Help wanted in sourcing sewing machine motor capacitor

[HacksawParticipant @hacksaw]

My daughters hair straighteners make the dog go potty when they're switched on…electrical noise must hurt his ears ? Anything that will stop it ? Do those clip on cable watsits on phone chargers and laptops stop the " noise "?

I might add we can't here anything ,so it must be high pitched !

Edited By Hacksaw on 06/10/2018 20:10:16

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 06/10/2018 19:00:41:

Testing capacitors used to require ouch expensive specialised equipment, but as with other electronics, prices have dropped amazingly. Not the best test kit in the world but this £15 multimeter can measure capacitance. Plenty of other examples on the web, also more sophisticated boxes that can test inductance, resistance, capacitance, ESR and other goodies.

Even with fancy kit, exactly as John explains, you still have to disconnect one lead to make sure the measurement isn't wrecked by whatever else the capacitor is connected to.

Dave

Don't bank on it I've been caught twice buying a multimeter with 'capacitance' in the description that don't actually do it… although the one you've linked does have it on the dial as well.

Neil

[Ian S CParticipant @iansc]

The capacitor on a sewing machine motor is for RFI suppression, and not required for operation of the motor. I can remember when the capacitor blew on Mums Singer, I must have been about 7yr old, my bed room was next to the living room where Mum was sewing, there was an all mighty bang, and I went out to see what had happened, Mum was sitting back from the machine in a cloud of smoke. About an hour later Dad arrived home, took one look at the machine, disconnected the capacitor, and that's the way it remained for the next thirty or so years, notably with less RFI than it had before the big bang. Mum did have a Jones, no electrical problems there, it was Grans treadle machine.

Ian S C

[egaParticipant @ega]

Ian S C:

Stirring stuff! You do wonder about a capacitor that failed so dramatically *and* which was apparently ineffective when in use.

The treadle machines must have been easier to use than the hand crankers.

Neil Wyatt:

I'm seriously considering that multimeter. Can anyone confirm its capacitance capability? I noted the capacitor symbol on the dial.

[Bill Davies 2Participant @billdavies2]

I concur with ega. The Maplin multimeter with capacitance I bought several years back could not cope with electrolytic capacitors used in simple power supplies. I can't find it at the moment so can't quote value. Motor caps will surely be much larger in capacity.

Bill

[John HaineParticipant @johnhaine32865]

That looks more like a battery symbol to me, though what it would measure I don't know. I think the way these meters measure capacitance is to charge it through a resistor or constant current and measure the time to reach a set voltage. Once you have a micro and the ability to measure time/frequency it's easy. Nothing like as accurate and versatile as the old Wayne Kerr LCR bridge I used to use though.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by ega on 07/10/2018 13:48:56:

I'm seriously considering that multimeter. Can anyone confirm its capacitance capability?

.

Me too … The 'view more product details' link would appear to confirm its capability.

MichaelG.

Edited By Michael Gilligan on 07/10/2018 15:21:28

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 07/10/2018 15:17:48:

Posted by ega on 07/10/2018 13:48:56:

I'm seriously considering that multimeter. Can anyone confirm its capacitance capability?

.

Me too … The 'view more product details' link would appear to confirm its capability.

MichaelG.

Edited By Michael Gilligan on 07/10/2018 15:21:28

The fact it give the capacitance range covered gives me confidence that one probably does do what is claimed.

Neil

[SillyOldDufferModerator @sillyoldduffer]

Duncan Webster pointed me at the LCR-T3 a while ago, which is less than £10. Although described as a 'kit', that just means it comes without a case.

The ZIF socket bottom left provides 3 contacts, the ebay picture shows a thyristor plugged into 1, 2 and 3 in a line, other gaps in the socket provide arrangements to suit various lead arrangements. 

Various models, I have a LCR-T3-H. It's pretty smart, recognises resistors, capacitors, inductors, PNP, NPN, diodes, mosfets etc. automatically and provides data. Remarkable for the money. Has to be used with a little intelligence, if you plug in a capacitor and it tells you it's a resistor you have a dud capacitor.

Dave

Edited By SillyOldDuffer on 07/10/2018 16:07:50

[john fletcher 1Participant @johnfletcher1]

Dave beat me top it. I have one of those testers from PRC and find it very useful. I bought a plastic box, cut the base so that the display fitted in together with the battery and fitted three different coloured leads with croc clips. One of the things these testers want do, is test the capacitor at working voltage, which is important for motors and mains operated equipment. I've not seen an LCR bridge which will do it either. John

[Neil WyattModerator @neilwyatt]

Oops, just ordered one to go with my cheap oscilloscope!

Despite what the listing says I'm not convinced it will test triodes though…

Funnily enough earlier today a fellow astro club member brought round the stepper and gearbox from his automatic focuser. The symptoms sounded like a duff coil or connection – noise but no movement. It was a unipolar stepper so we got it working with a 2.5V across the coils in the right order.

At least confident it was OK he was able to get it sorted by reloading the software.

Neil

[John RuddParticipant @johnrudd16576]

Guys, I've been servicing electronic equipment for more years than I care to remember………..

Test gear notwithstanding, the best way is by replacing the suspect component….I have found that in circuit measurement of 'suspect 'devices to unproven…

[Frances IoMParticipant @francesiom58905]

Hacksaw:
the doggy problem will most likely be excessive audio transmission prob around 20-25kHz from a power conversion circuit (mains to whatever low voltage the tongs use) – if you can gain access to the innards try seeing if a tiny transformer close to the mains input can be weighed down with some some magic putty though if the low frequency switched waveform is fed to the heating elements within the tongs then it is probably these that are radiating – I have a couple of small mains to dc converters that I can hear the whistle which changes with the load – a long time ago when I played around with TVs I could easily hear the line whistle from early 625line tvs but suspect I have long since lost that part of the spectrum.

[HacksawParticipant @hacksaw]

Thanks Frances , I doubt she'll let me take them apart….

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 07/10/2018 16:05:35:

Duncan Webster pointed me at the LCR-T3 a while ago, which is less than £10. Although described as a 'kit', that just means it comes without a case.

.

Thanks, Dave

The Seller's description …

Notice: this is kits, not assembled

… is refreshingly cryptic

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Rudd on 07/10/2018 20:25:00:

Guys, I've been servicing electronic equipment for more years than I care to remember………..

Test gear notwithstanding, the best way is by replacing the suspect component….I have found that in circuit measurement of 'suspect 'devices to unproven…

+1 to that. Measuring components already wired into a circuit can be highly misleading so to test a component you often have to remove it. And once you've stressed a delicate component by desoldering, it's not smart to put it back again even if it tested OK. If you've gone to the trouble of removing a suspect component, best replace it with a new one if you can. (Judgement required to decide if a component is 'delicate' or not!)

Dave

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