Selection of some components

Submitted by abr on Tue, 05/18/2021 - 09:48

I am new to this website and I don't know if it is the right place or section.  I will try again in "General" this time.

If it is not right could someone relocate it to the right place.  Thanks.

I am also new to electronic so my requests might sound strange to some of you.


Night light project. (My first project.)

I need some help to select some components.

I have a 240 VAC/50 Hz voltage supplying a Bridge Rectifier (4x1N4007) with an output of 24VDC for 6 blue LED in series.

I have to step down the voltage from 240VAC to 24VDC.  I don’t know how drop the voltage from 240VAC to 24VAC.  It is where I need some help.

From what I read, to step down the voltage, I need a Cx capacitor (?? µf 400V) and one resistor R1 (??Ω) in parallel  to discharge the capacitor and another R2  (??Ω) resistor on the other side.

I know the Bridge Rectifiers change the current from AC to DC and I will need another C1 capacitor to smooth the ripples and a resistor of R3 (120 Ω) before my 6 LED.

I could use the values already available on different websites but I would like know the step by step for selecting the Cx Capacitor and the 2 resistors before the Bridge Rectifier.

Thank you.

Please use a SMPS withisolation if possible. If not use a transformer less power supply. What is current rating for 24VAC?

  Joined February 12, 2018      696
Monday at 02:11 PM

Thank you for your time.

My power supply is 240VinAC/ 50Hz 1Amp. What would be the Vin AC at the Bridge Rectifier after the current goes through Cx, R1 and R2. The Voltage after the Bridge Rectifier should be the same, minus 2V because of the diodes. The Voltage and Current are to supply 6 blue LED rated at 20mA.

I would like to know how to work out and select Cx, R1 and R2.

Thanks again


  Joined May 17, 2021      4
Monday at 11:11 AM

The voltage will not be equal due to the RMS conversoin by the diode. Use proper capaictive power supply with the 120mA typical output. But 24V 120mA is a very high output? Why you need 24V? those LEDs needs to be connected on parallel with 120mA of current (6x20mA = 120mA). What is the forward voltage of the LEDs?

  Joined February 12, 2018      696
Monday at 02:11 PM