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Thread ID: 129256	2013-02-13 08:47:00	Very strange LED issue... almost supernatural!?	The Error Guy (14052)	Press F1

Post ID	Timestamp	Content	User
1327886	2013-02-13 20:45:00	LEDs take very little current to light dimly, you probably have some AC potential on your body from somewhere too small to feel but enough to create a tiny current flow from the PSU to your fingers. Small amounts of electrical current flow all over the place constantly and we don't notice. If we didn't conduct electricity we wouldn't get shocks.	dugimodo (138)
1327887	2013-02-14 01:16:00	+1 (or is that 2?) for Agent and Dugi.	linw (53)
1327888	2013-02-14 01:25:00	The light flickers, I'm guessing at 50Hz, it's probably to do with the power supply since the battery doesn't do it. You won't see a 50Hz flicker, it is too fast for the human eye to resolve, so it would have to be 25 Hz (half mains ferquency) but you don't say that your power supply output is AC or DC. You can run LEDs off ac if wired in alternate-polarity pairs (each conducts for one half cycle) with an appropriate series current limiting resistor and I'm picking that your dim and flickering illumination lighting is simply finger conduction/LED electrification via an AC supply. The LED rectifies the 50 Hz (being a diode of course) and lights up for the appropriate polarity 25Hz half cycle. Ergo, it flickers. Sounds to me like you need a beefy DC supply and a current limiting resistor or voltage regulator. The LED currents will be in parallel, so the demand will be the current value for a single LED multiplied by the number of LEDs you wish to activate, plus at least a 10% margin to avoid running the transformer at its maximum current rating. A little knowledge is a dangerous thing, and you may destroy all your LEDs if you are not careful. This is the key to your problem................Read my second sentence The only way they will both light is in parallel. Cheers Billy 8-{) Some may now be damaged so you should test each diode with a suitable battery and current limiting resistor. Your whole project will need some means to limit the current or the LEDs will probably be munted by overcurrent/overheating.	Billy T (70)
1327889	2013-02-14 04:30:00	Cheers for all your responses. I was simply using the 3.2v Laser PSU since it was near by and I wanted to quickly test each cluster before putting it together, these LED's were from ebay :p so I wanted to make sure there were no dud/dim ones involved. I ordered a bunch of IR LED's from eBay once, not sure if they were varying brightness's or if they were very inaccurate in their wavelength (hence appeared brighter with our IR cam with a 850nm bandpass filter) but they weren't very good. These one are much better :) So, I'll get the actual 12v supply I was planning on using, put in the resistors on each cluster and it should all work. Just wondering, since my maths and physics has gone out the window... If I make 10x clusters in parallel, with 5 LED's in each cluster with the following specs: LED's: FWD I: 30mA FWD V: 3.2 Supply V: 12v I would need to use 30ohm resistors? The total set-up should dissapate ~500mA, PSU is 1A so plenty of spare juice. I think thats about right! Thank's for all the help clarification. I need to use my maths more or else I forget it :rolleyes: some days I feel rather daft.	The Error Guy (14052)
1327890	2013-02-14 05:06:00	3.2v * 5 = 16v - A 12v supply won't cut it if you want five LEDs in each series string. With a forward drop of 3.2volts, and a 12v supply, the most you will get away with is three in series for each string. In that case you'd be dropping 9.6 volts across the 3 LEDs, leaving 2.4v you need to drop across their series resistor. At 30mA current (I expect that is maximum rating? You should probably go for 20mA) you'd need a resistor of 80 Ohms. LED resistor value calculation is; (VSupply - VForward) / LED Current, in your case: (12 - 9.6) / 0.03 Amps = 80 Ohms, or for a safer current of 20mA, you'd get 120 Ohms. (In series, they will all have the same current through them so it does not multiply, the voltage increases instead) Then, You should also find out how much power the resistor will need to dissipate. This is easier, Square the current and multiply by the resistance. So that's (0.02A ^ 2) * 120 Ohms = 0.048 Watts or 48mW. Even a little 1/8th Watt (125mW) resistor will do the job. Each string of 3 LEDs will take 20mA or 30mA each depending on the resistor you choose, so just multiply that by the number of strings you want to get the total current. Note that putting several in series is a problem if one fails. Obviously if one died open circuit the whole string goes out. But if one fails shorted, the other two will get an increased current and may also fail soon after. Of course in such a cheap circuit this is hardly a concern.	Agent_24 (57)
