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From: "Anthony Q. Bachler"
Subject: Re: How to do a 220V AC to 12V DC and 24V DC without a transformer ?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Thu, 16 Jan 2003 06:26:28 GMT
NNTP-Posting-Date: Wed, 15 Jan 2003 22:26:28 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Rather than poo-poo your request for help like some others in this thread, I
will actually point you in the right direction, since you came here askign
for advice, not criticism on your level of technical knowledge.
Transformerless power conversion circuits are almost invariably some flavor
of switching power supply if efficiency is in any way a concern, which it
obviously is since you mentioned heat being a problem ( heat is caused by
innefficient conversion of power in the supply ).
Radio Shack sells a very good manual for beginners on building power
supplies. It includes at elast one design for a switching power supply.
The method of action in a switching power supply is to have a high voltage
DC source ( rectified line voltage) connected to a capacitive charge circuit
with some method of swotching the high voltage DC on and off at a very fast
rate ( typically >100 KHz ) insuch a way that you can keep the capacitor
charged up above the minimum voltage, btu nto over charge it above the
maximum voltage. Typically MOSFET's are used for the switching portion.
When a MOSFET is turned off, current is very low, so very little power is
used, when it is on the voltage drop across it is very low, so again, very
little power is wasted. If the voltage across the capacitor is below
nominal voltage, the MOSFET switch is turned on,l allowing the high voltage
source to begin charging it, but only for a very brief period of time. When
the voltage level on the capacitor is at or above the nominal voltage, the
MOSFET's are turned off. The clock frequency determines how accurately the
circvuit can regulate the output voltage as well as how efficiently the
circuit switches. The input to the gate of the MOSFET's is a PWM ( Pulse
Width Modulated) signal generated by clocking a D flip flop whose input is
the output of a comparator circuit. The comparator cuircit compares the
output voltage of the power supply with a fixed reference, usually generated
with a zener diode.
"If a million people say a foolish thing, it is still a foolish thing."
Anatole France [Jacques Anatole Thibault] (1844-1924)
"Vincent Clerc" wrote in message
> I have to design a circuit that connects to the main power (220V AC) and
> transform it to 24V DC (3 amps) and 12V DC (3Amps too).
> Given the amps, a transformer based solution is to big and emits too much
> I thougth using a switching regulator but I don't know how to do that
> without transformer, lots of Amps).
> Does any one have a method (or schematic) to do that ??
> Any input or comment is welcome !!!
> Thanks a lot for your help by advance,
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