I'm using 3 strings of LED's each of which has its own AC/DC adapter wall wart that delivers 3.6V /800 mAh. I was hoping I could splice the LED lead wires together and find a 3.6V 2400 mAh wall wart that could drive all 3 instead of having to plug in 3 separate wall warts. (Note that the LED strings would branch from the power feed and would NOT be connected head to toe.)
Unfortunately, 3.6V wall warts aren't very common and certainly nothing that can deliver that much current.
I measured the current draw for a single strand and it is about 750 mAh so I really do need to supply that much current.
2 questions
Are the current ratings of wall warts conservative? Can one rated at 2.2 mAh deliver 2.25 mAh (=3 * 750 mAh) safely?
If not, can I treat wall warts like batteries and wire their outputs together? I could plug the individual wall warts into an out-of-sight power strip and build a wiring harness that would merge their outputs into a single feed wire. This would make a much neater installation.
Are the LED supplies constant voltage or constant current? For LEDs, they may very well be constant current supplies, so combining them would be potentially problematic.
There are 120 LED's attached to very thin copper wire. They are almost invisible when they are off and quite bright when on. A single string is powered by a 3.6V 800 mAh wall wart.
jimbarstow:
2) If not, can I treat wall warts like batteries and wire their outputs together?
No.
You can't do it with batteries either, and for the same reason. If one of the batteries discharges faster than the others then they're not putting out equal voltages and they'll start feeding power into each other.
jimbarstow:
Are the current ratings of wall warts conservative?
Try it, see if it gets hot...
jimbarstow:
Unfortunately, 3.6V wall warts aren't very common
How did you measure 3.6V? If it's a LED driver then it should be current regulated, not voltage regulated.
3.6V is the forward voltage of many LEDs, they might have just printed "3.6V" on them because they're required to put a voltage on there, not because they're 3.6V power supplies.
Bottom line: This isn't a simple situation. Don't do it.
I've got a string of 120 tiny white led's wired in parallel that are run off a 3.6V wall wart. The
entire setup was $20 so there is obviously no expensive led driver involved. If there are
resistors anywhere they must be embedded in the tiny led itself or a single one in the wall wart.
I've notice that the led's at the end are slightly dimmer than the ones at the beginning.
I read (on wikipedia, of course) that if the input voltage matches the forward voltage, then a resistor
is not necessary. There is a significant variation in voltage drop across led's, however, so this seems
to be risky when using multiple led's.
So is this led string just cheap or is there some truth to the wikipedia statement?
There is no well defined "forward voltage" for any diode, rather the entire curve of current (Y-axis) versus applied voltage (X-axis) is an exponential curve unique to every diode. What people usually call the forward voltage is an average value obtained at some current (often about 20 mA), averaged over a large number of devices.
Usually LEDs are powered by a source with higher voltage than any (reasonable) point on the curve, so a resistor is necessary to limit the diode current to a safe or desired value.
If you set the voltage across an LED to be any particular safe value, then the current through the LED will be determined entirely by the device characteristics. I don't know what is going on with your LED string, but it sounds like there is significant voltage drop within the inter-LED wiring itself. For high voltage series-connected LED strings, I've seen nonlinear resistors of some sort in parallel with each diode, which can take over if one or more diodes burn out.
The entire setup was $20 so there is obviously no expensive led driver involved.
That does not follow, no expensive led driver but what makes you think they are expensive.
There is a significant variation in voltage drop across led's
What do you call significant?
The point is, I DON'T think it is expensive. The whole string of 120 lights cost $20. The appeal is that they
are string on very fine copper wires and the less are tiny. (Each one bridges the wires and is enclosed in a tiny
bead of clear plastic.) During daylight, they are virtually invisible. At night, they look very cool.
I'm trying to combine 3 strings and hoping I can power them from a single source. In order to do that, I need
to figure out exactly how it works. The entire string draws 800 mA from a 3.6 V wall wart. (I measured it.)
I figured the easiest thing would be to splice the strings together in parallel and use a 3.6V 2400 mAh source. Unfortunately, I can't find one.
The ugly approach is to keep each string separate, use 3 wall warts and a power strip. I'm hoping to do something a little more elegant.
I figured the easiest thing would be to splice the strings together in parallel and use a 3.6V 2400 mAh source.
That would only work if the current limiting device was in the LED string and not in the power supply as it very well might be.
The entire string draws 800 mA from a 3.6 V wall wart. (I measured it.)
OK exactly what and how did you do your measuring?
If you measured 3.6V out of the supply then it is likely that current limiting is in the LED strip and the LEDs are wired in parallel.
However, with a cheap LED string it is likely that they have not been designed properly and will die after a short time.
I need to figure out exactly how it works.
You do indeed, only when you know how it works can you hope to be able to understand how to modify it.
I figured the easiest thing would be to splice the strings together in parallel and use a 3.6V 2400 mAh source.
That would only work if the current limiting device was in the LED string and not in the power supply as it very well might be.
The entire string draws 800 mA from a 3.6 V wall wart. (I measured it.)
