How supply power for changing number of LED?

D

-dave-

Guest
A friend wants to use some LEDs in something she is designing. She
may use 5 LEDs or she may use 20 LEDs. It may even turn out she uses
50 LEDs.

Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?
 
"-dave-" <-invalid@mail.com> wrote in message
news:Xns9CC1EA7471FD35D4AM2@feeder.eternal-september.org...
A friend wants to use some LEDs in something she is designing. She
may use 5 LEDs or she may use 20 LEDs. It may even turn out she uses
50 LEDs.

Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?
Click to expand...
A constant current power supply is one solution. Any number of series LEDs
(within reason, of course) would be excited within the desired current and
it should work fine.
 
"-dave-" <-invalid@mail.com> writes:
Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?
Click to expand...
The brightness of an LED depends on the current through it; the
voltage across it varies little over its useful range. so, if you
want the brightness to remain constant, you need a constant *current*
source, not a constant *voltage* source.

In the usual circuit, you have a constant voltage and a resistor.
Since the LED has a "fixed" voltage across it, the resistor does too,
and you can use the value of the resistor to determine the current
through the circuit. However, if you change the number of LEDs, you
change the voltage across the resistor, and thus the current.

If you want the current to remain the same regardless of how many LEDs
there are, what you need is a constant current power supply. You can
use an LM317 with a single 62 ohm resistor as a 20 mA constant current
supply, but you need to get one beefy enough to handle the *minimum*
number of LEDS (largest power dissipated by the regulator) and the
*maximum* (highest needed input voltage) at the same time. 50 LEDs
would require about 100 volts, but 20 could be doable... that's about
40v input, and a big LM317 can handle up to 57v input.

So, example... 20 LEDs at 2v, 20 mA = 40v output. Add 2v for the
regulator itself, and 1.25v across the 62 ohm resistor, say 44v input.
Now, 5 LEDs is only 10v, possibly as low as 9v. That's about 34v
across the regulator, or about 0.7 watts In a TO-220 package, it'll
run 50 C/W or 35 degrees C hotter than ambient, well under the 125C
limit.

Once she finalizes the design and you know how many LEDs are in the
string, you can use the same circuit but with a smaller input voltage
to reduce heat loss. If she needs more than 20 LEDs, just make two or
more circuits. I've got a project with 11 LM317s in it running 11
strings of 9 LEDs each off a 37v supply, works great.

Alternately, if the input voltage is fixed already, figure out how
many LEDs you can put in series before you run out of voltage drop,
and put strings of those in parallel with each other (each string has
its own suitable current-limiting resistor). For example, if you have
a 12v supply and need 18 LEDs, you could have three strings of five
LEDs (10v drop) each with a 100 ohm resistor, plus a string of
(example) 3 LEDs (6v drop) with a 300 ohm resistor. This is probably
the easiest solution if you don't *have* to have all the LEDs in
series.
 
A friend wants to use some LEDs in something she is designing. She
may use 5 LEDs or she may use 20 LEDs. It may even turn out she uses
50 LEDs.

Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?
Click to expand...
You haven't stated one critical fact: are the LEDs in series or in
parallel? If in parallel, is there a single dropping resistor, or one
per LED?

The critical facts are this: LEDs have a specific forward voltage
drop (depends strongly on the LED color, and somewhat less strongly on
the amount of current flowing through the LED). LED brightness is a
direct function of the amount of current flow. If you put LEDs in
series, their forward voltage drops add together.

Therefore: the trick is to choose the value of the dropping resistor,
so that the voltage across the resistor (which will be
TotalSupplyVoltage minus the LEDForwardVoltageDrop) results in the
amount of current you need to get the amount of brightness you want.

Several scenarios exist:

- LEDs are in series, with one dropping resistor. If you add an extra
LED to the string, you'll increase the total LED-string voltage
drop, decrease the voltage across the resistor by the same amount,
and you'll need to decrease the value of the resistor in order to
maintain the desired current.

Trying to drive one or two LEDs from a high voltage is wasteful of
power... most of the voltage is across the dropping resistor, and
most of the power is dissipated in the resistor as heat.

Trying to drive a long string of LEDs from a voltage which is just
barely above their voltage drop is efficient, but tricky. Small
variations in the supply voltage can result in a proportionally-
large change in voltage across the small-value dropping resistor,
resulting in a large change in current and brightness.

- LEDs are in parallel, with one dropping resistor per LED. No,
don't change the resistor value... just add one new LED and one new
resistor in series with it.

- LEDs are in parallel, and then hooked to a single dropping
resistor. You'd need to change (reduce) the value of the resistor
when adding LEDs. This is generally *not* a good hookup strategy,
because the LEDs would have to be exactly matched to get equal
current sharing, and they usually aren't. Some will usually have a
slightly smaller voltage drop than others, and will "hog" the
current and be brighter. In extreme cases, one can hog most of the
current, get VERY bright for a brief moment, and then burn out...
leaving the next one to hog the current, flash, phut... in this
mode they can be known as NEDs (noise-emitting diodes).

