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Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Hi guys, I’ve got this crazy idea today… I’d like to know your opinion whether something like this can work. The idea is to use a capacitor and charge it straight from the rectified mains (220V AC), but let it charge only to certain voltage. As a very simple solution including the feedback I came with an idea to use UCC37322 chip with the enable pin.
So how it could work: Once the mains are connected the UCC chip becomes open and the current charging the cap can flow. Once the voltage on the cap reaches the zener diode rating the enable pin becomes connected and the chip gets closed. Once the voltage decreases below the zener diode rating the UCC becomes open again. The whole system will oscillate and keep the capacitor charged close to the voltage of the zener diode. The proposal for such circuit is enclosed. I’d like to solve something like that for charging batteries but it could be use also for other loads as a very simple transformator. So, will this work???
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
What is the purpose of the lower rectifier diode and what is the purpose and value of R1? It's gonna see halfwave rectified 230V, is this what you wanted?
I really doubt the circuit will work as you drew it.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Dr. Kilovolt wrote ...
What is the purpose of the lower rectifier diode and what is the purpose and value of R1? It's gonna see halfwave rectified 230V, is this what you wanted?
I really doubt the circuit will work as you drew it.
Nothing in the circuit should see 230V. Maybe it is not fully clear from the scheme but there is a crossing (not contact) - one of the terminals of the cap goes straight to the neutral of the mains (that's why you need two diodes). The R1 is only for opening the UCC. I somehow need to get some signal to the IN pin. Maybe it is not the best solution; I plan the value pretty high (20K maybe even more). Once the circuit gets open it will stay open because the charged cap will always provide enough energy and R1 is not longer necessary.
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
Dr. Kilovolt wrote ...
But I really don't see how the capacitor's negative connect to UCC's negative, I think the design of this circuit is all wrong...
Finally I see your point... No ground for the UCC is a problem. I did not realize that UCC is not a simple switch. It would need completely separated power source for the UCC and drive a FET with it... I'll think about it more. Thanks
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Okay, so the waveform on the load will be saw/ramp wave (because the FET dumps the cap into it, then it charges again).
I think a much better approach for a simple transformerless PSU would be this: 230V mains -> series current limiting capacitor -> fullbridge rectifier -> filter capacitor with parallel zener diode for voltage clamping.
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Simple PSU for charging batteries from mains
