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Can't explain this phenomenon of ohm readings.

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MikeT1982

Thu Apr 26 2007, 08:20PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119

I took my Fluke meter and measured a 20Kohm Ohmite Wirewound resistor and not suprisingly I get 19.8 kOhms.

However, when I measure the primary side of my NST I get 5.1 Ohms. This confuses me as the Primary side draws 1.17 amps at 120 volts! All 3 France's read almost exactly 5.1 ohms and work fine. What I can't understand is why Ohm's law equation fails me here. If I plug in 1.17 amps and 120 volts into ohms law, the NST's Primary should be 102.56 ohms. Please help me to understand what the heck is the reason for this.

Thanks,

Mike

Ken M.

Thu Apr 26 2007, 08:24PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I would say because its a transformer and when power is applied the flux lines shift the resistive values...thats the best explaination I can give...Never really bothered to ask why, just know that coils act funny with resistivity (Probly not a word).

MikeT1982

Thu Apr 26 2007, 08:38PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Interesting, that makes sense man thanks. Maybe this explains why even these little 4kV@30mA Frances dim the room lights for a split second when plugged in. If 120 volts surged into the 5.1 ohms my Fluke meter gives then we'd have a current draw of like 24 amps...it must drop reeeal fast once the coil energizes if thats the case.

Reaching

Thu Apr 26 2007, 08:40PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

thats just the dc resistance you measured. but since you work with ac and an inductance, you cant use ohms law here to calculate the amperage.

Marko

Thu Apr 26 2007, 08:42PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Nothing is 'acting funny with resistivity', your meter only and rightly gave you the reading of DC resistance of primary winding.

When plugged into line, the inductive resistance X will be what will limit your current mostly.

102.56 ohms is your primary impedance and not resistance Z wich you apply the ohm's law to.
Inductance causes the current to lage the voltage for 90 degrees.

So Z = sqrt(R^2 + X^2).

The ohmic resistance of your primary winding should actually be smallest possible because it just represents loss; an ideal transformer would have no ohmic winding resistance at all.

r3d

Thu Apr 26 2007, 08:43PM

Registered Member #532 Joined: Sat Feb 17 2007, 05:00PM
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Posts: 11

Every transformer pulls a lot of current when pluged in, especially those with 300VA+! For them you have device called "soft start" that minimises the current they can draw!
And every single component has different resistance when measured without current passing thru it and with current passing thru it.

Steve Ward

Thu Apr 26 2007, 09:55PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

And every single component has different resistance when measured without current passing thru it and with current passing thru it.

NO

Just... NO!

Pleaaaase, i dont mind people trying to help, but if you dont know what you are talking about, you are just starting more misconceptions about the nature of electronics!

Simon

Thu Apr 26 2007, 11:49PM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

wrote ...

And every single component has different resistance when measured without current passing thru it and with current passing thru it.

It's strictly true but can be ignored in just about any practical situation, including this one.

ragnar

Fri Apr 27 2007, 12:17AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Thanks Steve! R3D, WhiteArc, don't make stuff up. That's my job

MikeT, Firkragg's explanation is correct. Or, less accurately (assuming a zero resistance), you can calculate the "inductive reactance" using the following formula:

R = 2pi * f * L

where R is your 'reactance',
f is your mains frequency,
L is the inductance.

MikeT1982

Fri Apr 27 2007, 03:12AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Thanks. This is what I like about this board is there's people willing to help out. I wanted to find a board less advanced since I often have "elemetary" questions but am doing a decently big project. But this place keeps me coming back because I know I will get educated answers, I appreciate all viewpoints too regardless of how precise. Orignally I had some thoughts that something funky was going on being that it was a transformer and a coil with AC current but couldn't figure quite what would do that. Now I can see that the ohms law equation V=IR can't be used alone on an AC driven coil like this to see how it reacts when live.

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