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Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Hi All,
I have been playing around with Jan's 555 flyback driver but I wasn't getting any good performance out of it. I figured that my psu couldn't deliver the amps required for the job, so I thought I might be able to rectify my small variable transformer. I put a bridge rectifier across the output, all good, but when I grounded the negative to mains ground there is problems. As I turn up the transformer it starts to hum and basically cook itself. What is happening here? I need to ground the negative to mains otherwise I get the 50Hz signal on the output of the 555. Perhaps I don't need to ground that side of things as the variable transformer is only feeding the flyback through the mosfet, and I have a separate supply for the 555 driver. I'm sure I'm missing something simple here. Any clues? Cheers, Michael.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
the problem with grounding the bridge rectifier is that it is shorting your variac to ground through one of its diode every half-cycle. If you look at a schematic drawn in with diodes you will be able to see what is happening.
One way to solve it is to run the varaic from an isolation transformer, another way is to connect the - from the 555 supply to the - of the variac and not ground anything (keep in mind that this method your circuit will be 'floating' and have a bit of voltage on it.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
I see now. I knew it was something simple i was missing. The problem I found with not grounding anything was that my wave forms got all screwed up by the AC cycle getting in. As soon as I grounded the driver to mains ground the wave form became classic. I think i'll try not grounding the HV side of things, but ground the driver to keep it clean. I also have a large 2:1 stepdown transformer out of an old TV that I could put before the variac. As the "HV" side is only 30-32V, I'll just turn the variac up to 60-64V. Thanks for the help.
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Rectifying Variable transformer output.
