Building regulated AC high voltage bench supply. - Page 1
Forum section: electronics / general

author: Dan Moos
author_rank: Frequent Contributor
timestamp: May 28, 2023, 07:58:34 pm
content: What is the standard architecture of an adjustable regulated AC bench supply, say 400v max. I have a transformer meant for tube amps, so I can get over 400v AC at lowest current. Also have the higher current 6v Heater windings.I'm not sure how to create a constant current circuit with AC. Also, how to adjust voltage. Dividers with high wattage resistors? I also don't really know how to regulate AC.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: May 28, 2023, 10:27:49 pm
content: Quote from: Dan Moos on May 28, 2023, 07:58:34 pmMost industrial sources use PWM or a combination of PWM/PAM today, so essentially the same as drive converters. The classic approach is rather akin to a very robust, glossly overdimensioned audio amplifier. In the latter case you would use a gain regulation stage (see VGA, AGC..) for amplitude control.Regarding current control, you have first to decide WHICH current. RMS, peak, or mean, which does also have an influence on the possible dynamic behaviour of your control loop.Then you have to bring your control loops for CV/CC together in a similar manner to a DC supply, but you have to keep in mind that you might have a non-constant phase shift depending on your load and that you might want to keep the control decision valid into the next period. This is where it is not trivial.

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2023, 10:32:30 pm
content: A class-B, class-AB (lower distortion), or class-D (higher efficiency) power amplifier can directly produce an AC output, or drive a transformer to produce an AC output. These days a class-D amplifier is the way to go and there are many suitable class-D audio amplifier ASICs from companies like TI which make it easy. This takes care of the hard part.So a variable amplitude sine wave source drives the power amplifier above to produce a line voltage output. Other features should include short circuit protection, current limiting, and thermal protection to protect the output circuits.

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 28, 2023, 10:37:54 pm
content: Traditionally, variable autotransformers (variacs) are used. These give decent but not amazing load regulation, but in cases you need better then a motorized variac with automatic adjustment can be used. Multitap transformers with relay selection can also be used, with the main caveat being that it can only do discrete steps, with a size depending on how many relays and windings you can justify.There are also linear (dissipative) and switching methods to regulate voltage. A linear pass transistor can be placed inside a bridge rectifier to for a pass element that works for both polarities of the AC, and the same applies for switching transistors.These days I'd also consider rectifying the voltage and re-converting it back to AC using a synchronous buck converter. It might seem a bit more complicated, but it can offer good efficiency and controllability, and it gives you the ability to do arbitrary frequency and waveshape as well.Which solution is best depends on the requirements of the application. Line and load regulation, THD, response time, overload handling, power level, regulation range and resolution and so on.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: May 28, 2023, 10:48:09 pm
content: QuoteOk,I thought about doing it this way. (class ab in fact). Correct me if I'm wrong, but the ripple on my high voltage DC supply would need to be pretty clean for this to work well, right? My experience in high voltage power supplies is with tube amps, and I'm not sure if a full bridge with some filter caps (and maybe a choke?) would get me clean enough DC for this. Maybe I'm wrong?Would building a discrete regulator for the high voltage DC be prudent?

author: Benta
author_rank: Super Contributor
timestamp: May 28, 2023, 11:09:42 pm
content: How much power do you need? MW? kW? W?

author: Dan Moos
author_rank: Frequent Contributor
timestamp: May 28, 2023, 11:24:14 pm
content: 10s of watts at most. Maybe 100 at most.

author: Benta
author_rank: Super Contributor
timestamp: May 28, 2023, 11:55:25 pm
content: Quote from: Dan Moos on May 28, 2023, 11:24:14 pmIn that case I'd simply take an LM3886 and let it drive a suitable transformer. Sine wave generator as input.

author: Dan Moos
author_rank: Frequent Contributor
timestamp: May 29, 2023, 12:12:34 am
content: QuoteI don't know why I didn't think of that. So Basically, I do all my work in low voltage, and then step it up. And I imagine input into the transformer is as regulated as the DC powering the amplifier.

author: mawyatt
author_rank: Super Contributor
timestamp: May 29, 2023, 12:50:43 am
content: If one needs direct coupling here's a thread on HV Buffer AWG Amps.https://www.eevblog.com/forum/testgear/hv-buffer-amp-for-awg/msg3623953/#msg3623953Best,

author: Wolfram
author_rank: Frequent Contributor
timestamp: May 29, 2023, 11:05:58 am
content: Quote from: Dan Moos on May 29, 2023, 12:12:34 amThe input into the transformer will be as accurate as your signal source and the gain of the amplifier, the supply voltage doesn't factor into this unless it's too low for the amp to generate the required voltage.The output of the transformer will have worse regulation, largely due to winding resistance and leakage inductance. The resistance tends to dominate for smaller transformers. The best way to work around this is to sense the output voltage and use that to regulate the amplitude going into the amplifier.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 29, 2023, 02:19:29 pm
content: Look up "IRS2092" for an amplifier board that's ready to use. Drive that with a DAC and microcontroller. Use a divider from the output back into the microcontroller ADC and use that to correct for voltage sag due to varying load.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2023, 05:49:23 am
content: Quote from: Dan Moos on May 28, 2023, 10:48:09 pmThere should not be any need to regulate the DC supply because feedback taken from the amplifier's output, or the step-up transformer's output, should remove any variation from the DC supply, except in the most demanding applications.

author: Xena E
author_rank: Frequent Contributor
timestamp: May 30, 2023, 04:27:58 pm
content: ...You could regulate the AC directly with a saturable reactor, also if you like tube circuitry and have a flair for design you could do the control with hollow state.Ill get my coat...

author: Benta
author_rank: Super Contributor
timestamp: May 30, 2023, 07:56:16 pm
content: Quote from: Xena E on May 30, 2023, 04:27:58 pmA better idea would be to get a Naloxone shot.

author: Xena E
author_rank: Frequent Contributor
timestamp: May 30, 2023, 10:33:23 pm
content: Quote from: Benta on May 30, 2023, 07:56:16 pmThanks for the suggestion! I've never had an addiction to opiates though, just alcohol and cock.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2023, 01:39:31 pm
content: Another topology which is seen in line stabilizers is to drive a buck/boost transformer with its output in series with the line input. This allows a full 120/240 volt output to be regulated within a limited range, and full power is available at 120/240 volts with a smaller amplifier.