OK exactly what and how did you do your measuring?
If you measured 3.6V out of the supply then it is likely that current limiting is in the LED strip and the LEDs are wired in parallel.
However, with a cheap LED string it is likely that they have not been designed properly and will die after a short time.
I need to figure out exactly how it works.
You do indeed, only when you know how it works can you hope to be able to understand how to modify it.
The wall wart is rated for 800 mah and went I measured the amperage with my meter, it measured 800 mah. (Seemed a little suspicious since things in life are rarely that regular.) the voltage before the first led is 3.87V. If there is a resistor with each led, it must be tiny and too small to see.
Could there be a single resistor inside the wall wart? I can't see how this would protect 120 separate led's but I have seen wiring diagrams that hook up parallel led's this way. Then again, I've seen lots of other stuff that is wrong, too.
Wall warts aren't rated in mAh, only batteries are, but I know you meant mA... It sounds like voltage drop is what is dimming your LEDs. How long and what size is the wiring between the wall wart and the string connection? Also, there will be losses inside the adapter especially if you are using it at its maximum rating. You should get a bigger supply and make the low voltage wiring as short as possible.
Another trick is to connect BOTH ends of the string to the supply.
Sorry that is not precise enough.
Do you mean the voltage across the two wires from the supply is 3.87V or the voltage across the first LED is 3.87V.
If there is a resistor with each led, it must be tiny and too small to see.
Yes you can get LEDs with built in resistors, normally you can see this as a small black square inside the LED at the side through the plastic.
I measured the voltage across the supply wires about 6" from the wall wart. The supply wires are about 6' long from the wall wart to the copper wire the LED's are attached to.
I looked at an LED with a 10x jeweler's loupe and there is something I suppose could be a resistor. I don't have any experience with such a thing so I'm not sure what it is.
The description claims 20 year lifetime on the LED. If they don't use any resistors, I don't see how they could claim this. Of, course, you can also by a watch that has a Rolex logo for $10...
'20 year guarantee' is easy to print if you don't intend to back it up. They probably have verbiage in the guarantee saying you have to provide the original packaging and receipt and pay $30 shipping and handling to receive your replacement. And file an engineers report verifying that the power supply isn't the problem because that's not covered...
if the voltage output from the supply is 3.87V and it's powering 120 LEDs, I can guarantee that they are in parallel, since if they were in series the voltage drop across each LED would be about 30 mV. They probably look like the (crude) diagram below.
As others have indicated, LEDs are current driven devices with non-linear voltage/current characteristics. That said, you can treat them as devices with a constant voltage drop if you stay well within the operating parameters and they don't overheat, etc. That's exactly what we do all the time with the standard 3mm Red LED we run off a 5V pin with a 220? resistor. My guess is that this is exactly what the manufacturer did with your LEDs.
3.8V _______________________
| | |
power R R R
| | |
supply LED LED LED
| | |
0 ------------------------
You could build your own power supply (not a wall-wall wart). You'd typically want to use a switching voltage regulator for anything more than about 1 Amp.
For example, the [u]LM2576[/u] can put-out up to 3A. There is no 3.6V version but there is an "adjustable" version that you can "program" to 3.6V with a pair of resistors.
You'd need to build the switching regulator circuit and also build a basic unregulated "linear" power supply with a transformer, a bridge diode, and a capacitor to convert AC power to isolated low-voltage DC to power the regulator. 6.3VAC transformers are common, and I'd look for one that puts-out about 2 Amps. (Switching power supplies are almost 100% efficient, so you can get more current out than you put-in, with less voltage out than you put-in).
That quit simply is a nonsense. There is a lot of math in determining MTBF ( Mean Time Between failures ) and to imply that every device can last 20 years is just a lie.
Sleepydoc - probably has it right in that diagram, from the way the photo looks it looks like simple parallel connections, therefore yes you probably can have a single power supply at three times the current and wire the strings in parallel.
If it's just a bunch of resistors/LED pairs in parallel you really don't need that good of a power supply; even if it's got a fair amount of ripple, the LEDs will handle it just fine. They may appear slightly dimmer, but if they're flickering slightly at 120 Hz, your eye won't notice much.
If your leds operate at a specific temperature/heatsinked, then driving them at a specific voltage would be safe, I don't believe "time" is a concern.
Eg a 10watt 4.2v rated led would be happy at a constant voltage and not die, but if the temp rises you risk destroying it... I have ran leds for days on end without any form of current regulation.
Mike although correct, is also wrong if you can keep the temperature at a constant which difficult to do with leds under a watt or so.
Mike although correct, is also wrong if you can keep the temperature at a constant which difficult to do with leds under a watt or so.
No Mike is also correct and correct.
It is totally impractical to keep a power LED at a constant temperature.
Even if you could keep a constant temperature, you can not stop the forward voltage changing with the age of the device.
However this is nothing to do with this thread as the LEDs involved are not power LEDs, they are just normal low current LEDs with built in resistors so no external resistors, or other form of current control, are required.