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| Building regulated AC high voltage bench supply. |
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| Dan Moos:
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. |
| Neomys Sapiens:
--- Quote from: Dan Moos on May 28, 2023, 07:58:34 pm ---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. --- End quote --- Most 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. |
| David Hess:
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. |
| Wolfram:
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. |
| Dan Moos:
--- Quote ---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. --- End quote --- Ok,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? |
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