1327891	2013-02-14 06:20:00	Well I'm on stunning form for maths today aren't I! Completely decimated what little faith I had in my electrical skills! Right, so if I use the 12v supply, I can go with a 3x LED cluster and 120 ohm resistors. If I step it up to 24v, I'm hoping I can find a 24v wall wart somewhere! I think I threw them out, I should be able to get 10 lots of 5 LED's in series using 470 ohm resistors drawing 200mA. This is of course using 20mA as my fwd current. You are correct in that 30mA is the max rated fwd current. Thanks Agent, re learning everything from now on. Not going to trust myself again!!	The Error Guy (14052)
1327892	2013-02-14 08:24:00	If it's any consolation, I'm not doing any better following all this. My inherent notions are all wrong. Thanks to all for the wake-up. :waughh:	Paul.Cov (425)
1327893	2013-02-14 09:08:00	Nope, you are not there yet. If you have a 12 volt supply, and each LED has a forward voltage of 3.2 volts, then the maximum number of LEDs per series-string will be limited to three (3 x 3.2 = 9.6v), because 4 x 3.2 is 12.8 volts and on load the suply output may drop below thatand the string would extinguish. This means you will probably need 33 series-clusters of 3, using 99 LEDs. However, alot will depend on the voltage output, current capacity and filtering of your chosen power supply. Per the above, each string will require a resistor of suitable value to drop 2.4 volts at 30mA, which would be an 80 ohm resistor at 0.5 watt. The closest preferred value is 82 ohms, which would be fine. This would also protect against catastrophic failure should one LED in a string go short circuit, and some may already have been damaged by your earlier experiments so premature failure is a possibility. The output of the average DC plug pack is not regulated, which makes it very likely that there would be significant voltage sag, which in turn would reduce the light output, so a supply rated for 2 amps output would be advisable. I'd be inclined to add some filter capacitance to the output of the supply as well, because if if it is unfiltered, the ripple will translate into light-level fluctuations at 100Hz which could produce interesting effects, or maybe not, as the case may be. There is no thermal inertia in LEDs so they will pulse their light output at 100Hz if there is no smoothing. The resistors will dissipate 2.4 watts across the array, which will hardly get warm, but filtering may keep them cooler as well. That is a rough breakdown and I don't have the information provided to you, but it is a reasonable estimation. You can mock up a couple of 3-diode strings and see what light output level you get, but remember that diode polarity is important. If digital scanning is involved, I'd be trying to provide a constant light output level, so the extra filtering may be required on the power supply output. Cheers Billy 8-{)	Billy T (70)
1327894	2013-02-14 23:59:00	I'd be tempted to use a 3 pin regulator and wire them all in parallel.	dugimodo (138)
1327895	2013-02-15 02:27:00	I'd be tempted to use a 3 pin regulator and wire them all in parallel. That would be ok too, but then he'd need a operating supply current of at least 3 amps, and allowing that running the supply source at 100% capacity is not a good idea, we are getting well out of plug-pack territory. The format you suggest would need a 5 amp rated ac supply of higher voltage (to give headroom for the regulator), plus rectifier (full wave), filter caps etc, heatsink for the regulator, fuse protection, and some fairly substantial wiring between LEDs, then when all put together, that starts to get a bit complicated for a novice, which TEG does seem to be. Cheers Billy 8-{)	Billy T (70)
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