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jpb
09-01-2008, 09:14 PM
I need 24v DC to run some LED lights via a DC SSR and don't have a suitable power supply but have plenty of 24VAC

If I run the AC through a bridge rectifier will that be good enough for LEDs? or should I stick a smoothing capaciter behind the rectifier?

thanks in advance
Jon

magott
09-01-2008, 10:19 PM
I have been running a lot of converted led light strings quite happily through just a full wave bridge rectifier and a current limiting resistor with no problems . If its a half wave you will get a hsimmering/flickering effect. Not sure if you really need the smoothing capacitor. Maybe someone whose used one could comment.

moose
09-01-2008, 11:46 PM
24Vac threw a bridge rectifier will output 45Vdc depending on load.
Most 12Vdc batt chargers are half wave, you could install a bridge rectifier to make 24Vdc. Seeing as you have lots of 24 Vac your choices are Voltage limit resistor "which is directly tied to load and won't work if you load is constantly changing" or Some form of voltage control circuit which could get quite warm depending on load.

jpb
09-03-2008, 06:00 AM
Moose are you sure that 24vac through a bridge rectifier will give me 45vdc? Maybe I have my terms wrong, I am talking about a full wave rectifier using 4 diodes, commonly drawn like a diamond.

Now for a secondary question about the limiting resistor. If the light string was originally across 24vdc and I am just going to source my power from somewhere else do I still need the resistor? In my small understanding, if the lights were originally across 24v they must already have the correct resistance. But I have been known to be wrong before.

Jon

P. Short
09-03-2008, 05:08 PM
Under full (rated) load the peak output voltage will be 1.4 (sqrt(2)) times greater, or 34V (minus the diode drops). Under light load the peak output voltage will be quite a bit higher, perhaps as much as 40V, but I would be surprised to see it as high as 45V. That is using the conventional four-diode full-wave bridge rectifier that you described.

jpb
09-03-2008, 11:59 PM
Thanks all,

I cracked open the original controller ( I wanted to avoid this) and found it was pretty much just a full wave rectifier with a .1 uF cap across the AC side. The smart bit was one of those blob ICs driving things labled PCR606J which I think are Thyristers or something.

Anyway, I have decided to run the controller off my 24VAC and and then run from rectified side of the conroller into my SSRs and then to the light string. Fingers crossed.

Also, my apologies to Moose, I have gone back to my first year apprenticeship notes and found that a bridge rectifier will indeen give 1.4 times the AC voltage as DC and .62 times the current. In my defence that was 25 years ago that I was supposed to have learned it.

Jon