Variac Transformer 0-120VAC  for homemade AC PSU ?   - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 20, 2011, 03:05:26 pm
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: October 20, 2011, 03:13:18 pm
content: 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.

author: ElektroQuark
author_rank: Unknown
timestamp: October 20, 2011, 03:25:56 pm
content: 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?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 20, 2011, 03:40:52 pm
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: October 20, 2011, 04:04:54 pm
content: Quote from: Kiriakos-GR on October 20, 2011, 03:40:52 pmVariacs 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.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 20, 2011, 07:28:47 pm
content: What about just make a high voltage (0-400V or so) DC bench power supply?

author: IanB
author_rank: Super Contributor
timestamp: October 20, 2011, 08:06:19 pm
content: Quote from: NiHaoMike on October 20, 2011, 07:28:47 pmBecause then it wouldn't be AC?

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2011, 09:19:27 pm
content: 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.

author: ciccio
author_rank: Frequent Contributor
timestamp: October 20, 2011, 09:57:32 pm
content: 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 adual 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

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 20, 2011, 10:30:48 pm
content: Nice design ...The digital display takes away all it glory, but yes it is handy = less guess work.

author: david77
author_rank: Frequent Contributor
timestamp: October 21, 2011, 01:26:25 am
content: 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.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 21, 2011, 01:54:44 am
content: Oh my !!Variac Transformer designs, had start to jump out of everywhere like rabbits LOL..Thanks people, I do appreciate all the advices.

author: ciccio
author_rank: Frequent Contributor
timestamp: October 21, 2011, 05:36:56 pm
content: Quote from: Kiriakos-GR on October 20, 2011, 10:30:48 pmIn 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

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 23, 2011, 01:22:55 pm
content: Quote from: IanB on October 20, 2011, 08:06:19 pmWith 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 23, 2011, 06:21:10 pm
content: Quote from: NiHaoMike on October 23, 2011, 01:22:55 pmThe 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.