simple power supply question
simple power supply question
(OP)
I'm trying to figure out Microchip's application note TB008. It describes a simple nonisolated power supply that can be used for PICs. The author starts out with a simple circuit: The line voltage across a series connected resistor and capacitor. He states that as long as the reactance of the capacitor is significantly larger than the resistance of the resistor you will have a nearly constant current. I don't get that. Wouldn't the current still be a sine wave (not constant)?
I'd like to use this circuit but I'm uncomfortable not understanding how it works.
I'd like to use this circuit but I'm uncomfortable not understanding how it works.





RE: simple power supply question
Same thing here. The AC current RMS value is more or less constant because the voltage drop in the RC combination is many times higher than the voltage drop across the following rectifier/capacitor/zeber combination.
So, I think that it is all a question about how you look at it. The AC current is, of course, time-varying. But its RMS value is constant.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: simple power supply question
Unless you're intending to produce the powered circuit in quantity, so cost really is an issue, I'd strongly suggest you just buy a DC wall wart instead of using a line- powered nonisolated supply.
Mike Halloran
Pembroke Pines, FL, USA
RE: simple power supply question
For reliable operation the capacitor would need to be rated above 1000V AC (on 240V rails).
RE: simple power supply question
Mike - I want to use the PIC to measure line zero crossing and RMS value and also to measure the current (via a series sense resistor) in my circuit. I don't have much room on the board to do all this. Using the nuetral as a common would work nicely. I too am worried about the line transients.
RE: simple power supply question
Mike Halloran
Pembroke Pines, FL, USA
RE: simple power supply question