power factor of linear transformer

Thread Starter


Joined Dec 10, 2013
Hi all,
I am curious what is the power factor of a linear power supply - the classics- linear transformer to 12v AC (or lower) then rectified and filtered. How it changes when regulated by linear voltage regulator or LDO?
I didnt find any info on transformer datasheets, possibly because it depends on load type?
Thanks !

Thread Starter


Joined Dec 10, 2013
Also can an averaging (non RMS) current meter algorithm/meter be used to measure currents of loads with PF 0,9; What would be the error then compared to RMS meter readings - is it cllose to 10%?

Thread Starter


Joined Dec 10, 2013
I think I found a nice answer here:
thanks to Jony130
But my second question remains

So for bridge bridge rectifier + capacitor and load we have.

This situation looks like this

As you can see the source voltage is sill a sine wave but the source current no longer looks like a sine wave. And this is why PF is not equal to 1.

Vin_rms = 7.07V and Iin_rms = 161.3mA
So the APPARENT POWER = S = 7.07V *161.3mA = 1.14VA
And the REAL POWER is transfer by first harmonic RMS source current.

I_V1_rms = 104.21mA * 0.707 = 73.5mA

So the REAL POWER = P = 7.07 * 73.5mA = 0.52W

And finally

PF = 0.52W/1.14VA = 0.45

After some more thoughts about this I think that the we can find REAL POWER in ltspice directly.

All we need is to plot V_In * I_in = V(N002,N003)*-I(V1)
and move the mouse to the label of the trace, hold down the control key and left mouse click.

So the P = 728.47mW


Joined Apr 24, 2011
A transformer by itself is a marvelous device that presents whatever complex impedance on its output to the input, adjusted (approximately) only by the turns ratio.

For a simple fw bridge and capacitor the situation is horrible as the ap is only charged during short slivers of time where the AC nears the peak. It is not a linear current at all so power factor does not apply.

A recent development is a power factor controller stage added to an off the line switching supply. The purpose of this stage is to cleverly control the charging of a supply cap so the current drawn off the AC line is a very nearly unity power factor. The ones I have seen consist of a step up converter (so the input current is continuous) that moniyors the AC line while controlling the pulse width of the switcher. By running at a frequency higher than the line they draw an almost perfect current a ar as power factor is concerned.

The one drawback (aside from cost and complexity) is these are necessarily step up stages, so the 115 v in 28v out supply we make first steps up the line to 360 volts.