Diode check

[alanporter]

Can anyone advise me whether I can check my alternator diodes with an ordinary Volt Ohm Meter. My meter does not have a diode check mode, and , as a skinflint, I would rather not buy a new meter. Thanks in advance.

<hr width=100% size=1>Alan Porter

 

[Strathglass]

Alan
Meters with a 'diode check' are no different from those without. They just have a diode symbol stamped at the range switch position compatable with reading the forward current on that particular meter.

Switch your meter to a range that displays 1K (1000 ohms) at somewhere between 30 and 50% of full scale and expect a reading of about that in the foreward mode and no reading in reverse.

Iain

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[alan]

As Ian says you should get readings of a few hundred ohms in the forward direction and a few Megohms in the reverse direction.

Alan
Nettuno, Italy.

<hr width=100% size=1>Alan Cloke

 

[halcyon]

Make sure there not connected when checking, or at least one end is free. You can have a return path via other components that will show you a faulty diode.

Brian

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[john_morris_uk]

Because of the way that meters work on the ohms scale and the different ways that diodes are manufactured, there is no simple reliable 'correct' reading for the forward resistance of a diode when tested by a multimeter. If it conducts one way and doesn't conduct the other it is probably (99.99%) perfectly ok. (0.01% for when the junction has failed for the designed current, but the the multimeter doesn't provide enough current to break down the junction and show the failure!!)

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[VicS]

I recently compared the way two very different digital multimeters operated on the ohms and diode check ranges. One was a recent cheapy for Maplins the other a very old Avo.

The polarity of the voltage applied varied between the two and between resistance and diode check ranges. The voltage applied varied through the ohms ranges, higher voltages being appied on the higher resistance ranges.

It was possible to check diodes using the higher resistance ranges but not the lower ranges.

It confirms your opening statement and that you have to look for a situation where you get a very high reading one way and a very low reading the other. It would be a good idea to check a known good diode first, if posible, just to be sure the meter is on a suitable range and to give an idea of the readings to expect.

BTW Maplins have their cheapest on offer at half price at the moment. Who can afford not to get one or even two?

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[William_H]

Hello Alan I can't help thinking you are making a mistake wanting to check the diodes in your alternator. I don't believe they give much trouble and any problem with your alternator is far more likely to be in the brushes and slip rings. However in answer to your question yes you can but it is probably far too much trouble. There are 6 diodes in a 3 phase bridge. The cathodes of 3 go to the positive output terminal while the anodes of those 3 go to the 3 windings. The other end of the 3 windings are joined together and go to a auxilliary terminal or no connection just to one another.
The cathodes of the other 3 diodes also go to the 3 winding ends where the other diodes connect. The anodes of those 3 diodes go to ground. The diodes are pressed into 2 aluminium plates one connected to ground one to the output terminal. On the first 3 diodes the body of the diode is the cathode while on the other 3 diodes the body is the anode. There are 2 different types of diodes the same only oposite connections. There may also be a further 3 diodes possibly smaller anodes connected to the 3 windings which provides current for the regulator and rotating field coil.
To check the diodes you would need to disconnect the lead of each diode or press them out of the plate so that one end is free otherwise the connection of the coils will confuse your reading. The proffessional would run the alternator on a test bench and check the output on an osciloscope as any single diode failure will result in an output which is low in current and shows a gap in the supply for one phase. The classic syptom of a failed diode is extreme radio interference when the alternator is charging and low output.
I am a dedicated DIY however I would suggest you get an overhaul shop with a test bench to check it if you really think you have bad diodes. In practice it might be easier cheaper to replace the alternator with one from an auto wrecker.
My guess is that you have no charging. Pull the brushes or brush and regulator box off the end plate other end to the pulley. If you have only brushes on the plate use your multimeter to check for continuity from one brush to the other, before you remove. Check as the alternator is rotated. This should be several ohms only. Look at the slip rings if they are deeply grooved more than .5 mm then that is likely to be your problem. If you remove the end plate it should expose the slip rings which can be turned down in a lathe or even turned by had which pressing emery paper against it to clean up the slip rings.
These comments are are all guess work based on normal alternator theory, might give you some food for thought. I agree with comments re multimeters except my multimeter reads voltage drop in forward direction ie .7 volt on diode test and overange in reverse. Where on ohms scale it shows in ohms as calculated from the current forced through the diode dependant on the voltage applied. The moral is check a known good diode to be sure although any reading showing very high resistance one way and a bit the other way probably means its good. Regards ole will
PS the anode is the connection of the diode where if a positive is applied it can flow through to the other end called the cathode. On a circuit symbol the cathode has the point of the arrow and the horizontal line. The anode is the tail of the arrow. ie current flows in the direction of the arrow.

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[alanporter]

G'day William. Many thanks for your long,and obviously highly qualified, reply. From what you say it seems much more complicated than I thought at first. I think a call to a marine electrician is in order.
It is very nice to be able to get knowlegable advice on this forum, and I am grateful to all who have responded.
Fair winds
Alan

<hr width=100% size=1>Alan Porter