Replacement light source for a fibre optic Santa - Page 1
Forum section: electronics / projects

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: December 19, 2013, 08:02:56 pm
content: Hi everybody.What's the worst thing that can happen at Xmas with your Xmas lights.Well the halogen bulb in the fibre optic Santa has died, and i can't find a quick replacement anywhereSo i wanted to ask which would be the best and safest optionIt's powered by a 12v ac 10va transformer.I have a box full of 1W white leds ( 3.2v 300mA )Option 1.1W led in series with a 7ohm 1w resistor in series with a 1n4001.Option 2.Full wave rectify the 12v ac, use a smoothing cap, resistor and then the led.Option 3Full wave rectify the 12vc ac, smooth and use a buck converter to control the led current.Option 4Use a series capacitor of approx 470uf ( i think i would need two back to back ) series 1n4001 and the led.Or another option I’ve missed, apart from buy another unobtainable halogen.What would you go for as a quick fixps original was 12v/7wMany thanks for your time

author: Kjelt
author_rank: Super Contributor
timestamp: December 19, 2013, 08:08:02 pm
content: Rectify the AC, large elco, LM317 connected as currentsource for the exact current for the specific leds and probably you can put 3 in series if they are white.If you need more leds, two identical currentsources for two strings.Halogen is 25lm/W so the original probably had around 150 to 175 Lumens. How many Lm/W do your leds have and is the color correct or will Santa be mucht brighter white then previous years?Pictures of the old bulb, why is it unobtainable?Also look in a lighting store, Philips makes a lot of led replacements for halogen bulbs, might be a standard replacement you can use 1:1.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: December 19, 2013, 11:15:10 pm
content: Hi,The led's i bought last year and the sellers long gone, but, looking around; on average they are rated between 90 and 120 lm.I have one running in diode/resistor/led mode and as expected its only getting half the cycles so flickers, but when in Santa the colour changing disk masks the flickering ( it's fibre optic, with a motor for the disk and the rocking motion ( he's on a sleigh) ).What i want to do, because it's going to be enclosed in a plastic/cloth toy, is keep the heat down as much as possible.I know the halogen was pumping out 5w, so the item must meet the fire requirements, but the lower the better for piece of mind.For the lm317( spit ) if i full wave rectify and smooth I’ll get ( 1.41x12 ) 17v (16.9v), take away the 3.2 for the led and a bit lost across the rectifier at 300mA will result in approx 4 watts dissipated in the 317. And 1w in the led for a total of 5w, the original bulb, but what a waste of 4w.If i single wave rectify with no smoothing cap i get ( 12x0.45 ) 5.4v, with 3.2 across the led, that gives 0.66w for the resistor ( if i put a diode across the led to protect the reverse breakdown voltage, which is 5v. Then the resistor dissipates 0.66w lighting the led and 1.62w on the opposite cycle. So the total power will be 1.66w either way). If I single wave rectify and smooth with a cap i'll get the same for the 317 ( 17v ), but my current drops down to 0.28xIac which will be about 336mA.There's no fan in Santa, the led is pumping out 1W and for a quick see if it works; on one of those small flat star led heat sinks which gets quite hot.That's why i was thinking MC3406 in buck mode ( got lots of em ), but it's more in BOM, and the series cap as some use in 120/240v ac droppers. But i'd need around 470uF to get an impedance of 7ohm.Am i right in thinking two 1000uF polarized, wired back to back with V above the ac voltageSanta's coat is a lot brighter than last year because of the dispersion of the led compared to the directional halogen

author: Zero999
author_rank: Super Contributor
timestamp: December 20, 2013, 02:42:02 pm
content: How about connecting LEDs in reverse parallel so they light on alternate cycles?

author: Kjelt
author_rank: Super Contributor
timestamp: December 21, 2013, 09:31:44 am
content: Quote from: Mr Smiley on December 19, 2013, 11:15:10 pmThat is why my thoughts were about three or four leds in series. If you use three led's you have aprox. 9.6V (going up to 10+V over lifetime) at 300mA you dissipate around 17-9.6 = 7.4*.3 = 2,2W. Four leds only 1,2W loss, very acceptable for a cheap 1:1 solution I think.I wonder if the 17V is what you really get, if you load it it usually drops some, since the 1,4 is the top of the voltage and that lasts only a part of a ms, so if you do not use to large elco's it probably is lower end voltage.If you really want to do it nice you scrap the units (rip transformer etc) and put a cheap mains (dimmable) led driver 300mA in there and a small heatsink for the leds. And if you run the led's at say half the current they don't get that extremely hot, still for 24/7 operation check the temperatures over the day and make sure they stay < 70 degrees C. Probably much faster, cheaper, reliable then a DIY buck.