15v 500mA transformerless capacitive power supply circuit

[saileshdewangan]

I need 15v 500mA transformerless capacitive power supply circuit. What will be the components value for this. What is the formula to get value of capacitance. Can 12v relay work with 15v 100ma power supply.

Thanks

 

[BradtheRad]

Sure, transformer prices have gone up, up, up. But it's a tall order, for you to try to do away with them. They are still a much safer method for reducing mains AC to low voltage.

In case you are determined...

The capacitor will be about 15 uF. It will have bursts of 1A continually going back and forth through the thin foils. Be surprised if it doesn't heat up.

You need to put a resistor inline, to limit startup surge current. A 10W rating should do it.

 

[btbass]

There is an application note, AN954 at Microchip that covers this subject.

 

[saileshdewangan]

Which part of circuit is dangerous to touch. I know source part is dangerous. But I want to know that if output is 15v dc then Is it also dangerous to touch directly? Because when we use transformer for step down the voltage, we can touch it directly.

 

[BradtheRad]

With a transformer you only touch low voltage on the secondary side. That is a good thing, because you want to avoid touching mains AC which is high voltage.

If you go transformerless, you need to be cautious at all times, since it is possible for high voltage to reach all the wires.

Even with a capacitor intervening between you and mains, it can carry a fatal amount of current. Just 1 mA, and your life is in danger if you are grounded, or if your other hand touches the opposite mains wire.

Furthermore there is the possibility that the capacitor will overheat (and perhaps explode), if you are not careful in your design and construction.

Furthermore the capacitor needs to be non-polarized. It will not be easy to find one at the value needed, in order to transmit the amount of power you are asking.

Furthermore the capacitor needs to be rated for your mains ACV, plus a large safety margin. If your mains AC is 240V, then it peaks at 340V. Your capacitor should be rated 400V or so.

So, do not expect a single 15 uF electrolytic capacitor, with a 20V rating, to do the job.

The capacitive drop technique is better suited for current draw at the level of led's, say 20 mA.