Low voltage AC power supply

[AndrewBourhis]

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

 

[Simon Bridge]

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]

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

You already know the answer.
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

 

[AndrewBourhis]

You already know the answer.
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

 

[alphy]

Hi Andrew,

It does matter what order you step the voltage down in your power supply. The variac transformer should be placed before the stepdown transformer in order to control the input voltage. This is because the variac transformer is designed to vary the input voltage, while the stepdown transformer is designed to reduce the voltage to a specific level. If you were to place the variac after the stepdown transformer, it would not have the same effect on the input voltage.

Additionally, it is important to consider the input voltage rating of your stepdown transformer. If it is rated at 120 volts, you should not exceed this voltage when using the variac. It is always important to follow the specifications and ratings of your equipment to ensure safe and proper functioning.

I hope this helps and good luck with your adjustable power supply!

Best,