Thread Starter

Shahzad Javed

Joined Oct 18, 2008
Why in a step down trasformer changing the load resistance changes the output voltage of transformer which should remains constant........?


Joined Dec 27, 2007
I'm actually not sure why this happens, it is a good question.

I will guess for fun :)

The lowering of the load increases current through the windings. This will cause an increase in the flux density of the magnetic circuit. As flux density increases, the permeability of the magnetic material falls, and energy starts to get lost in the material. This also makes the hysteresis loop bigger, whose integral is the power lost.



Joined Nov 9, 2007
The alternating voltage in a coil is composed of two components. One is due to the resistance drop of the windings, the other is equal but opposite to the voltage induced in the coil by the change of flux linking the coils.

When the magnetic field is wholly in air the flux linkages are proportional to the current. However when there is an iron core, as in a transformer, the flux is related to the current by the non linear magnetisation loop.

This non linearity introduces phase lags between the electric current waveform and the magnetic flux cycling in the core. So the waveform suffers harmonic distortion These harmonics appear as eddy currents in the core which dissapate energy as heat, increasingly with flux density.

I haven't itme to post the maths at the moment but if you are interested I will expand on this again.


Joined Mar 20, 2007
Shahzad Javed

Why in a step down trasformer changing the load resistance changes the output voltage of transformer which should remains constant........?
Step up, step down, or 1:1, it makes no difference. Change load --> secondary current change --> IZ voltage change by transformer secondary impedance --> change in secondary output voltage.



Joined Dec 20, 2007
A cheap low current transformer has high resistance in its windings causing the voltage to be double with a low load current.
A good quality higher current transformer has much better voltage regulation because its resistance is much lower.

I have a cheap 9VDC/200mA adapter with 20V output without a load and exactly 9V with a 200mA load.
I have another adapter with 9VDC but with 500mA current. Its voltage is only 11V without a load and is exactly 9V with a 500mA load. It was not expensive.


Joined Aug 8, 2005
...the output voltage of transformer which should remains constant........?
You're premise is incorrect -- who says it should remain constant? A xfmr is typically not regulated.

If a xfmr is rated 9VRMS @ 1A, then it should produce 9VRMS @ 1A and within the design tolerances of the xfmr. However, the output voltage will change depending on a lighter or heavier load relative to the 1A rating.

You also have to be careful of which units you are using, say RMS vs. peak. If the xfmr is being used in a linear power supply, then the rectifier and capacitor act as a peak detector. So, a 9VRMS xfmr may actually produce 12VDC or more at the output (but, again, voltage will depend on the load).