Outboard electrical question

[VicS]

If you were to attempt to test the output from an outboard engine charge coil before the rectifier using a digital multimeter set to an AC volts range would you expect to get a meaningful reading or would the meter just not respond to something so far removed from a sine wave?

If a digital meter doesn't work an analogue one would wouldn't it.

The evening is young ... anybody with an outboard handy at a loose end?

 

[Ken_Irby]

Well now, are we talking simple pair of coils like on a small outboard or...

the multi-phase array as on a larger, say 50hp motor? if the former then i can't see how it is anything other than a sine wave output. howsever the larger engines have, generally a three phase multi-coil setup which uses a seperate stator feeding into a three phase rectifier. does that help?See:-http://www.crowleymarine.com/mercury_parts/11493/40.cfm item 5 so the answer is yes, it would be a proper reading.
Chas on Kentrina's login so not necessarily her views.

 

[VicS]

The engine in question is a 2 cyl 40hp with a single pair of coils, #16 in the diagram below, feeding a 4 wire, 6 diode, rectifier #80.

Diagram

 

[William_H]

O/B lighting coils

My old Johnson has 2 coils mounted under the rotating flywheel which has permanent magnets mounted in inside of the rim. I imagine without testing that the waveform would be essentially a sine wave but with a gap between waves. Certainly it will not contain square waves or anything nasty because the o/p is generated by the magnet approaching then departing from proximity of the coil.

Presumably you are interested in how much charge you can get from the coils. So the best bet is to set up rectifiers as appropriate and use an analogue meter to measure into a battery of various degrees of charge.

Use of schotky diodes should get you a little more voltage hence a little more charge. olewill

 

[VicS]

Presumably you are interested in how much charge you can get from the coils

No interested if measuring the AC volts from the stator coils was valid thing to do in trouble shooting an engine that's not charging its battery despite the rectifier having apparently been tested and found to be OK

A digital meter apparently gives an unreadable display.

Wondered if an analogue meter would be more sensible or a good old fashioned bulb on a couple of wires.

 

[jamesbell]

As far as I know, you need a DVA Meter to read any voltage from the electrics on a outboard, The Marineengine.co.uk as them at £180+vat or an adapter to fit a multimeter for £53+vat.

The only thing a standard multimeter is any good on a outboard is testing Ohms and continuity.

Hope this helps.

James

 

[john_morris_uk]

As far as I know, you need a DVA Meter to read any voltage from the electrics on a outboard, The Marineengine.co.uk as them at £180+vat or an adapter to fit a multimeter for £53+vat.

The only thing a standard multimeter is any good on a outboard is testing Ohms and continuity.

Hope this helps.

James

This isn't true! Multimeter means multi (ways of measuring) meter.

Even my cheapest ones measure DC and AC volts as well as continuity.

To answer Vic's question, the answer is yes - you should get a meaningful reading, but the actual voltage will be dependent on a number of factors.

Another test would be to check the continuity of the coils. If the coils are intact and the magnets in the flywheel haven't lost their magnetism, then the thing will work or otherwise its defying the laws of physics...

 

[jamesbell]

This isn't true! Multimeter means multi (ways of measuring) meter.

Even my cheapest ones measure DC and AC volts as well as continuity.

To answer Vic's question, the answer is yes - you should get a meaningful reading, but the actual voltage will be dependent on a number of factors.

Another test would be to check the continuity of the coils. If the coils are intact and the magnets in the flywheel haven't lost their magnetism, then the thing will work or otherwise its defying the laws of physics...

Don't want to burst your little bubble, I never said it was true, I said "as far as I know"

And it still stands, the output voltage from a stator is too varied for a normal standard multimeter to read, (by the way, the OP has stated this), you need a DVA METER to read the peak voltage from all* electrics on a outboard.

* by all I mean, the trigger, stators and switchbox/cdi units

 

[earlybird]

[QUOTE
A digital meter apparently gives an unreadable display.
Wondered if an analogue meter would be more sensible or a good old fashioned bulb on a couple of wires.[/QUOTE]

My understanding, FWIIW, is that a typical diy DVM just rectifies the voltage and gives a peak reading. If the waveform is regular, then that reading should be reasonably steady and fairly independent of that actual waveform. If the reading is varying randomly and widely, as OP implies, I would suspect something wrong with the coil output, eg a series of random spikes? Perhaps an analogue meter might average this to a steadier reading, but I don't know if that would be of any real value.
The power, or charging capability, depends on the area under the voltage curve, this depends on its shape. A more sophisticated meter, reading RMS voltage, is required to measure this value. Vic's suggestion of a light-bulb is a move towards this.

 

[john_morris_uk]

Don't want to burst your little bubble, I never said it was true, I said "as far as I know"

And it still stands, the output voltage from a stator is too varied for a normal standard multimeter to read, (by the way, the OP has stated this), you need a DVA METER to read the peak voltage from all* electrics on a outboard.

* by all I mean, the trigger, stators and switchbox/cdi units

No little bubbles to burst - but with respect you are wrong.

You don't need a special DVA meter to read the unrectified output from an outboard motor charging circuit. Nearly any cheap multimeter is quite capable of measuring the AC volts developed by the coils and magnets as the thing is running.

You might want to use a DVA meter to measure the ignition circuit,but that is another matter. Are you confusing the two functions of the magneto and the charging coils?

Th OP was asking about the charging circuit.

 

[WindermereColvic]

Know anybody with an oscilloscope?

 

[pampas]

or a moving iron instrument,

 

[VicS]

Thanks for the responses. I decided to visit my boat today . Scrubbed off some of the turnstone poo, filthy little b*****, and took the opportunity to clip the meter on to the outboard.

Yes you can measure the AC volts from the charge coil with a digital multimeter( one of Maplin's cheapest ) to test it.

 

[DaveS]

AC voltage reading

I would agree that for most practical purposes a cheap DMM will give perfectly acceptable results for any reasonable waveform. Cheap meters (i.e. not true RMS types) are subject to waveform error, however, since they generally work by taking the average value of the rectified waveform and multiplying this by the ratio of sine wave RMS / average voltage to convert that to an RMS value. Of course this only gives the right answer if the waveform is in fact sinusoidal. So a square waveform will produce a reading that's a bit high, and a triangular waveform will give one that's a bit low.

 

[john_morris_uk]

I would agree that for most practical purposes a cheap DMM will give perfectly acceptable results for any reasonable waveform. Cheap meters (i.e. not true RMS types) are subject to waveform error, however, since they generally work by taking the average value of the rectified waveform and multiplying this by the ratio of sine wave RMS / average voltage to convert that to an RMS value. Of course this only gives the right answer if the waveform is in fact sinusoidal. So a square waveform will produce a reading that's a bit high, and a triangular waveform will give one that's a bit low.

I agree - and there are two reasons why I insisted that a cheap multimeter will be adequate for the job that VicS asked about. One is that the waveform produced by magnets passing coils will be close to sinusoidal, and the second is that the absolute reading is almost irrelevant. The output voltage reading (open circuit) will depend on internal resistance of the coils, speed of the outboard motor, impedence of the multimeter etc etc. But the meter WILL tell you whether the thing is working or not if it gives some sort of sensible voltage (perhaps 20 or 30 volts open circuit might be possible?)

I will put a pound to a penny that the bridge rectifier in any cheap multimeter will rectify the output of the charging coils of an outboard motor. It will certainly tell you whether it is working - which is as VicS eventually reported back.