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

From: Rolavine (rolavine_at_aol.com)
Date: 02/04/05 

Date: 04 Feb 2005 16:22:31 GMT

>Subject: Re: High AC voltage convert to DC voltage on driving IC
>From: Jamie jamie_5_not_valid_after_5_Please@charter.net
>Date: 2/4/05 11:07 AM Pacific Standard Time
>Message-id: <bHMMd.26542$VA7.14748@fe06.lga>
>
>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~
>yes, the 7812 is fine for your needs how ever, your input voltage must
>be dropped down using a transformer which is much safer in the long run.
> get a transformer that will give you a drop to around 16.. 24 Vac. then
>pass that to a bridge rectifier, Cap and then 7812.
> the 7812 is rated for 1 amp and needs a heat sink., in your case of
>200 mA, keeping the input voltage 3 volts (after filtering) above as a
>min should produce a smooth stable regulation for the 7812. if you keep
>the input voltage not much above the output voltage , the 7812
>will be in near saturation state and thus can be operated with out the
>heat sink at 200 mA . the lower resistance effects will produce much
>less heat.
>
Take a look at Power Integrations, at

http://www.powerint.com/PDFFiles/lnk302_304-306.pdf

This buck design works very well and is fairly cheap compared to the cost of
the big non polarized cap you would need for the cap coupled off line supply.
Just a guess, you would need at least 5uF for 200 ma output, maybe 10.

Rocky

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