So I've been winding and unwinding, all to get the current right but it's not going well, so I'd like to know how I can limit the current going into the primary side of my transformer, since the winding ratio is 1:2, 160 turns on the primary side, and 320 on the secondary, I'm supplying 80v to the primary but the winding only has a resistance of 4ohms. I'd like to know how I can still be able to supply 80v to the primary and limit the current to 1 or 2 amps, to get 160v on the secondary. I am pretty sure the voltage will drop on the primary since the resistance is so small and I'm worried it will also make the output voltage less than what I need it to be...

I'm pretty sure a resistor in series with the primary winding will result in a voltage division? Which is also bad. I don't have much more wire to wind, can you please help me with a solution? Would a series resistor work?

Oh and the frequency is 60hz

Found this:


You can measure the inductance of the winding, but assume it is 10 henries. Reactance=2. * pi. * f * L
F= 60 hz
L = 10 henries
So reactance = 2 * 3.14159 * 60 * 10 = 3769 ohms

So does this apply to a square wave 60hz alternating current supply?

  • 1
    \$\begingroup\$ Make sure your magnetizing inductance is big enough (I.e. the open secondary inductance of the primary) to give you enough impedance so that you're not drawing excessive current from the mains. \$\endgroup\$
    – John D
    Commented Mar 29, 2021 at 16:11
  • \$\begingroup\$ Regarding the recently added information about square wave: electronics.stackexchange.com/questions/132115/… \$\endgroup\$
    – devnull
    Commented Mar 29, 2021 at 16:58
  • \$\begingroup\$ When you say limit the current, which current are you referring to? The no load current? If yes, see Andy’s answer below. Do you have some load on the secondary you are trying to limit? \$\endgroup\$
    – winny
    Commented Mar 29, 2021 at 17:22
  • \$\begingroup\$ BleedingEdgeLab, are we done here? Do you have an answer? \$\endgroup\$
    – Andy aka
    Commented Apr 7, 2021 at 13:16
  • \$\begingroup\$ Not really, I guess I'm not satisfied \$\endgroup\$
    – BELSmith
    Commented Apr 7, 2021 at 13:55

2 Answers 2


the frequency is 60hz


I'm supplying 80v to the primary


160 turns on the primary side

To avoid excess current into your primary you probably need more turns. A typical medium size standard mains transformer used in old fashioned household goods might have enough magnetic permeability to produce 10 μH for one turn and, given that inductance is proportional to the square of the turns, 160 turns will produce about 256 mH of magnetization inductance.

At 60 Hz, that's an impedance of about 96.5 Ω and, from an 80 volt supply, the magnetization current will be about 829 mA RMS. The primary RMS ampere-turns will be about 133 (take note)

That might be too much current and cause core saturation problems (to be avoided).

A pretty standard middle of the road 230 volts mains transformer might have a magnetization inductance of about 10 henries and it would have about 1000 turns on the primary. The current from 230 volts would be about 60 mA and so it would have a magnetic field strength of 1000 x 60 mA = 60 ampere-turns RMS.

So, your transformer (all other things being equal) looks to have over twice the ampere turns and might saturate. You definitely need to do some math on this.


You need to find out what your core will do, in terms of volts per turn, or volts across a winding. That's limited by saturation, and is an absolute limit.

There are two ways to do it, measurement, and calculation. Without knowing the saturation level of the core, it can be easier and more accurate to measure.

With the transformer's secondary unloaded, measure the V/I curve for the primary. If you have a variable voltage source like a Variac, or function generator and amplifier, then great. If not, you can use resistors in series to drop the voltage from a high voltage supply. Filament lamps make good resistors for this purpose. You don't need to know what the resistance is, just limit the current into the transformer, and measure the current drawn and the transformer voltage.

While the transformer is below saturation, you'd expect to see a more or less linear graph of I against V. As you approach saturation, the I will start to increase more rapidly than the V. When you get to saturation, I will increase by an order of magnitude or two. Operate the primary at a voltage at or below this knee in the graph, and let it draw the current it needs to.


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