Re: Why voltage drops with very small load?

[Aaron323]

I built a very simple AC/DC transformer with the following parts

2 x 9V 20VA transformer
4 x MUR860 diodes
4 x 560uF 50V caps (just have these on hand)

I used various resistors to load the power supply and the output DC voltage changes as follows

No loading 12.84V
175 ohm loading/current shows 65mA/voltage reads 11.85V
125 ohm/95mA/11.78V
100 ohm/115mA/11.66V
50 ohm/225mA/11.27V
36 ohm/280mA/11.14V

Considering the transformer is 20VA with other parts well exceed 1A, why does the voltage drops when the current increases to such a small loading?

Thanks

[AudioFreak]

I'd expect it to settle at around 10.5V at rated load. A transformer with low VA like that will have poor regulation which explains the higher than expected no load voltage. At only 9VAC secondaries, the diode drop is a significant portion of the rectified voltage which accounts for the deficit between VAC x 1.414 vs. the actual loaded output.

[moamps]

Originally posted by Aaron323
Considering the transformer is 20VA with other parts well exceed 1A, why does the voltage drops when the current increases to such a small loading?

Hi,
That's normal for unregulated power supply.
For more info see
http://hyperphysics.phy-astr.gsu.edu...rectct.html#c4

Regards,
Milan

[dnsey]

If you take into account the internal resistance of the supply, the variation is easily explained.

[Eva]

Voltage drop is not proportional to the load current, the apparent resistance is reduced as the load is increased. Voltage will also drop if you (or your neigbours) turn on appliances connected to the same mains line, this is another funny feature of unregulated power supplies.

[Aaron323]

Thank you for all the information. If I changed the transformer from 20VA to 500VA, will the voltage also drops with a load of 250mA?

[Eva]

It will also drop. Consider it this way: the first few volts of voltage drop appear very easily in that kind of unregulated power supplies. A much smaller change will happen when the load is increased from, say, 250mA to 500mA. It happens due to the shape of the mains sine wave and the non-linear characteristic of the diodes.

[richie00boy]

Voltage will always drop with load, just that the higher the VA rating the less it will drop. Or looking at it another way, the rated voltage is usually at the rated load current, so if you don't load it up fully it will be higher. And the lower the VA rating the worse the regulation so the greater the difference between loaded and loaded output voltage.

[AudioFreak]

Originally posted by Aaron323
Thank you for all the information. If I changed the transformer from 20VA to 500VA, will the voltage also drops with a load of 250mA?

Yes, as richie and Eva have already said.... It will still drop but regulation for a 500VA transformer will be far superior to that of a 20VA transformer so the difference between no load and loaded voltage output will be less with the 500VA transformer (ie. It will produce a "stiffer" supply).

[Aaron323]

Thank you for all the information