# power factor of linear transformer

Discussion in 'General Electronics Chat' started by _dan_, Oct 13, 2015.

1. ### _dan_ Thread Starter New Member

Dec 10, 2013
20
1
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 !

2. ### _dan_ Thread Starter New Member

Dec 10, 2013
20
1
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%?

3. ### _dan_ Thread Starter New Member

Dec 10, 2013
20
1
I think I found a nice answer here:
thanks to Jony130
But my second question remains

4. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,335
1,573
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.

_dan_ likes this.