Author Topic: How is a u controller powered directly from AC  (Read 4663 times)

supermankid

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How is a u controller powered directly from AC
« on: June 28, 2013, 03:31:08 PM »
I opened up my remote control switch of my room. Surprisingly there was no transformer...and there was no SMPS or anything visible....I was wondering how they power the micro controller from the ac mains so simply using rectifier and caps....once again.....how do they power the micro controller .. :)

I have attached the top and bottom picture of my ac- dc powered microcontroller

Jason

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Re: How is a u controller powered directly from AC
« Reply #1 on: June 28, 2013, 04:55:52 PM »
This supply looks like a resistive power supply. Note how R9 has a safety gap with an identical value resistor beside it. You can think of it as voltage divider where the capacitors are smoothing out the signal. Though it may also be using resistive and capacitive systems in combination to achieve the desired voltage and current. You can use a transformerless power supply generally speaking for any load that draws less than about 500ma, any larger current will often require a transformer. I think this document may be of value to you: http://ww1.microchip.com/downloads/en/appnotes/00954A.pdf
If it ain't broken, let's open it up and see how it works.

SeanB

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Re: How is a u controller powered directly from AC
« Reply #2 on: June 29, 2013, 02:54:53 AM »
It uses a capacitive power supply, and this gives enough to power the 24V relay, and then there is a simple resistor and zener diode to supply power to the remote receiver chip, which then provides the signal to operate the relay. The power supply normally provides about 33V across C14 regulated by zener diode Z1 to pull in the relay, but this is going to drop to about 18V with the relay on, as it is charge stored in capacitor C14 right next to the relay. The actual micro has it's 12V supply provided by the small diode Z2 and the capacitor there. The antenna and radio receiver is all the circuitry under the board to the right, with the 2 coils on top providing the antenna and the input tuned circuit. I would guess the range is no more than 10m for this.

dr_p

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Re: How is a u controller powered directly from AC
« Reply #3 on: June 29, 2013, 05:10:23 AM »
I was wondering how they power the micro controller from the ac mains so simply using rectifier and caps....

same way as this:


supermankid

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Re: How is a u controller powered directly from AC
« Reply #4 on: June 30, 2013, 11:28:28 AM »
hey. thank you all. I simulated the circuits from the microchip appnote.... awesome...
....now i was wondering....is it possible to obtain current as much as 1A from circuit as this....

....just a curiosity.... :)

SeanB

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Re: How is a u controller powered directly from AC
« Reply #5 on: June 30, 2013, 12:44:53 PM »
Yes, but at the expense of having to always draw 1A from the mains, irrespective if it is on or off. It is best for a draw of under about 20mA, as then you do not really have to worry about power draw in the on or off condition, as they will be small. As well remember that this entire circuit is at mains potential, and all parts and anything connected to it that is not isolated by either an optocoupler, high voltage mains rated capacitor or at least 2mm of plastic in 2 separate layers ( basic Class II insulation requirement) will be at either mains or some other potential that is close to it, and with the capability to deliver a fatal shock.

Drawing 1A means you would find a transformer, rectifier, smoothing capacitor or a proper isolated switching power supply will be smaller and lower cost. To draw 1A means the capacitor has to be a value close to 22Uf 400VAC rated, a capacitor that big is at least the same size as a can of cool drink, and about 5 times the weight of a full can. Switching so big a capacitor on as well, and discharging it when the power is off as well, is not easy, the switch has to be rated for 20A at least and then you need inrush limiting and a 10W bleeder resistor as well across the capacitor.