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jeepwm69

Can I test an AC capacitor with a multimeter

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Dad handed these to me yesterday and asked if I knew how to test to see if they are good or not using one of my multimeters.

 

Anyone have any idea how to test these things?    About as far from ATV's as you can get, but we have a very talented group here, so figured I would ask here first.

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With the proper multi meter you can. You need a symbol that looks like this -|(- that setting can test a capacitor for uf. You will need to look on the side of the cap it will say something like 5/30uf +/-5%. That would mean it has to be tested common to herm and common to fan. Common to fan would be around 5. Common to herm would be around 30. If its a single cell cap it would just have one uf and just test across the 2 terminals and so on. We have universal caps with 5 or 6 cells in them and that gets alot more complicated.

 

Now beware. If they have been sitting for a long time they could read as dead. But still be good. You gotta test them when they have a full charge and that can drain off over long periods of time.

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You are testing for farads which are units of electrical capacitance(ability to hold an electrical charge).  I am assuming you are trying to test a single phase start or run capacitor.  This video gives pretty good instructions.

 

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I remember having fun shocking people with automotive point ignition capacitors , on  a 12v battery , you touched the metal case on the negative and the lead on the positive with a extension wire , it would hold the charge till both the  lead and case were touched at the same time , so if you threw it to someone and said catch , when they caught it , it would zap them hard 

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An over simplified test is to use an old analog meter set to ohms.  After shorting the cap start on your lowest meter setting and touch the leads to the terminals for a few seconds.  Reverse the leads and watch for the needle to jump.  Then repeat the test with the leads reversed.  Anytime the meter shows a low resistance reading  other than the spike, the cap is shorted. 

The way this works is the ohm meter puts a low voltage through a circuit in order to read the resistance.   This puts a slight charge into the cap.  When the leads are reversed that stored charge is read on the meter as a short pulse and swing of the meter.  Starting low scale prevents a hard and fast swing of the needle and prevents meter damage.  Tiny caps might need you to move up into higher scales to see the needle swing.

Capacitors will go out of spec or short out.  This simple test only tells you it's working but not how well.

 

Or do as Fishfiles recommends,  charge and toss em to someone. 

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Well crap, I thought I posted pics yesterday.

 

No telling how long these have been sitting, and given that my only multimeters are of the cheap digital variety, with no -l(- symbol, looks like my best bet is to see if dad has an analog meter.

 

 

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hey jeep, don't tell me you never had to kick start and electric motor working at the grainery......we had a grain Ben floor unloading auger..... never could figure out what was wrong with (wouldn't start) it...... but a friend of my pop said grab the pulley have some one send power to  the motor, and give the pully a spin and it worked every time lol! ...... I'll tell you farmers (some ) will take the shortest most dangerous short cuts. (I THINK) 

 

Quote

Dad handed these to me yesterday

 

 

i was wondering and looking his dad handed him what ? no pic.... lol! 

 

lol! fish..... did y'all do the electric fenceer deal ? some friends and i nailed a friend one time with a range runner electric fencer . right off hand i don't recall how many of us were inline...(holding hands) .. it's hard to explain how this gag works...... but you get some friends and one sucker...... each of you hold hands, the LAST one standing next to the sucker just has to touch his skin while the person 1st inline grabs the fence wire . doesn't make any deference how many people are linked inline the last person always gets nailed..lol!! the more  people inline holding hands the more powerful the shock is..... the trick is..... the person next to the man to be shocked HAS TO TOUCH HIM 1st while the head  man grabs the wire...... and the chain cannot be broken till the head man turns loose of the wire...... and this was a range runner box ether 100 mile, or 500 mile, i can't recall which, but they were extremely powerful, and painfull!! lol! no telling how many people we nailed over the years...... moser and i came up with the idea to set one our service truck... and those range runner boxes we used were solar powered boxes that charged its own 12 volts battery ...so moser tells me his plan,....... sigh...... we head to lunch at my uncles (Wayne's country store) place......we lieve the box on..... and he walks in says he's locked his keys in, well...... you know how helpfull farmers like to be........ no telling how many got shocked that day......so.... Richard (got nailed) came up to me and said there's something wrong with the electrical system on that truck...... i don't think I've ever as laughed as hard in my life.......and to this day we have NEVER told them about the joke..... we both should been jailed over that gag haha!!!! 

Edited by _Wilson_™
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3 hours ago, _Wilson_™ said:

hey jeep, don't tell me you never had to kick start and electric motor working at the grainery......we had a grain Ben floor unloading auger..... never could figure out what was wrong with (wouldn't start) it...... but a friend of my pop said grab the pulley have some one send power to  the motor, and give the pully a spin and it worked every time lol! ...... I'll tell you farmers (some ) will take the shortest most dangerous short cuts. (I THINK)

Yep, and that's how we started the fan on the AC unit till the new capacitor came in too!

 

But he wasn't sure if these were the old ones or extra new ones he bought.  Of course, if he would THROW AWAY the old ones as soon as he took them off it wouldn't be a question!

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This looks like what you need to do with the capacitors posted in the pic.  It can be done with a digital meter that does not have a farads setting.  This guy shows exactly how to do it. 

 

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Looks like I need to make a discharge thingmabob first.  Thanks man!

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I discharge capacitors when I'm in a hurry at work by using a RUBBER or WOOD handle screwdriver by shorting the + terminal to the side of the case.  Not really advising you go that route just letting you know it can be done.  If you decide to do it you could also wear rubber gloves in addition to the rubber or wood handle screwdriver

Edited by Scotticus
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The automotive condensers I use to play with  , which are about the size of 1/2 a roll of Life Saver candy , will snap  when discharged , but nothing like that video above , wow , that would sting 

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3 hours ago, jeepwm69 said:

Yep, and that's how we started the fan on the AC unit till the new capacitor came in too!

 

But he wasn't sure if these were the old ones or extra new ones he bought.  Of course, if he would THROW AWAY the old ones as soon as he took them off it wouldn't be a question!

 

 him throw something away ?? i don't see it happening.... nope.....

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Man i was almost spot on with the cap size without a picture even lol. All the analog test will do is tell you if its shorted or dead. Not weak. Only way to test if its weak is testing for uf. Take them into any hvac place. Or stop any hvac truck you see. They will test it in 3 seconds.

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