# Regulated AC power supply?

I've noticed mains voltage tends to be 120V +/- 10% for 108-132VAC.

How would you go about building a regulated AC-to-AC power supply which works off of mains and outputs 120VAC with a much lower (less than 1%) variation?

The output must be a sine wave at 120Vrms.

Edit: For those asking, the output would be fairly low, 1.5A - 2A, 2.25A max.

Circuit to be powered: Expected input: 0V-5.5KVrms sine wave at 60Hz

Simulated output: +/- 0-100KV DC.

The objective here would be to connect components in the order:

mains --> power conditioner --> variac --> 1:46 center-tapped transformer insulated for high voltage --> Cockcroft Walton Multiplier outlined above.

To achieve a variable 0-100KV DC (effectively 0-200KV DC with both channels factored in) power supply.

I am not certain the power conditioning stage is the way to go but assumed it would be since it is much easier to stabilize low voltage (120Vrms) than it is to stabilize high voltage (5.52KVrms.) Relatively minor ripple of 5% will grow into +/- 10% by the end of the CW stage at full power, yielding around 20KV off of the expected value - not an insignificant amount.

• It's gonna be hard. And depends how many power (and reactive power) you need. What do you need such a supply for? Dec 24, 2016 at 2:19
• Powering high voltage devices such as Cockcroft Walton generators and Tesla coils in a stable manner. Dec 24, 2016 at 3:07
• @CoryG: Short answer: you don't need this. Voltage is unlikely to change at your place from one moment to another, if you don't switch a big load besides your tesla coil yourself. Just use a hand-adjustable HV transformer as the first stage. Dec 24, 2016 at 11:10
• @RichardCrowley The question is fine, that's the output of the particular component the question is about, not the entire project. The question is focused on a single component (constant AC voltage stabilization) and thanks to the wonder that is the Stack Exchange community you can rarely get a straight answer without describing exactly why you want to do what you want to do, which is irrelevant to the original question requiring 2-2.25A max output. Dec 24, 2016 at 17:49
• It is because most of the high-reputation people here have designed 10000x more circuits than you have and they know all the other details that must be considered for a proper answer. Details which you are not aware of (or you wouldn't be coming here for help.) Just because YOU think these details are "irreleant" doesn't make it so. And conversely, things that you think are important don't have any clear reason to be. For example your insistence on a sine wave? Or the value of your capacitors which seems much more appropriate for the typical high-frequency circuits. Not the 60Hz you want. Dec 24, 2016 at 21:13

The classic solution adopted by the TC community is a Variac, but they can tend to get quite heavy for kW throughput.

If you only want to cope with a small range of input variation, say 10% = +/- 12v, then you might consider a small Variac, followed by a 120v->12v transformer, the secondary of which is put in series with your mains supply.

Let's say you had a 1A variac and a 120VA transformer. The transformer could deliver 10A, so could trim a 1200 watt supply, using 120 watt components. There's no free lunch of course, the increase in power throughput goes together with the decrease in range.

If you wanted to be clever, you could custom wind the 12v transformer to give you (say) 0.5v taps, and electronically switch them with (say) back to back MOSFETs, depends what your level of skill and requirement for automatic operation was. A binary tapped 12v transformer and some manual switches would make a cheap half-way house.

• I'm looking for a solution that will handle mains fluctuations without manual tuning every time it changes. The application will actually be in the low hundreds of KV range, but a steady low voltage (120VAC) supply is critical to tuning the high voltage as 5% +/- variation can be the difference of tens of KV. Dec 24, 2016 at 6:53
• What's the VA of your application? Are you looking for something you can purchase, or something you're happy to design. Would a servo-motor driven Variac be OK, or do you want something with no moving parts. None of those questions are answered in your OP or the comment above. Dec 24, 2016 at 9:08
• I added more details to the question. Dec 24, 2016 at 16:28
• Would a servo-motor driven Variac be OK, or do you want something with no moving parts? Dec 24, 2016 at 19:18
• Now we are getting towards a specification, it's often the case that we need to play 20 questions to tease out your requirements. How about an electronically switched tap transformer that tracks a synthesised sinewave, or a PFC corrected DC supply followed by a sinewave inverter, or a switch-mode variac? Dec 25, 2016 at 5:44

The classic answers were ferroresonant regulators and motor/generator sets. Nowadays, a good sinewave generator and a class D audio amplifier into a stepup transformer will manage the same thing, at limited power output.

At the high power end, this is often done with magnetic amplifiers, but those aren't usually available as off-the-shelf building blocks. Big battery chargers are a typical application mag amp batt charger.

I think you are looking for a ferroresonant transformer like this. But that one has a 3% specification. I have one in my lab as it also isolates the line from the work. If you can live with the likes of 3% you may find a used one on ebay at a reasonable price.

• Might be overkill though the word power conditioner lead to some interesting results - I'm looking for around 2A-2.25A max, additional details in the edit. Dec 24, 2016 at 16:29
• They come in all sizes. Search ebay for Sola Constant Voltage Transformer. I saw a few at 250VA. Dec 24, 2016 at 17:17
• Thanks, so far this would be my top pick but 3% is still more than I'm looking for (ideally under 1% since 3% is still about +/- 6KV on the high voltage end at full power - +/- 2KV I could tolerate but that's about it.) Dec 24, 2016 at 18:03
• That 3% was the spec on the first link and that would be worse case. Regulation error is load dependent. If regulation is important you may want 1000VA transformer for a 250VA application. Check out some spec sheets line this Dec 24, 2016 at 18:38

What time constant are you after? If you can live with second long recovery times, a motorized variac and a controller for the same will give you very well regulated 120 V output over time but slow transient recovery.

If you need faster, a suitable sized AC/DC power supply/battery charger followed by an inverter will give you dead flat 120 V, if your budget allows.