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I have some appliances which I am trying to convert from 120 Vac, 60 Hz to 220 Vac, 50 Hz. In particular, it is an LED light. I think I figured out how to do it. I modeled the power supply in the simple simulator on falstad.com.

enter image description here

Basically the power supply is separated into 2 sections.

  • The "left" side in this picture is the supply for 16 LEDs. With a Zener diode with 30 V breakdown voltage, and a 100 ohm current limiting resistor.
  • The "right" side in this picture is the supply for 8 LEDs. With a Zener diode with 15 V breakdown voltage, and a 30 ohm current limiting resistor.

In the simulation, the current across the LEDs is around 50 - 60 mA. Since the LEDs seem to be in series, I think I should maintain that current limit when changing this supply to 220 V.

In the simulations, when I apply 220 Vac without making any changes, the current across the LEDs jumps to 100 - 120 mA. This would probably burn them out.

My initial idea was to increase the resistance values of the current limiting resistors. But that way, they would just get very hot (much more power wasted in the resistors according to the simulation).

There are two big initial film capacitors in the supply. In the picture they are modeled as 1 uF. But in the original 120 Vac version, they are 1.5 uF. When I change these capacitors to 1 uF and apply 220 Vac in the simulation, then the current across the LEDs seem to be around 50- 60 mA again.

I already ordered 1 uF film capacitors. But just to be sure, do you think this would be a good solution for converting this power supply to 220 Vac?

Link to simulation.

Transistor
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hmmmniek
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    Very poor schematic layout. Perhaps redrawing the schematic in a more conventional way will get you more help. – Unimportant Dec 24 '21 at 12:42
  • There is a partially obscured diode in each circuit that looks offensive the way its drawn. Having said that, the Zeners in each serve the same purpose so think about what I'm saying. To make comparisons (as you are requesting), it makes it more straightforward to have the schematic more acceptably drawn and, with superfluous components removed. – Andy aka Dec 24 '21 at 13:45
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    FYI https://electronics.stackexchange.com/q/28251/2191 – RedGrittyBrick Dec 24 '21 at 15:46
  • Thanks for actually pointing me to resources to help me improve my question @RedGrittyBrick :) – hmmmniek Dec 24 '21 at 19:58
  • You've already ordered film capacitors? Are they rated for mains connection? You do realise that mains has spikes of up to 1500 V don't you? That's why I specified X rated caps, which are specified to handle these spikes without failing short circuit. – Neil_UK Dec 24 '21 at 20:18
  • I did not know it could have spikes up to 1500V. I ordered [these](https://www.amazon.de/gp/product/B07RTGJDP7/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 630V105). Visually they look similar to the capacitors currently on the circuit. Besides that, the circuit is also fused. – hmmmniek Dec 24 '21 at 22:07

1 Answers1

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With a 'capacitor dropper' AC power supply, reducing the value of the dropper capacitance will reduce the current supplied to the load, equivalently it will increase the voltage you need to supply to get the same load current.

I don't understand what the zener diodes are doing, unless they are there to protect the capacitors downstream of the rectifier should the LEDs go open circuit. They are not needed to protect the LEDs, the 200 uF will absorb any initial surge through the droppers, they're not needed to regulate the LED current, the input voltage, frequency and dropper value does that, and they don't regulate the LED voltage, the LEDs do that given a current. I'd ditch the zeners and their adjacent 15 ohm resistors (keep the input 15 ohm resistors). I don't see what the 100m resistor is doing either.

Appalling schematic by the way, not least because it has no reference designators (which 15 ohm resistor do I mean, the one next to the zener, ugh). I'd tend to redraw it like this, without the extraneous components. Do use an X-rated capacitor for C3. Positive to the top, negative to the bottom, flow from left to right.

schematic

simulate this circuit – Schematic created using CircuitLab

Neil_UK
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  • Thanks @Neil_UK , for showing me what a good circuit diagram would look like. Based on what you are saying, it seems like my simulations are at least close to the truth. – hmmmniek Dec 24 '21 at 20:07