Capacitive power supply help request

[v8dave]

I have a lamp module from Everspring for controlling my lights in my home. They work great as long as the voltage is stable and above 210Vac. The unit is rated on the label for 230Vac

I live in Indonesia and have a home power monitor and often see the voltage drop as low as 202Vac and this is where the units start to switch off and back on every few seconds. The speed of this gets faster as the voltage becomes lower. At around 208Vac is switches of and back on every 3-5 seconds.

I've opened one unit up and found that the supply is based on a capacitor drop type with a bridge rectifier, Zener diode and reservoir capacitor on the DC side. The reservoir capacitor is a 220uF 35V electrolytic.

I don't know much about this type of supply other than what I have read but I do know from research that they are not very good with varying input voltages and this is likely why I am seeing the effect of switching off and on as the output drops below the reset point of the microcontroller on the board.

I want to ask if anyone knows if I can simply put in a bigger reservoir capacitor, say something like 470uF 35V and if this will give better storage of the chopped DC wave form from the bridge rectifier? My concern is that this larger value may appear initially as a larger load on initial switch on of the unit although once power is on, it will always be on and the controlled output is done via the home automation system.

Is there any issues you can see in changing this cap to the 470uF or is the design probably optimised for the 220uF one currently fitted?

I have attached the underside of the PCB and you can see the bridge D and Zener Z1. R1 and R2 make up the resistor on the AC input with the capacitor underneath this. It's a high voltage X2 type rated for 275V

[Ian.M]

Not a good idea.  Undeer 210V, the module's capacitive dropper power supply circuit is supplying less current than it needs.  This is causing the voltage to drop and the MCU to reset.   Increasing the reservoir cap will simply cause it to reset at a slower rate.

The only possible fix would be to increase the class X2 cap for more current, but you'd probably need a higher wattage Zener and might have to up-rate other components as well.

[Seekonk]

It probably wouldn't take much of a capacitor to bridge with C23.  Z1 zener is probably big enough if some extra heat sinking is added.  I have a collection of old consumer electronics.  I like the tin plated shielding to use for heat sinks.  Easily cuts with a scissors and a surprising small amount is quite effective at lowering heat.  Just solder a piece onto the diode tabs.

[v8dave]

It probably wouldn't take much of a capacitor to bridge with C23.  Z1 zener is probably big enough if some extra heat sinking is added.  I have a collection of old consumer electronics.  I like the tin plated shielding to use for heat sinks.  Easily cuts with a scissors and a surprising small amount is quite effective at lowering heat.  Just solder a piece onto the diode tabs.

There is no tabs to solder onto. The Zener is SMD package as you can see in the photograph. It looks quite a high current type but I can't find any data on it. The code is TRR LV with which a search comes up empty.

Also, you mention C23? C2 is the high voltage cap and C7 is the reservoir cap.

[Seekonk]

Actually it looks like C22 if it follows convention.  PCB copper isn't very good at getting rid of heat.  Attach HS to solder blob on each side.

[v8dave]

Actually it looks like C22 if it follows convention.  PCB copper isn't very good at getting rid of heat.  Attach HS to solder blob on each side.

I think C22 and the transistor next to it, Q2 are something to do with zero crossing detection. The X2 rated cap is on the other side of the board. It's much larger. This is the one Ian.M is referring to, to be changed.