use of a resistor to drive l.e.d. from 30 volts supply

[rajaram04]

Hello sir
i made a transformer less power supply to drive it from direct a.c. source so as to drive some l.e.d. lights for normal use & i measured the voltage as 30 volts with no load connected between the output terminals , now please tell me how to drive a single white or green l.e.d. from this supply ? what parameters of a resistor must be there ? , please give all the specifications to obtain a full light here .
thanks

 

[karthick1987]

depending on the current you want (or according to spec) you need a resistor of value

(30-3.5)/I for white or
(30-2.1)/i for green

Use I in Amps for R in Ohms

hope it helps
cheers

 

[IanP]

Hello sir
i made a transformer less power supply to drive it from direct a.c. source so as to drive some l.e.d. lights for normal use & i measured the voltage as 30 volts with no load connected between the output terminals , now please tell me how to drive a single white or green l.e.d. from this supply ? what parameters of a resistor must be there ? , please give all the specifications to obtain a full light here .
thanks

It will be safer for you if you presented here the whole circuit, as the no-load voltage of a transformer-less ps can be very misleading.
Compare your circuit with the attached one: it has Zener across the output that sets the level of the output oltage ...
:wink:
IanP

https://electroschematics.com/5678/capacitor-power-supply/

 

[thebadtall]

the idea of direct connection to mains is very bad, in some cases can be hazardous. Just wonder why all the electronics use transformers.

anyway, maybe you could use a variable regulator (lm317) and drive the leds without resistor, just drop voltage to 3v