used the more expensive oscilloscopes you find in labs and have not worked with these hand held meters. I keep watching these online and will get one at some point. Thanks for sharing!

 

[twecomm]

The ripple you see is typically and certainly not an issue. Sometimes when after market radios or stereo systems get added to these cars that don't have sufficient DC filtering, the AC ripple varies with alternator speed will be heard as a "alternator whine" noise in the audio devices. Usually it can be solved by installing a choke coil in series with the DC feed to the devices. If I recall, many years ago Ford had this problem with radios in certain cars and had
a choke they provided that eliminated the whine..

 

[RJRENTON]

Personally I wouldn’t be concerned with 55mV peak to peak of ac ripple especially if your car is keeping the battery charged and you have good voltage when the engine running without problems.

Looking at your oscilloscope picture, I’m seeing the vertical grid is at 100mV (100mV between grid lines vertically) which makes 55mV peak to peak seem correct. I don’t know if your oscilloscope can go to a lower voltage range such as 20mV to better see the signal you are getting.

For your voltmeter where you measured 0.1 volts of AC, I would expect it to be similar to the oscilloscope. Now how they get the AC measurement can vary in terms of if they are a true RMS measurement or not. Some meters can do interesting measurements with some waveforms since they are usually thought to measure more sinusoidal waveforms. I have also seen some AC voltage meters not really measure below 0.1 volts so it could be round 55mV up to 0.1 volts.

There is a lot of potential for electrical noise in your measurements in an engine bay due to things like ignition systems and electric fans/motors if present. Your oscilloscope cables can easily pickup electrical noise and make things appear worse than they are.

As a note, I have only used the more expensive oscilloscopes you find in labs and have not worked with these hand held meters. I keep watching these online and will get one at some point. Thanks for sharing!

Nice writeup.....BUT....what was the alternator's RPM when you took your measurements?? Remember, the alternator's ripple will be determined by its RPM, as ripple/frequency its speed related (even after rectifying the AC generated by the stator). The rectifier is a three phase full wave bridge configuration, which has a tendency to minimize ripple. The lead acid storage battery is a great capacitor/condenser and will absorb and minimize any ripple effects. Personally, I would not be concerned about 0.55 mv. P-P ripple ....just my thoughts and opinion.
BOB RENTON

 

[Canadian1968]

The test was done just at idle so approx 800rpm. I will do another test at various rpms and see what I get.

Also I probed at the battery for this test. I was curious as to if this would effect the readings. For a more accurate reading should I go directly off alternator stud ? Should the AC ripple be consistent threw out the system ?

 

[Paul_B]

AC ripple will likely vary a bit depending on where you measure. For example, if you’re probing close to electrical connections of loads (e.g. ignition systems, electric motors like fans…) that aren’t a constant smooth current draw, you could pickup voltage ripple from their varying current demands.

Other locations you could probe may be more uniform if inductance and/or capacitance is present in the wiring/circuit nearby to your probe point. These could potentially filter the ripple making it effectively smaller.

 

[twecomm]

Think of it this way.. If you didn't have a scope, you would never know there was any ripple present.

 

[RJRENTON]

Think of it this way.. If you didn't have a scope, you would never know there was any ripple present.

Measuring ripple is a "nice to know information", but to the average vehicle owner, who, likely does not know (about ripple or cares about ripple), now has an additional "worry point"......too much ripple (and how much is too much).....Will affect the vehicle's performance.....especially true of the "knit-pickers" who dwell on obscure things (K-D ?) ......."my 0.57mv RMS ripple" is causing _________(fill in the blank) ECM failure or Right rear tire valve stem cap to loosten, or, is ripple causing bulkhead connections to corrode at a faster than normal rate?
The exercise in Measuring ripple is interesting and I believe, providing information only......or, will now as a result of the information presented, cause a rush to purchase an oscilloscope to measure my car.......just my opinion of course.......
BOB RENTON