Variac Transformer 0-120VAC for homemade AC PSU ?

[Kiriakos-GR]

From long time now, the idea about one adjustable AC PSU puzzles my brain. 

I was had read some schematics about making one with electronic adjustment,
but today I found that I can avoid the use of excessive electronics, just by getting one Variac Transformer .

I think that an output of 5A max is all that I will ever need.

But even so , you are free to comment the positives and the negatives of either designs.     

[IanB]

Variacs are a standard solution and have always been used, for example, when doing TV repairs.

However Variacs are not isolated, so if you want an isolated supply you would need an isolation transformer in addition, or an electronic solution.

[ElektroQuark]

Yes, it is useful for repairs.

I have a 220-220 400W isolation transformer and a 0-260 500W variac. I think I will be doing a variable, and very heavy, AC power supply.

Any advice of protection schemes I should implement?

[Kiriakos-GR]

Well I do have an isolation transformer 220-220 & 220-110V .

But my seconds thoughts had to do mostly with the robustness of the movable knob on those  Variac Transformers.

[IanB]

But my seconds thoughts had to do mostly with the robustness of the movable knob on those  Variac Transformers.

Variacs are old school. They have been in use by repair technicians since forever. They are not any of this new-fangled fragile electronicky stuff. (Although it could be that modern cheap Chinese ones are not going to last 50 years like the old ones did. Perhaps better to find a used one from the 60's or 70's when they knew how to make things properly.)

[NiHaoMike]

What about just make a high voltage (0-400V or so) DC bench power supply?

[IanB]

What about just make a high voltage (0-400V or so) DC bench power supply?

Because then it wouldn't be AC?

[Zero999]

I have a 2A variac which is pretty well built. I say go for it. The only protection you'll need is a slow blow fuse or time delay thermal circuit breaker. An isolation transformer is a good idea but that won't totally protect you against a fault on the variac causing a much higher voltage on the secondary than expected.

[ciccio]

I have an old (1980) ISKRA Variac that is still operating perfectly.
I use it for the maintenance and for the first test of power amplifier, where a reversed big electrolytic could be very dangerous if reversed connected and powered from direct ac mains.
I think I cannot do any audio work without it..
When buying it, please consider that most of the times you'll use it at full power only when it will be in "unity ratio" position (e.g. output voltage equals input voltage), and in this condition it can delivery more power than it's nominal value.
I use a 2.5 amps Variac (575 VA at 230 V ac)  to power big amplifiers that  draw more than 1.5 kVA, and the Variac  works well, without saturation and sagging of the ac waveform.
Obviously you should not drive the amp when there is a big input to output ratio of ac voltage.
Put the variac inside a vented cabinet, with a panel meter showing output voltage, an  ac outlet, a time delayed fuse (or better a resettable thermal switch) and a dual circuit switch on the input side, switching both live and neutral.
See the attached photo of my "dinosaur". built in 1980, and never finished...
The Fixed Output was not a good idea,  i do not suggest to make previsions for it: it's confusing.
regards
 
 

[Kiriakos-GR]

Nice design ... 

The digital display takes away all it glory, but yes it is handy = less guess work.

[david77]

I have thought about a variable AC PSU myself for some time and found the solution in an isolating variac. Building something myself allways struck me as a bit of a waste of time, after all you don't need AC as often as DC. For some time I used a transformer with a secondary that has outputs in 1V steps from 1...36V. These things used to be quite common, they were sold as "experimenting transformers" here.
The isolated variac is far more usefull, especially for test & repair as it's also isolated .

These things are quite expensive when bought new, so look for a used unit.
This one has a downside: The inrush current of the big tranny nearly allways trips the 10A breaker to my lab. One day I will have to make a zero crossing detection circuit for it. Or replace the MCB with a 16A one.

[Kiriakos-GR]

Oh my !!         Variac Transformer designs,  had start to jump out of everywhere like rabbits LOL..

Thanks people, I do appreciate all the advices. 

 

[ciccio]

Nice design ... 

The digital display takes away all it glory, but yes it is handy = less guess work.

In effect it was an analog AC voltmeter, but it was anything but usable, so I replaced it with a demo kit of I don't remember what converter IC..
Regards

[NiHaoMike]

Because then it wouldn't be AC?

With most large power supplies, the first thing that is done is to convert to DC. If you really need AC, just add a H bridge inverter.

[Zero999]

With most large power supplies, the first thing that is done is to convert to DC. If you really need AC, just add a H bridge inverter.

The disadvantage with that is you'll need a filter if you want a sinusoidal waveform and it won't be as rugged as a transformer.

A good variable AC source is required to adequately test mains appliances over a wide voltage range. Even if the appliance just converts it to DC, AC is still good because it ensures the rectifier and filter capacitor are working correctly.