capactive power supply understand

[yassin.kraouch]

Hi, i designed a power supply like in the link below,
have question :
- if i connect a resistor in parallel with the 1000uF capacitor, what should be the power rating of the cap ( assume that the Vf of LEDs is 140V)
- if i disconnect one of the LEDs, what will be the voltage in open circuit ?
https://www.electroschematics.com/5678/capacitor-power-supply/

 

[FvM]

Presumed there's no zener diode, the capacitor voltage would be 140 respectively 322 V (230*1.4).

 

[yassin.kraouch]

Presumed there's no zener diode, the capacitor voltage would be 140 respectively 322 V (230*1.4).

Sorry in my first question i mean what should e the power rating of the resistor that we put in parallel with the filtering cap

 

[FvM]

"resistor in parallel with the filtering cap", why?

 

[yassin.kraouch]

In fact to discharge the 1000uf capacitor,

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i have another question, if i want to connect a transistor as a switch in series with the LEDs, do i have to take care about the voltage across it ? because you said we have 322 V open circuit,
and what is the requirment for this transistor ?

 

[FvM]

A LED power supply would usually omit the filter capacitor, or at least use a small one only, for component saving reasons. And otherwise you have problems with a switch, it discharges the excess capacitor voltage into the LEDs, transistor and LED pulse current ratings must be observed.

For the transistor specification you have to consider maximum voltage and current, possibly power if you don't manage to control the transistor clearly on or off. Refers to the question what's the control voltage or current source? A small 400V MOSFET might be teh best option.

 

[yassin.kraouch]

how can u calculate the discharge time of the capacitor?normally when i swictch off the light, current of capacitor wil flow throught LED and resistor in parallel,
can i use a capacitve power supply to supply a microcontroller (with zener), and the microcontroller will be connected to a gate of a MOSFET, and switch the light on or off ?

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A LED power supply would usually omit the filter capacitor, or at least use a small one only, for component saving reasons. And otherwise you have problems with a switch, it discharges the excess capacitor voltage into the LEDs, transistor and LED pulse current ratings must be observed.

.

the problem if we omit the filter cap we will have a flickering problem, and thats why this cap is recommended,

 

[yassin.kraouch]

Hi Fvm, have you idea on that ?