# Low voltage AC power supply

Hi all,
I need to create an adjustable power supply (AC) that ranges from 0-6 volts with roughly 1 ampere at 1 volt. I have a variac transformer (0-130 volts), an isolation transformer and a stepdown transformer. My question is, does it matter what order I step the voltage down? In other words, could I keep the variac in front of the stepdown transformer, and vary the input voltage of the stepdown transformer? Or should I step it down first then use the variac (even though the input voltage is rated at 120 v...)

Or does it not matter at all?

Thanks,
-Andrew

Related Electrical Engineering News on Phys.org
Simon Bridge
Homework Helper
You can fine-adjust the amplitude of the AC wave using a resistor or potentiometer. You are using a variable transformer though. The relationship between voltage and current depends on the load - you cannot just supply a fixed voltage and current.

For your question - which order you step down will affect the performance ... but you should certainly step down first, and then use the variac. It may be rated at 120V, but what about spikes? Put the highest rated transformer first.

jim hardy
Gold Member
2019 Award
Dearly Missed
Hi all,
I need to create an adjustable power supply (AC) that ranges from 0-6 volts with roughly 1 ampere at 1 volt.
Does that mean it'll be 6 amps at 6 volts?

I have a variac transformer (0-130 volts), an isolation transformer and a stepdown transformer. My question is, does it matter what order I step the voltage down? In other words, could I keep the variac in front of the stepdown transformer, and vary the input voltage of the stepdown transformer? Or should I step it down first then use the variac (even though the input voltage is rated at 120 v...)

Or does it not matter at all?

Thanks,
-Andrew
As you step voltage down you step current up. The voltage drop in your transformer windings is in proportion to current, it'll amount to a few percent of nameplate volts at nameplate amps..

What is the current rating of your variac? Probably it'll have a volt or two drop at nameplate amps and max setting. That's not bothersome at 130 volts out, but it sure could be at 6 volts out.

Placing variac first will give you better regulation out of it because it'll be handling less current.

old jim

Last edited:
1 person
As you step voltage down you step current up. The voltage drop in your transformer windings is in proportion to current, it'll amount to a few percent of nameplate volts at nameplate amps..

What is the current rating of your variac? Probably it'll have a volt or two drop at nameplate amps and max setting. That's not bothersome at 130 volts out, but it sure could be at 6 volts out.

Placing variac first will give you better regulation out of it because it'll be handling less current.

old jim

Great, I measured the output of using the variac first, and it was fairly stable. Now I need to rectify this into a DC...

Thanks for the help