If I connect 8 leds @ 1.6v vf onto a power supply producing 12vdc, I won't have to use a resistor, right? Theory being that the 8 led's will use .8 volts more than the ps can provide. They won't burn as bright, but they won't burn up the ps, they won't destroy themselves, and they won't require resistors?
Wrong. Why get rid of the resistors? Are they expensive or something? LEDs "like" to be driven by a constant current source, the cheapest approximation being a voltage source with a resistor.
With the right voltage, a resistor free LED powering is possible. With the wrong voltage, thermal drift into POOF!
D from BC
To elaborate, they behave somewhat like a zener with a broad knee. Personally, I think a voltage regulated supply to LEDs is not a good idea, it's not good design practice and would have to be pre-set to batches of leds to ensure at least consistent brightness across the product line.
Like the first reply asked, are resistors expensive or something?
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D from BC
Under certain circumstances, even I leave out current limiting resistors. There is enough inherent resistance on the supply side of whatever is driving the resistor to 'internally' limit the LED current.
There is nothing wrong with this.
However, the OP was *way* too broad in his description of "a power supply producing 12vdc" and "8 leds @ 1.6v vf".
The supply is very nice round number, so implies either something that is regulated, or, at least has a lowish internal impedance.
"1.6v Vf" sounds like something that came direct from the spec sheets. In real life, it means little for this purpose.
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The "1.6V" will be the voltage drop across the LED at a particular current - often 20mA - and a particular temperature - usually 25C.
The forward voltage of a LED - as with any other diode - is roughly proportional to the logarithm of the current, and decreases by about
2mV/C as the junction temperature rises. At high currents the resistance through the LED can become significant.-- Bill Sloman, Nijmegen
A perfect LED with a perfect voltage source will either burn-out or stays dark.
What it makes possible in reality to run a LED with a voltage source without a resistor is just the combined internal resistance of the LED and the voltage source. This acts as a small current source.
- Henry
-- www.ehydra.dyndns.info
That's using one model. There are other, more sophisticated and accurate, models which can be employed. See, for example, PDF page 5 here:
LEDs behave similarly to other diodes...
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
A power supply labeled "12vdc" could actually be 13V, or something else. And whatever it is, can change appreciably when it is drawing current, or has had a few minutes to warm up. And, diode current changes quite dramatically with small changes in voltage.
There is a small chance you could get away with what you are proposing, but it's not a very good idea. If you do, at least measure the actual power supply voltage, at 20 mA load, and then measure the actual LED current after installing them (and be willing to sacrifice the LED's for the sake of experimentation.)
Regards,
Mark
p.s. They do get away without adding a resistor in coin-cell operated LED flashlights. This is because the series resistance of the cells provides a fair amount of current regulation.
On a sunny day (Wed, 26 Dec 2007 19:43:19 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :
They wont burn at all, as 8 x 1.6 = 12.6, LEDs are diodes, current will only flow once you equal or exceed the forward voltage drops.
On a sunny day (Wed, 26 Dec 2007 21:07:34 -0800) it happened D from BC wrote in :
Exactly, I tried it with taxes once, and that turned out very expensive. OTOH one guy here declared the value of his house wrong, comma sort of thing, and got 3 million Euro back from the taxes, automatic. Years later they figured out his error, and asked it back. He sued them, and the judge decided he did not have to pay it back. (Tax office should have spotted the mistake). Was in the paper last month. Now I have this idea ;-)
Spehro Pefhany schrieb:
That is for experts. They know to add a resistor.
- Henry
-- www.ehydra.dyndns.info
Not necessarily. See, for example, US 6313589
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Spehro Pefhany schrieb:
That patent is a temperature compensation circuit. Not bad idea.
- Henry
-- www.ehydra.dyndns.info
"Constant" light output.
...Jim Thompson
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Ofcourse I agree with those that say its a current driven device.. Just funny .. I bought a lightchain with 480 diodes for less than(equal 20$) a few days ago,as the supermarkets have turned down the prices . It works well conected to a transformer that is included in the box.. but I think the trafo will have an airgab ,to give a high R out from trafo. Also ,they use AC output,and LED's in anti parallel. Buying LED's with socket this way is actually cheap.. and the transformer is worth the price too.:) Alex from Sonderborg in Denmark.
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