resistor used to convert 220V DC to 12V DC

i made a pwm inverter which takes a little portion of the generated

220V AC voltage as reference. the 220V AC is rectified with 1N4007 bridge and given to the ic using a 220K Ohm resistor. is it safe to use a resistor this way to step down 220V DC? the ic operates at 12v dc.
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** Desperate Groper Alert

** Only safe as long as human contact is impossible with the inverter or whatever it powers.

So no inputs or outputs.

Fully plastic encapsulated.

........ Phil

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Phil Allison

wtf r u talking about? did someone pee on your mother's face?

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For safety most ac line driven power supplies use a transformer to isolate the circuit from the line voltage. If you have any problem with your circuit it could put 220V on the input, output or the case. It's not a good idea, get a wall wart. Mike

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He's built an inverter -- it generates the 220V from some other supply (probably battery). It's not operating on mains supply, it *is* the mains supply.

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Greg Neill

yes that is true. the transformer output voltage is rectified using

1N4007 bridge. then given to optocoupler ic 4n35 using 220K ohm resistor.. then the output of 4n35 is given to pwm controller ic sg3524.
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Just ignore Phyllis. She's always on the rag, and she hates everything in the universe, but her favorite thing to hate is anything posted through Google.

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Michael A. Terrell

You are in Bangladesh, correct? Greetings.

About that PWM inverter you made - is it a square wave generator? If it's not, you might be making a bit more than 220 VDC after your bridge rectifier. More like 220V x sqrt(2).

That said... from V=IR, the current from a 220k resistor looks like it will be about 1 mA. Will your IC be happy with 1 mA?

heater. Can your resistor take 0.22 W? (Get a big one: rated at 1W, say.)

Good luck...


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I have dropped voltage with a capacitor in series with the line voltage. I only did it to drive the LED in an SSR but it should work for an IC if you used the right regulation on the far end. Certainly a transformer will work.

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If the circuit is:

220VDC ----- [R] ----- [opto] ---- GND

then be aware that you're only getting about 1 mA to the opto. If you want to use that to regulate it, I'd do something to reduce the amount of influence the opto characteristics have on the quality of the feedback.

How about just a voltage divider? 220K with, say 5K in series will give you about 5V for about 220V in. Buffer that, and then drive your opto.

But, since it's already an inverter, why do you need the isolation in the first place? Or is it a second level of isolation?

What is it you're trying to accomplish?

Cheers! Rich

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Rich Grise

i have got the answer from another site. the voltage goes to a optocoupler ic and it has a LED and photo transistor inside it. so it has no direct connection to the circuit so it cant harm the circuit. thanks for your help.

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