Re: High AC voltage convert to DC voltage on driving IC

From: Tim Hubberstey (bogus_at_bogusname.com)
Date: 02/04/05 

Date: Fri, 04 Feb 2005 02:00:09 GMT

Electronic Swear wrote:

> Currently I am designing a circuit for controlling motor.
> And I need to convert a 230Vac to 12Vdc for supply low voltage IC. In
> the past, because of low power consumption for IC, I just use some
> resistors and transistor for this convertion.
> ________ ___ C E
> 230Vac o-----| |---|_R_|--+----+------\ /-----o 12V
> output
> | diode | | | ---
> | bridge | --- | ___ B
> | | --- +-|_R_|--+
> o-----|________| | |
> GND |
> 12v Zener
> |
> |
> GND
>
>
> Now, I have changed some components and it need more current to drive
> it. And the past design is not enough to do so. Now, I want to know
> any similiar simply circuit can convert 230Vac to 12V dc with arround
> 200mA current.
>
> If I replace the transistor to the TO-220 7812, can it supply enough
> output for me to use? Thanks~

First, please go find a sturdy wall and recite the following while
banging your head on the wall:

"It is a very, very, very, very bad idea for a hobbyist to use a
non-isolated power supply".

The point you have labelled as "GND" actually has about 110 VRMS on it
when plugged in. There is also no current limiting of any kind so it is
a lethal combination.

Go to your nearest electronics store and buy a "wall wort" (wall plug-in
supply) to use as a starting point. You can either get a DC output that
already has the diodes and filter inside (very easy to find) or, if you
want the design challenge, get an AC output unit and build your own.
Given your apparent level of experience, I suggest you get a DC wort.

To you get a regulated output from a 7812, you need at least 14 VDC on
the input when under load. The more input voltage you have available,
the hotter the 7812 will get. Try to get a 15 VDC wort but anything up
to 24 V should work, as long as you put a big enough heat sink on the
7812. With 24 V in and 200 mA out, the 7812 will be dumping 2.4 watts
and will require a decent heat sink. Google for "thermal resistance" if
you want to calculate what kind of heat sink you'll need. 

-- 
Tim Hubberstey, P.Eng. . . . . . Hardware/Software Consulting Engineer
Marmot Engineering . . . . . . .  VHDL, ASICs, FPGAs, embedded systems
Vancouver, BC, Canada  . . . . . . . . . . . http://www.marmot-eng.com
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