Simple HV inverter with 60Hz transformer - Page 1
Forum section: electronics / projects

author: Kevelade07
author_rank: Newbie
timestamp: June 11, 2020, 12:54:26 am
content: Hi everyone,I want to build a simple HV inverter to test the DC leakage of the bus capacitors on the various devices I repair at work. I found an old 575VAC to 24VAC transformer in a pile of scrap. There's no info except voltage written on it, and I assume it's around 100-200VA looking at its size. I'd like to use it as a step-up converter, to hopefully bring the rectified voltage up to around 750VDC. I don't need a lot of current, 100mA would be plenty.At first I was thinking of using a simple 555 circuit, feeding a ~60Hz square wave into the transformer with the help of a drive transistor. However, I'm afraid the lack of efficiency will overheat it, or prevent my output from reaching my desired voltage. Is there a way I could filter the output of my transistor to approximate a sine wave, and if so, how can I calculate the value of the parts I will use (capacitor, inductor or even resistor)? These equations are faaaaaar away in my brain, and to be fair I'm far from an expert in analog circuit tuning...My other solution would be to program a PIC microcontroller to output a modified sine wave to the tranformer, and maybe adding a touch of filtering. I guess it would be more efficient to do it that way, but I'm a bit lazy and I'd rather not test the operational limits of the computersaurus (read: v.slow computer) I have at work, while trying to program a few lines of codeOh! And I almost forgot... It needs to be variable too! No need for fancy doodads, varying the input voltage (or drive voltage) can do just fine.I'd like to know your opinion on the subject, and maybe even good ideas I did not think of. The simpler the better!Thank you

author: profdc9
author_rank: Frequent Contributor
timestamp: June 11, 2020, 03:19:56 am
content: An efficient method can be to use a half-bridge with two transistors like IRF540s. One side of the transformer primary is connected to the half bridge, and the other is connected to two capacitors in parallel, one connected to the positive and the other to the negative so that the nominal voltage is at half the supply voltage. By choosing the capacitance, you can prevent too much current from passing through the IRF540s, and the body diodes of the IRF540s serve to shunt any residual inductive kick of the transformer. This is also a useful way to drive other transformers, such as flyback transformers.If you use a PIC, you can use PWM to vary the duty cycle of the half bridge and vary the output voltage as well. More dead time will decrease the output voltage. The main disadvantage of driving with a square wave is ringing.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 11, 2020, 03:55:32 am
content: Connect the 24V winding to an audio amplifier with a series resistor (ideally the minimum resistance the amplifier is designed for) as a current limiter. Connect the 575V winding to a bridge rectifier. (Or a doubler if you're having difficulty getting enough voltage.) Give the audio amplifier a 60Hz or so sine wave and adjust the volume to adjust the voltage. (Experiment with the frequency, might be able to go as high as 500Hz before losses become excessive depending on the transformer. You'll probably find a point where core losses start to offset the reduction in magnetizing current.)

author: Kevelade07
author_rank: Newbie
timestamp: June 22, 2020, 07:20:13 pm
content: Hi profdc9,Sorry for the delay of my reply. I somehow did not receive a reply notification.That's a pretty good idea. I've been thinking about it, and I'll probably use a PIC to be able to fine-tune the output as much as I want, and maybe add features if needed in the future. I'll try experimenting with that concept.Thanks a lot!

author: Kevelade07
author_rank: Newbie
timestamp: June 22, 2020, 07:27:49 pm
content: Hi NiHaoMike,I thought about that concept too, but I want to limit the number of parts involved. And to be fair, that setup would be too simple for my complicated mindIt's still a very good 'plug-and-play' idea, and I'll keep it in mind if the first try doesn't work as I want it to.Thank you!

author: james_s
author_rank: Super Contributor
timestamp: June 22, 2020, 07:33:06 pm
content: How much current are you wanting to draw? It doesn't sound like much.Using a simple oscillator to drive a H-bridge feeding what was originally the secondary will work fine. There were power inverters made this way back in the 70s-80s to get 120V from 12V back before the modern style with a high frequency ferrite transformer took over. They were bulky and heavy and the transformers driven by a square wave tend to buzz but they worked.

author: bin_liu
author_rank: Contributor
timestamp: June 23, 2020, 01:37:30 am
content: It seems simpler to use power frequency transformer to make oneself.1. Use the MCU to output a pair of complementary 50% duty cycle PWM as a push-pull topology to increase the voltage, and open-loop work;2. The push-pull circuit with SG3525 as the core can also raise the voltage.

author: Kevelade07
author_rank: Newbie
timestamp: June 23, 2020, 12:14:55 pm
content: I'm looking to get at least 50mA @ ±750VDC once rectified.I'll start with a simple H-bridge setup, and go on from there. Once it works I can always add/tweak stuff on the go with the magic of the PIC microcontroller.I'll update as soon as something works!Thank you all!

author: Nauris
author_rank: Regular Contributor
timestamp: June 23, 2020, 05:47:14 pm
content: I would just feed it with 24 VAC from another transformer. Voltage could be easily adjusted by feeding the whole thing with a variac.