Blue Ring Tester and Automotive / Garden Equipment Coils

[R Lamparter]

The magnetic clutch on my lawn mower probably died, and replacing this expensive part will prove or disprove that it was the culprit (having followed the troubleshooting advice of the local service tech)  Nevertheless, it's pretty hard to get accurate readings on things like coils when the test leads of your multimeter give a reading of 1 ohm when shorted together.  I was wondering if the Anatek Blue Ring Tester would determine whether the coil in the clutch is bad.  Ditto for checking the spark coils on small gas engines.  Will the ring tester tell you directly if the coil of an ignition module is bad or do you need a known good one with which to compare readings?

[Dabbot]

If you at least know the operating voltage and current of the model in question, you will be able to use Ohms' Law to calculate how many ohms the coil is. Better yet if it straight up tells you in the specs.

Regarding your multimeter reading 1 ohm with the leads shorted, check the manual and make sure you know how to zero it.

[Circlotron]

For your magnetic clutch you should be able to just stick 12 volts across it and see if it engages. Watch out if it has a diode across it - apply the 12 volts so that the diode is reverse biased.

[Ian.M]

The trouble with using a ring tester (pulsed 'Q' tester) on coils in wound components other than a simple ferrite core inductor or switched mode transformer, is unless you have an identical 'Known Good' specimen to compare with, you don't know how high 'Q' the coil should be so the results are usually ambiguous.   If there's a core you can add an external shorted turn round, you can create a 'Known Bad' result to compare with, but although a no or insignificant difference can definitely eliminate many coils with internal shorted turns, a significant difference doesn't actually prove the coil is good. e.g. it may still break down at higher voltages/energies than the ring tester applies.

For spark coils on small gas engines, it depends on if they use magneto, Kettering or some variant of electronic ignition.   If its got points and a battery, you can do a functional test of the coil on the bench by duplicating the Kettering primary circuit and loading the secondary with a spark plug^* set up so you can readily observe the gap.   Be very cautious if using an adjustable gap type spark voltage  tester, as an excessive gap can cause insulation breakdown in the coil (as can operating the coil without a load on its secondary winding).  If its magneto or no points electronic ignition, you will probably have to test it in-situ, also with an exposed spark plug as a load, decompressing the engine by removing plugs or arranging to hold valves open, then spinning it at a suitable speed with a large electric drill and socket adapter on the flywheel nut.

* By Paschen's law, the breakdown voltage of a spark gap under pressure will be multiplied by approximately the compression ratio, so the test plug needs its gap multiplied by the same ratio to get a similar breakdown voltage in free air.  It doesn't need to be the same type as the one in the engine, but does need to have similar electrodes in reasonable condition and a good clean insulator. If it isn't practical to gap a plug wide enough, an adjustable gap tester can be used, but set the gap appropriately and lock it there (use bluetack if there's no locking mechanism), don't just increase the gap till the spark stops.

[R Lamparter]

Thanks for your prompt response.  I had ordered the replacement clutch, based on a minimal resistance difference between what it should read and what I got with my multimeter.  The replacement clutch came in today and solved the problem, so my measurement was correct. 

I had been thinking of the ring tester before this because of a problem with a battery charger for a battery powered drill in which a thermal fuse within the voltage transformer blows when the battery is charged for just a few minutes.  I disassembled the transformer and unwrapped it till I got to the thermal fuse and didn't see any burned areas, but the replacement thermal fuse blew when it was reassembled.  I don't find any shorts in the diodes that the transformer feeds making me wonder whether there was a shorted coil or two within the transformer that made it provide the correct output voltage, but possibly drawing more current than it should.  I've wondered whether a ring tester would tell me in this case whether the transformer was faulty or is this again a situation where you'd need a known good transformer with which to compare it.

[Ian.M]

With enough experience with ring testing similar transformers, and if in doubt, by applying a shorting turn threaded through the core to set a 'known bad' state for comparison, you can definitely identify most bad ones.  However proving that one is O.K is a lot harder.

For line frequency transformers, the best option (if you don't intend to attempt to rewind it) is to remove it from the equipment, and smell it!  If it smells burnt it probably is and you should never trust it again.

If it doesn't smell burnt place it on a loose concrete slab outside (or other non-flammable sacrificial surface), in a location safe from pets and children, bypass the thermal fuse if its blown and hook it up to power with an in-line fuse, and wait and see if it cooks itself with no load on the secondary.  If it runs for an hour without overheating, the original fault must have been due to an overload on the secondary.  If it catches on fire and blows the fuse it must have been bad.  Don't try to do the test inside unless you've got a good fume cupboard and some way of containing the expected transformer fire.  Testing it in-situ is not advisable as the heat and smoke may cause extensive damage.

If it turns out to be O.K., don't forget to replace the thermal fuse and trace and rectify the cause of the overload before refitting it.

[floobydust]

A shorted winding is a problem for magnetics with an alternating flux, like SMPS transformers and TV flybacks, motors, ignition coils.
Anatek Blue Ring tester is good for finding that. Measuring low ohms is difficult and hard to say if there is a shorted-turn based on milliohms.
But if the short appears "at voltage" like an ignition coil arcing internally, then the ring tester would not catch that.

The clutch is an electromagnet, flux is on or off. A shorted turn would lower DC resistance a bit and make it sluggish but still good field strength. It's usually heat that makes the magnets fade, or wear increasing the air-gap, not the coil or the coil shorts and burns up.
I don't know if the Blue Ring would say the windings have a problem, on the clutch. It's another low cost tool.
You could try the other method of injecting an AC flux to the component (by adding turns and AC source) or using a solenoid and rod.

[R Lamparter]

For your magnetic clutch you should be able to just stick 12 volts across it and see if it engages.

It engages, but blew the fuse while mowing.  It happened again with a replacement fuse - engaged and worked for less than a minute till it blew the fuse again.  Disconnecting the clutch and pulling the PTO switch on didn't cause the fuse to blow, so presumably the problem was going to be in the clutch.  I don't do TV work but knowing about the ring tester made me wonder whether I could have used that to confirm the clutch coil was bad.  From the other comments, it looks like the ring tester is really only good for flyback transformers, not the sort of coils and power transformers I'd be testing.

[R Lamparter]

If it doesn't smell burnt place it on a loose concrete slab outside (or other non-flammable sacrificial surface), in a location safe from pets and children, bypass the thermal fuse if its blown and hook it up to power with an in-line fuse, and wait and see if it cooks itself with no load on the secondary. 

Thanks. Sounds like a good suggestion.  As you guys can see, electronics is not my forte.  I've got a lot to learn.