# High AC voltage convert to DC voltage on driving IC

• posted

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~

• posted

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Thanks,
- Win```
• posted

Actually, I have changed to use 7812 on supply regulated voltage for low voltage IC. The circuit is as following:

________ ___ ______ 230Vac o-----| |---|_R_|--+-------+--| 7812 |-----o 12V | diode | | + | --+--- | + | bridge | --- | | --- | | --- zener | --- o-----|________|--+ | | | | | GND GND GND GND GND

Is it better to do so? However, the R will be in very high WATT and what can I to do so? Sorry, I am not familiar with such voltage convertion circuit. :(

• posted

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.
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Tim Hubberstey, P.Eng. . . . . . Hardware/Software Consulting Engineer
Marmot Engineering . . . . . . .  VHDL, ASICs, FPGAs, embedded systems```
• posted

How can you tell? I can remember doing this kind of thing when I was too young to know better so I know it could be real. I do _hate_ to waste time on trolls, though.

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Tim Hubberstey, P.Eng. . . . . . Hardware/Software Consulting Engineer
Marmot Engineering . . . . . . .  VHDL, ASICs, FPGAs, embedded systems```
• posted

Some ideas ... Unless you're interested in investigating your sensory and muscular response to electrical stimulus you need to put an optoisolator at the output and better still connect it to a triac or a relay... make sure the closing time of your relay is less then that of your heart beat to avoid premature death. Place a resistor and a fuse there too. Further precautions would be ... At the 240V output I'd place a fuse 100mA or whatever you need as current and a fusible resistance 200ohms.

Other ideas ... to avoid heat dissipation is to use a capacitor of the suppressor type then a resistor before the diode bridge as power dissipated by a capacitor is less then a resistor of the same impedance voltage and current being out of phase. I'd connect a capacitor then a regulator or some zeners to get a low ac voltage in that point go for something like a total of 16volts .

maybe this is clearer

fuse fuse resistor live o-o_/ \o--|___|-------| .-. resistor | | 1k | | '-' |+ Capacitor === +230v input /-\ \. to diode |------------- bridge | /´ V z zener | | z A zener neutral---------------------|

Well the part after the diode bridge is simpler... add another capacitor above 40uF then in parallel a Resistor plus a 12 V zener across which you'll have your load connection. do your calculations .... I = E/ 1/2*pi* 50(or60) *C . IMO you can do without the transistor. If you need current above 50mA .... your capacitors become very big and so expensive. Not much point then of doing without a transformer.

BTW Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user.

• posted

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.
• posted

• posted

On 3 Feb 2005 23:24:25 -0800, swear snipped-for-privacy@hotmail.com (Electronic Swear) wroth:

___ ________ ___ ______ 230Vac o--- |_C_|--| |---|_R_|--+-------+---| 7812 |-----o 12V | diode | | + | --+--- | + | bridge | --- | | --- | | --- zener | --- o-----|________|--+ | | | | | GND GND GND GND GND

Add a capacitor between the 230Vac and the diode bridge. That will drop the voltage without getting hot. Use a non-polarized (not electrolytic) capacitor. The voltage rating should be 400 volts and the value probably needs to be at least 1 microfarad. The R value can then be reduced so that it doesn't get too hot.

Jim

• posted

I agree with the previous poster. It is very dangerous to use power from the lines this way. You cold kill youself or so unsuspecting person that happens across your device.

I would HIGHLY recommend using the Wall arts idea followed by a regultor or puttin a dedicated transformer between the 230VAC and your curcuit. A fuse would also be highly recommended so you don't burn the place down if things go wrong.

In the current design below you will most certainly blow the 7812. When there is no load on the output, the input will likely rise above its input voltage rating and kill the device.

If you need help sizing a transformer or wallwart for the application, thet good folks here would be glad to help of you give voltage output current requirements for the power supply.

• posted

Take a look at Power Integrations, at