Electronics > Beginners
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.
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