inductance of a transformer.

Thread Starter


Joined Feb 11, 2007
I own a small transformer (bought for $3 from Sayal Electronics).
and it has 5 connection pins to it. Three are for I think the secondary winding of the transformer, and two are for the primary winding.

Can anyone give me a good estimation of the inductance of the primary winding, and the inductance of the secondary winding (you can exclude the center tap pin)?

I bought a 100uH inductor not too long ago, and I wanted to replace my transformer with something smaller.

I want to know this info because I am working on my PI filter to get rid of the AC interference coming from the power supply.



Joined Dec 20, 2007
A PI filter doesn't do much good. Use a voltage regulator IC if the unregualed voltage is high enough for it.

Or make an FM radio to pickup FM instead of a super-regen AM radio.

Thread Starter


Joined Feb 11, 2007
in my design, it works very good.

If I omit the filter, I will pick up a whole stash of interference, and I'll be lucky if I pick up any part of a remote station. When I made a PI filter with 220uF capacitors and the primary end of a transformer, the signal came in better.

That's why I wanted to know the inductance of the transformer windings.

I'm looking for the best inductance in the smallest package. So far, my largest in a small package is 100uH.

Ron H

Joined Apr 14, 2005
I recently measured the inductance of a power transformer that I pulled from a TV. The primary inductance was almost 1Hy.


Joined May 16, 2005
Put the coil in series with a resistor of known value. Apply an AC signal of known EMF and frequency. Measure the EMF across the resistor. Use Ohm's Law to calculate current. Use algebra to calculate EMF across the coil. Use Ohm's law to calculate impedance of the coil. Use L = XL/(2*pi*f) to calculate the approximate inductance of the coil.

There will be some error in your measurement, unless you have access to very good test equipment.


Joined Jul 17, 2007
Does the transformer have a part number on it, or at least voltage and current or power ratings? If not, what is the physical size of it? (L, W, H)

In general, a coils' inductance is porportional to the number of turns and the average diameter of the turns. The shorter the coil is for the same number of turns, the greater the inductance.

If you took a roll of 24 gauge magnet wire and wrapped 100 turns around the barrel of a Bic pen (which is 0.33" in diameter) in 8.4 layers for a 1/4" length, you would have a coil of 133uH. However, if you wound it single-layer, you'd wind up with a coil just over 2" long with an inductance of 13.7uH.

You can use sewing machine bobbins to wind inductors. If you use plastic bobbins, you don't have to worry about scratching the insulation. You can get sewing bobbins just about anywhere. Just don't swipe Mom's stash :eek: ;)