Ohm's Law and example that doesn't jive

Thread Starter

abstruse1

Joined Jan 18, 2021
6
There are many YouTube videos of people cutting apart microwave power transformers to make electromagnets, using the primary coil. It seems that they use line voltage (120 vac) across the primary. But these primaries measure around .6 ohms resistance. So Ohm's law says they will produce a load of around 200 amps when powered with 120 vac. This is impossible with ordinary household circuits.

What am I missing here? It seems that I'd need around 6 ohms resistance if I wanted to limit the current draw to, say, 20 amps, so I'd need to use 10 transfomers in series or put a 5.4 ohm resistor in series with one transformer.

If I did the latter, what wattage rating would the resistor need?

Apologies for a very shallow understanding of this subject.

Thanx!
 

wayneh

Joined Sep 9, 2010
17,152
There are many YouTube videos of people cutting apart microwave power transformers to make electromagnets, using the primary coil. It seems that they use line voltage (120 vac) across the primary. But these primaries measure around .6 ohms resistance. So Ohm's law says they will produce a load of around 200 amps when powered with 120 vac. This is impossible with ordinary household circuits.

What am I missing here? It seems that I'd need around 6 ohms resistance if I wanted to limit the current draw to, say, 20 amps, so I'd need to use 10 transfomers in series or put a 5.4 ohm resistor in series with one transformer.

If I did the latter, what wattage rating would the resistor need?

Apologies for a very shallow understanding of this subject.

Thanx!
The impedance of a coil is the sum of the DC resistance and the product of inductance times frequency. So as frequency increases, so does the impedance. In AC applications, the impedance due to frequency typically dominates the DC resistance. If you want to see this for yourself, find an old wall wart and measure the DC resistance of the primary, then the current draw. You'll see the current is far less than the ohms value would suggest.

That said, be very careful before connecting anything to line power. I'd be tempted to put a 60W bulb in series first. This will limit the current to no more than 1/2A.
 

Sensacell

Joined Jun 19, 2012
2,821
The coil has a DC resistance that is very low, but a much higher AC REACTANCE that actually limits the current.
The transformer core stores magnetic energy, this energy returns to the power source every cycle, opposing the applied voltage.

Google "inductive reactance" - it's pretty deep, but that's what is going on.
 

MrChips

Joined Oct 2, 2009
23,520
There is something called reactance. The unit of reactance is ohm, same as resistance.
The reactance of an inductor (and a transformer is an inductor) is given by

XL = 2πf x L

where f = frequency in Hz
L is the inductance in henry

For example, an inductor L = 10H @ 60Hz
XL = 2 x 3.1416 x 60 x 10 = 3800Ω approx.
 
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