I want to desgin a 5V,0.5A transformerless supply (minimum components) for mains 100-260Vrms, 45-65Hz?
I found several ways to do it which are mentioned below
1. resistive power supply - Microchip AN954
2. capacitive power supply - Microchip AN954
3. using dc-dc converter - using NCP1200 pwm
4. using capacitive voltage divider after rectifier
Which method should i adopt? (what about capacitive voltage divider)


closed as unclear what you're asking by Olin Lathrop, PeterJ, user17592, Matt Young, Chetan Bhargava Sep 5 '14 at 16:13

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    \$\begingroup\$ You specified the power supply output, but there is little to say without knowing what its input will be. How can you possibly imagine this information wouldn't be needed? \$\endgroup\$ – Olin Lathrop Sep 5 '14 at 12:15
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    \$\begingroup\$ There are major risks playing with mains electricity so if you are considering that don't: buy one, much safer. If you are not looking for a mains powered device we will need more details. \$\endgroup\$ – Warren Hill Sep 5 '14 at 12:18
  • \$\begingroup\$ @Olin Lathrop Universal mains \$\endgroup\$ – anishkumar Sep 6 '14 at 6:22
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    \$\begingroup\$ A 5V 2.5VA transformer would be quite small. Why do you absolutely need to avoid it? \$\endgroup\$ – Vladimir Cravero Sep 6 '14 at 16:23
  • \$\begingroup\$ @VladimirCravero i have very tight space constraints \$\endgroup\$ – anishkumar Sep 7 '14 at 12:04

Transformerless power supplies are not suited for 0.5A output at 5V even disregarding safety - cost and performance would be inferior.

Use a flyback supply like everyone else in the world.

You can consult Power Integrations data or look into self-oscillating converters with tiny transformers.


Firstly, a word of caution - a transformerless power supply is a recipe for death if it isn't built and insulated properly.

  1. This will burn many watts of power because if you require 0.5 amps on the output then 0.5 amps is drawn from the input AC and this has to flow thru the resistor divider PLUS you have to have slightly more input DC current to provide the zener with enough current to regulate the load voltage. Not a good idea for a 0.5 amp power supply. Imagine taking 220V AC and extracting 0.5 amps - that's a power dissipation of over 100 watts - find your self a big resistor or simply forget this topology.
  2. A capacitor dropper is much more likely to be able to provide 0.5 amps at the output because the AC impedance of the capacitor acts like an AC current regulator with very little power dissipation. 220V and 0.5 amps at 50 Hz implies a capacitor of value of greater than 7.23 \$\mu F\$. Probably best to choose a 10 \$\mu F\$ capacitor but you will still have to add a resistor in series with the capacitor to reduce inrush currents and this will dissipate a few watts.
  3. A dc-dc converter uses a transformer but you may not think it does because you don't understand the circuit - look-up flyback regulation. The bonus of a dc-dc converter is that the transformer can provide isolation from input to output DC voltage. To run on AC it needs a rectification and smoothing stage prior to the DC input.
  4. A capacitive dropper after the bridge is total nonsense - to get continued energy thru a capacitor it needs an AC component to the input voltage and as a bridge rectifier doesn't provide this it won't work.

I can't advise you on a non-isolating method to adopt because I'm not going to be liable should someone be electrocuted - use a transformer OR prove to me you are a competant engineer to undertake this task.


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