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Old 19th October 2010, 11:39 PM   #1
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Default how to test chokes for current and voltage

got a few chokes of unkown charateristics.

the inductance was easy...inductance meter

dcr easy...ohm meter


current easy using ohms law...load the choke...check temps.

voltage rating. Thinking of using a variable dc power supply (0 to 500 volts) and running the chokes up the ladder. to determine what the voltage rating is, i'm thinking of using various size fuses. I would compute for 95% of the fuse rating and use that voltage to test across a known load. if the fuse pops, i'm assuming that means I have a voltage leak which is causing a short. the highest voltage that resulted in no blown fuses, would be my voltage rating.

anyone have a better way of doing this?

thanks
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Old 19th October 2010, 11:49 PM   #2
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problem is, once you HAVE a leaky winding, it will progressively get worse...
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Old 19th October 2010, 11:54 PM   #3
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Hi there. I am a newbie.

I found your test very interesting. About the voltage leak part though, will the air humidity of the test day, play any role as for when the voltage will spark somewhere within the coil?

anyone has any other thoughts?

Marvin
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Old 20th October 2010, 12:30 AM   #4
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Originally Posted by aardvarkash10 View Post
problem is, once you HAVE a leaky winding, it will progressively get worse...
not sure I understand. These are new chokes with unknown values.

so far I got 1H, .250ma cold, .500ma warm,28 ohms.

so If I use the 150ma side of my 0 to 500 volt DC power supply, and compute a voltage that will produce 95% of my fuse current rating, are you saying that will permenately damage the choke so that it will always leak?
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Old 20th October 2010, 12:34 AM   #5
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Originally Posted by mrivera982 View Post
Hi there. I am a newbie.

I found your test very interesting. About the voltage leak part though, will the air humidity of the test day, play any role as for when the voltage will spark somewhere within the coil?

anyone has any other thoughts?

Marvin
I agree on the air resistance. But that would just contribute to current once you pass the insulation rating, I don't think that will lower the insulation rating.

I don't think that climbing up the ladder will produce a spark. more like what happens when you touch an insulated wire that is carrying more than the rating of the insulation...you get a tingle...but touching the wire against ground does not mean you will get a spark.
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Old 20th October 2010, 12:35 AM   #6
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I'm not a fan of your fuse method as you force substantial leakage (aka dielectric breakdown) in the inductor. Fuses are to prevent fire, they're not current limiters. I suggest inserting a sizable series resistor (1 Mohm for example) and measuring the leakage of the inductor with an ammeter.

~Tom
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Old 20th October 2010, 12:49 AM   #7
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Originally Posted by tomchr View Post
I'm not a fan of your fuse method as you force substantial leakage (aka dielectric breakdown) in the inductor. Fuses are to prevent fire, they're not current limiters. I suggest inserting a sizable series resistor (1 Mohm for example) and measuring the leakage of the inductor with an ammeter.

~Tom
Thats it Tom - or use a megger - a device designed to perform this test...

A megger applies a high voltage to the device and measures any leakage to ground. This allows you safely to assess the breakdown voltage. You can then calculate the safe working voltage (typically you'd apply a factor of 3 so if breakdown occurs at say 1200V your SWV is 400V)

A motor rewinder or industrial electrical contractor in your area may be your best bet.
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Last edited by aardvarkash10; 20th October 2010 at 12:57 AM.
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Old 20th October 2010, 02:28 AM   #8
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Isn't it a possibility that your core will saturate before you overheat the choke? When the core saturates, inductance will drop. I was under the impression that most power supply filter chokes current rating was limited by core saturation not by heat production. Am I wrong?
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Old 20th October 2010, 05:19 AM   #9
ChrisA is offline ChrisA  United States
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Originally Posted by speakerfritz View Post
got a few chokes of unkown charateristics.

the inductance was easy...inductance meter

dcr easy...ohm meter


current easy using ohms law...load the choke...check temps.

voltage rating. Thinking of using a variable dc power supply (0 to 500 volts) and running the chokes up the ladder. to determine what the voltage rating is, i'm thinking of using various size fuses. I would compute for 95% of the fuse rating and use that voltage to test across a known load. if the fuse pops, i'm assuming that means I have a voltage leak which is causing a short. the highest voltage that resulted in no blown fuses, would be my voltage rating.

anyone have a better way of doing this?

thanks
No. Inductance is not so easy to measure. The problem is that on a power choke inductance goes down as current goes up. The iron saturates. Your meter completely ignores this because it uses very little current. We are not taking about a few percent error either. Your meter is giving you "best case inductanc which you will never see in use.

I don't understand your voltage rating test. voltage on a choke is just the point the insulation fails. You don't want to test that, estimate it then don't go close. Once it fails the choke is junk and you'd have to rewind it with new wire.

Crrent is different. You can estimat that from the size of the wire and comparing to other chokes and then test it with 1/2 your estimate and then move up slowly.
You need a test curcuit that can load the choke with varying amounts the DC curent and on top of that a 120Hz AC component. So in a series you place a transformer secondary, the choke and your DC current source. This will place DC throught the choke and also the transformer, so make sure the transformer can handle it., Next place an AC source in the transformer primary.

Next get out the DMM and measure the drop on AC volts and in DC volts. From this you compute the reactance of the choke and them solve for inductance.

Next turn the DC current up and measure reactance again. it should be less. With enough current yu could drive it to zero, But the choke will get warm well before that. When it is just slightly warm and temperature remains stable for, 20 minutes or an hour use that current as the rating and then measure reactance, that is the chokes "minimum inductance that you can use for power supply design
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Old 20th October 2010, 05:26 AM   #10
ChrisA is offline ChrisA  United States
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Originally Posted by SpreadSpectrum View Post
Isn't it a possibility that your core will saturate before you overheat the choke? When the core saturates, inductance will drop. I was under the impression that most power supply filter chokes current rating was limited by core saturation not by heat production. Am I wrong?
If the choke is well designed. by this "well" means good performance to cost ratio. both limiting factors occur at about the same current. But saturation is like heat, both are gradual things and there is alwasy some seat and some core saturation and it is a judgment call to say how much is enogh

If these are older, salvaged chokes that you really do not know the exact specs for it is best to just de-rate them from your estimates by some safety factor.
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