# Re: Current Limiting and Impedence

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• Subject: Re: Current Limiting and Impedence
• From: "Tesla list" <tesla@xxxxxxxxxx>
• Date: Thu, 21 Apr 2005 10:24:15 -0600
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`Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>`

`Hi Mark,`

`Here is my shot on this one...`

`At 06:22 AM 4/21/2005, you wrote:`

`All:`

`Please follow my math and explain my confusion.`

```My TC power transformers have an impedence Z = 1.2 Ohms(secondary
shorted).
```

Ok, so if the output is a dead short, you draw 100 amps. Too high ;-) I think you are using microwave transformers if I remember right.

```I built a Current Limiting Inductor(CLI) with L = 15 mH, thus at 60Hz
X = 2*Pi*60*.015 = 5.65 Ohms. The R for the CLI is negligible. So the
inductor Z = 5.65 Ohms. The system therefore has total impedence of Z =
5.65 + 1.2 = 6.85 Ohms.
```
`It might be more like SQRT (5.65^2 + 1.2^2) = 5.77 ohms.  But close enough....`

```This limits current to 120 VAC/6.85 Ohms = 17.5 amps.
I have confirmed this through testing.
```

Ok perfect!! The coil does not always run into a dead short so a 15 amp circuit should be fine. I suppose you could even sneak by with less inductance in that case, but best to start out real safe.

```Measuring voltage BETWEEN the inductor and transformer I get around 90
to 100 volts(Mains 120V). So if I break down the circuit and consider
the current the individual components...
Transformer I = 90 VAC /1.2 ohms = 75 amps
CLI   I = 30 VAC /5.65 Ohms = 5.3 amps
```

That's almost right. It is like a voltage divider. 120 x 5.65 / (5.65 + 1.2) = 99 volts

`120 x 5.77 / (5.77 + 1.2) = 99 volts too.`

The current is the same in both the inductor and the transformer. I think you got the voltages backwards... It is the voltage across the device divided buy the resistance.

```30/1.2 = 25 amps
90/5.65 = 16 amps```

`Probably some significant phase angle error now in this case.`

```Obviously, one can't analyze the components this way.  Is it because the
voltage measurements are not accurate due to the phase angle?  Am I not
allowed to analyze individual component impedence?  What don't I
understand.
```
`I think you just got a little confused.  It all seems right to me.`

`BTW - Here is a \$30 meter that can measure real power and power factor.`

`http://www.themeterguy.com/advertising/Kill%20A%20Watt/killawatt.htm`

But don't overload it too much. No relation to "me" even though they use my test report with permission.

`Cheers,`

`        Terry`

```Thanks.
Mark
```