115 volt DC power supply for LED holiday lights

Nobody likes flickery LED christmas lights! You can put a full wave bridge rectifier and a capacitor on them but then you’re then supplying a lot more power than they are designed for since that will be about 160 volts DC give or take and the duty cycle will be 100% rather than the 50% duty cycle of regular AC. They will likely be nice and bright for a short time before they burn out. Years ago I experimented with rectifying the output from a regular dimmer switch and this seemed to work pretty good and let you reduce the output to a reasonable level. But not all dimmer switches wanted to come on turned down so low and if they got bumped or reset by an interested child it could burn out your lights. I tried to duplicate that setup recently for a friend who’s flickery LED lights I had brought to his attention and which have bugged him since (hi Bill) and I just couldn’t get it to work anymore. I have no idea what I was doing differently. I blew up a whole box of dimmer switches which just kept burning up on me. Turns out that the rectifier I was using was damaged and was a hard short... So that explains that. But in the meantime I was doing other research and decided that the proper way to do it with less danger to life and limb was to start without about 80 volts AC and rectify and smooth that out.

80 volt transformers seem difficult to find but I did find 40 volt .25 amp ones inexpensively and so 2 of them with their secondaries in series and the primaries in parallel yield 80 volts. Which when rectified yields 115v DC which is perfect.

.25 amps at 80 volts output might mean that this can supply 20 watts of power into strings of LED lights, which is plenty. This is a very dangerous test setup obviously, don’t do it this way. Important considerations are that the 2 transformers should be carefully wired the same polarity to the mains so that the output is in the same phase so that it adds up to 80 volts instead of unadding to 0 volts... I wouldn’t go more than 2 of these, I don’t know what the internal insulation is rated for. 80 volts doesn’t seem to be a problem but it will break down eventually if you start adding them up. That little 47uf 450v cap smoothes out the voltage very nicely. At least for this test string. The setup this will eventually go on is larger and so I might need to increase that value. I will also be adding a .25 amp fuse so that this doesn’t overheat. I don’t think it can fail in a spectacular fashion, the transformers will just heat up and eventually short themselves out and blow the fuse. I will also add a high value bleeder resister to the cap so that any voltage left in it when you turn it off is bled off safely. Not that cap will store a whole lot of power, but LED’s don’t conduct much at all once you get below their threshold and so 50 or 60 volts might stay on the cap for some time after turning it off.

In generally messing with mains voltage and high voltage is a real bad idea. I’ve got this plugged through an isolation transformer and a ground fault sensor just in case, and though it doesn’t look it from there the cord is clamped down so that it can’t be pulled off the desk and such. I’ll post more pictures when I’ve got it mounted in a nice case.

If you decide to do something like this you’ll also have to test it with the specific strings of lights you’re using. Obviously it won’t work if there is a controller or sequencer box attached. Some strings also wire some of the LEDs one way and some another way so that they are on in opposite swings of the AC. If you run those kind of strings on DC then only half the LED’s will light. There may be other dangers and issues I haven’t though of too...

UPDATE: the device as delivered:

working great, well within specs I think and making everyone happy.