Kragen Auto Parts has these white led replacement lamps for turn and brake lights for about 7 bucks. Claims super intensity for off road use only. I want to run them off a transformer rated 24v 40va. Will this work? Would like to know before I hand over 15 bucks. For illuminating inside a sandblast cabinet. Thanks Dan
Yeah, it'll work great - for a very short time. At a guess, I'd say you'll have a wonderful lighting system for about... Oh... 5 minutes, tops.
Chances are beyond high that they're designed for a 12-14 volt DC supply. Which means that your nearly double-rated-voltage supply (and AC, no less...) should cook them off quite nicely in short order. Precisely HOW short order is hard to say without knowing specific details of the units, but it isn't a question of "will they survive?", but "How long before the combination of double-the-rated-voltage flipping from forward to reverse, plus Ohm's law cause them to go "Fzzzt" and let all the magic smoke out?" As I said, my bet is on "not very long at all". Definitely under an hour, probably under 5 minutes, possibly under 10 seconds. (Side note: It's not an absolute certainty, but the very real possibility of an outright explosion at the moment of powerup with that oversized supply does exist)
If you're going to go that route, you want a proper 12-14 volt *DC* supply, preferably regulated. I'd expect that these things want to draw somewhere in the neighborhood of 20-30 ma per physical LED, perhaps a shade more - 50-ish ma would be about the top end, I'd estimate, but the new white diodes seem to prefer lower amps than the traditional reds, greens, and ambers. Without knowing *EXACTLY* which diode they use, though, it's impossible to do more than give you some ballpark figures, since each maker's product is specced a bit different than the next one.
Get a supply rated for those kind of numbers, and you've at least got a fighting chance of keeping them lit for more than a few minutes before they go up in smoke.
Don and Richard K are right that 24VAC would wreck a
12V DC LED unit in short order. Two units in series with a diode and the 24VAC transformer would last longer, but the 34V peak from 24V RMS is excessive.
Three units in series would work fine, hooked to the DC side of a full-wave bridge, like the first item at
It sounds like you [D.P.] want to use 2 units (~$7 each, ~$15 total) and to do that with the 24VAC transformer and a full-wave bridge you would need to add a series resistor sized to drop 10 volts (ie, 34V - 24V) at whatever current the LED units draw. For example, if they draw .2A, R = 10V / 0.2A = 50 ohms. For DC, power = I*V = 2W, so you would need a power resistor a couple of cubic cm in size, although a 12V light bulb of that wattage would work ok. (Eg, if your LED lights drew .3A, use a 3W 12V bulb, etc)
Probably you will find it easier to follow Don's advice about using a 12VDC power supply. Eg, click the "transformers" link on the allelectronics page above and pick out a $5 12VDC wall wart, or get one for half a dollar at Goodwill/Salvation Army/etc.
Thanks guys. I'll hunt thru my collection of little power supplies for a more suitable device. I looked at the bulbs again and there is a resistor in the shell with the 6 led's I suppose to limit the power from the auto circuitry.
Why don't you just buy a set of under-the-cabinet low voltage lights from Home Depot or Lowes? Complete kit, pretty cheap. Usually halogen, but they may have LED versions. I'd prefer halogens - brighter, better light distribution.
Actually, the RMS average is 17V for 24V half-wave rectified. But the average is irrelevant here -- what matters is peak current draw, which would occur at peak voltage, which is
24*sqrt(2) or about 34V. This is too high even with 2 12V devices in series; they'd be running about 50% over-current for a few milliseconds at a time, and that would probably shorten the LED lives from decades down to mere years.
Don'cha love the term "actually" as in "and now the truth"? It's such a nice sorta Mr. Peabody sort of preamble.
Actually, "RMS" and "average" are different measures. Actually, the average value of a half-wave rectifed sinewave is .318 * peak value
-- more accurately, peak_value/pi. Peak voltage from a 24VRMS xfmr is 33.9 volts, average after halfwave rectification is 10.79 volts minus 1 diode drop. RMS is relevant to the dropping resistor, but not to the LED because the LED is essentially a constant-voltage device. It only cares about current. Peak LED current will be considerably higher than rated, but avg current will be slightly less than at 12 volts DC. Most larger-die LED's will tolerate that even down at 60 Hz.
If the power rating on the resistor is marginal, that could be a problem -- I didn't think of that until later.
I agree that a regulated 12 to 14.4 volt supply would be a safer bet.