# transformerless power supply help

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#### devil6600

##### Full Member level 5
can anyone advice if this circuit can be used for ac-dc conversion? i needed 30v and 3amp output.

i have used bridge rectifier circuit with voltage divider, after voltage divider i will be using 2 1000uF 50V capacitors.

#### WimRFP

This circuit will not give you 30V at 3A.

You will fry the rectifiers becaue of too high current and too high voltage, the output isn't regulated and the heat generation is massive (even under no load condition).

If you want a linear solution, go the traditional way with a mains transformer, rectifier, capacitor and a regulator circuit.

#### FoxyRick

I guess you have recently learned about voltage dividers?

You need to think about the circuit as a whole, not just one effect of it (dividing a voltage).

You have calculated the resistors correctly for 220V (well done for that) but not accounted for the fact that 220Vrms mains is not the same as 220V DC.

To convert AC (sine wave) to its peak voltage, multiply it by 1.41. So, 220V mains is about 310V peak. That's what you will get as DC after the bridge rectifier.

So, your resistances will not give 30V.

In any case, think now about just those resistors. You have 310V across 87 ohms. From Ohm's law, that will cause a current of 3.6A though the resistors. Or, work out the power from P=V*V/R = 310*310/87 = 1100 watts. Wow, you've just made a 1KW heater! Put a fan behind it and warm your room up - for about 1/10th of a second before it sets itself on fire.

See what I mean about looking at what happens to everything in the circuit?

Using a voltage divider is never a good way to power anything significant.

I hope that helps your understanding.

devil6600 and Din0

Points: 2

Points: 2