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I want to drive a led with the mains. Using a resistance in series works, but some power is lost in the resistor. Would it be more efficient if i use a capacitor in series with the led? Any other option for driving the led with the less power comsumption lost in the driver itself? I dont want to use commercial drivers by the way.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Never done it, but I think the cap trick does basically work, and that seems to be what is said here, but it looks like you also need a zener and a reverse diode/led to deal with transients and the reverse cycle:
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
johnf wrote ...
Just use two leds back to back and a capacitor of the right value to get the current you need 1/2x pi x f x c will get the capacitance you need
... and it will send the led domes flying across the room the second time you turn it on. You also need a series resistor to limit the inrush current that the capacitor can take if it's switched on at peak mains voltage. Consult the LED data sheet for the largest surge current it can stand, and choose the resistor accordingly. You might be surprised at how large the resistor has to be to limit the current to 100s of mA, or less.
I've used an electrolyitc capacitor across my unipolar LED string, partly because I don't like the 100Hz flicker and partly because it will absorb a large transient inrush without over-volting the LEDS. Even a cooking grade silicon bridge rectifier diodes will handle 10s of amps for one half-cycle, which means the series resistor can be a few ohms.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
This should work too. I think it will work with just one diode, too. It has a minimum of components, has a 'bleeder' resistor to prevent the electrolytic from staying charged (unless the LED fails 'open circuit'), and to limit the current to the LED, and will only draw sufficient power to keep the electrolytic 'topped up', I think..
If it won't work, maybe someone could explain why?
EDIT: Maybe it should have a second 'current limiting' resistor on the mains side?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
What you've got there Ash is a rectifier for the mains, so 170 or 340 volts on the cap depending on which side of the pond you live. Then all the power gets dissipated in the resistor.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Dr. Slack wrote ...
What you've got there Ash is a rectifier for the mains, so 170 or 340 volts on the cap depending on which side of the pond you live. Then all the power gets dissipated in the resistor.
OK, thanks.
So adding a shunt capacitor (or two?) would limit the power drawn from the mains, presumably?
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Driving a led with mains.
