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Circuit for driving 36 LED string on 240VAC?

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240v ac led circuits

I am interested in driving a string of 36 White LEDs (Forward Voltage: 3.2VDC @ 20mA) directly on 240VAC, 50Hz supply. I donnot want o have a tranformer as the circuit would be very heavy and bulky. Can any one recommend a circuit fo this application, I need to use this instead of the conventional flourescent tube for lighting.
 

36 LED String on 240VAC

36*3.2= 115 Volts
240*√2= 336 volts (peak value)
240 input voltage produce 336 volt peak value but we can use RMS value to select correct resistor.
whit this approximation:
use one diode bridge to 240 volt and use one resistor about (240-115)/20mA= 6K = 6.2 Kohm in input path.
resistor power dissipation will be 125*125/6.2K=2.5Watts
(use minimum 5W for resistor)
also you can use one resistor and one capacitor (in series) in place of resistor.
in this case power dissipation in resistor is lower.

Important note:
this formulas are not precise for this application,
because voltage on resistor isnt sinosoidal, but
for first try we can use these.
 

Re: 36 LED String on 240VAC

Resistor(s) is one of methods of droping voltage(s) .. There is, however, another way of achieving the same gol without generating heat: capacitor's impedance (reactance) .. see picture below ..
For more details visit:
**broken link removed**
or
http://www.marcspages.co.uk/tech/6103.htm

Regards,
IanP
 

36 LED String on 240VAC

Having so many leds in series in not really very good design practice. Really it would be better with all in parallell each with its own series resistor worked out to give the correct current to each led, you could then adjust these as required to even out the brightness. Even though you are looking for a less bulky solution, I would really not advise using mains directly. A plug in SMPSU would be not much bigger than a mains plug, and much safer.
 

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