- LEDs are fed in series-parallel - e.g. they might be arranged in
strings of 5 or 6 LEDs, with each string being fed through a single
dropping resistor. This is probably the most robust and efficient
and flexible arrangement - if your friend wants more LEDs, she
would just add as many strings of 5 (each with its own dropping
resistor) as the design requires.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
 
dplatt@radagast.org (Dave Platt) writes:
the number of LEDs in series

You haven't stated one critical fact: are the LEDs in series or in
parallel?
Click to expand...
Uh, series...
 
"Charles" <charlesschuler@comcast.net> wrote in
news:hdi5br$lvr$1@news.eternal-september.org:

A friend wants to use some LEDs in something she is designing. She
may use 5 LEDs or she may use 20 LEDs. It may even turn out she uses
50 LEDs.

Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?
Click to expand...
The resistance is fairly critical but constant current control takes care of
that by changing it as a result of maintaining current. Another way to make
it less critical is to use hyper-bright LED's at very low currents,they
should work ok at less than 5 mA and likely withstand up to 45 mA without
serious degrading of life expectancy.

Whatever route you choose, start with basic calculations: Find out the Vf
(forward voltage rating) for your LED's (which will be connected in series
for this calculation) and don't mix LED types if you want this to be easy.

Rs=(VS-Vf)/If

Rs is series resistance, VS is DC supply voltage. Vf is total Vf of string,
and If is forward current in string. Calculate power dissipated in a series
resistor, or a regulator, by (VS-Vf)*If, to avoid a fire risk. Try the LM317
regulator as a constant current source (see its datasheet for this). LM317's
are cheap, and have safety limiting built in so you get good protection.

Lastly, if you're using a lot of LED's the voltage grows too high to be safe.
It depends what you're making but if this is to be safe around children as a
toy or display, you really need low voltage and be sure that currents are
low, so maybe use a 15V supply, buy a few of the smaller 500 mA version LM317
regulators and keep total LED Vf within 12V. This way you also get energy
efficiency.

With small strings of 6 or so LED's in each, the control might be easy enough
with simple resistors if you have a power supply that won't sag its output
voltage much as you add extra strings in parallel as load. You might find a
handful of 500 mA LM317's on eBay or elsewhere, cheaply enough that it pays
to buy them, and build a MUCH cheaper supply than you'd have to if you wanted
it to have fine control of high currents! As you also get good protection for
each LED string it's a better choice than series resistors if safety is
critical.
 
In article <xnfx8jjh06.fsf@delorie.com>, DJ Delorie <dj@delorie.com> wrote:
dplatt@radagast.org (Dave Platt) writes:
the number of LEDs in series

You haven't stated one critical fact: are the LEDs in series or in
parallel?

Uh, series...
Click to expand...
50 LED's = high volts.

A constant current supply say 10 ma. is not going to do a lot
of damage, but what if it fails.

CAUTION

You may need strings like 24 volts maximum or about 14-16 Led's max in series.

If you gave the application, you could be better informed.

greg
 
"-dave-" <-invalid@mail.com> wrote in message
news:Xns9CC1EA7471FD35D4AM2@feeder.eternal-september.org...
A friend wants to use some LEDs in something she is designing. She
may use 5 LEDs or she may use 20 LEDs. It may even turn out she uses
50 LEDs.

Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?
Click to expand...
Ans: Yes. The resistor sets the current depending on the voltage drop and
the particular LED arrangement. Are you asking a design question: i.e., how
to pick the resistor value for a particular unchanging combination of LEDs?
Or, do you want to change the number of LEDs on or off at any given time in
some short of an array like in a sign? In the second case, each LED is
switched independently and requires it's own resistor and transistor or FET
switch. It is possible to arrange the LEDs in a matrix, one possibility
being where the number of resistors is the square root of the number of
LEDs; e.g., 7 resistors for 49 LEDs. The value of the resistors does not
change regardless of the pattern of lit and un-lit LEDs. The switching is
more complex requiring time division, however, another job for a PIC
processor.
 
Lostgallifreyan wrote:
"Charles" <charlesschuler@comcast.net> wrote in
news:hdi5br$lvr$1@news.eternal-september.org:

A friend wants to use some LEDs in something she is designing. She
may use 5 LEDs or she may use 20 LEDs. It may even turn out she uses
50 LEDs.

Would the resistence of the dropping resistor need to be changed if
the number of LEDs in series is changed (for example from 5 to 20)
or is the resistence not critical?


The resistance is fairly critical but constant current control takes care of
that by changing it as a result of maintaining current.
Click to expand...
What a great idea - now when she puts 50 in series she can drive it all
with a very dangerous 100 volt or more power supply!


Another way to make
it less critical is to use hyper-bright LED's at very low currents,they
should work ok at less than 5 mA and likely withstand up to 45 mA without
serious degrading of life expectancy.
Click to expand...
45 ma? as long as her life expectancy for the LEDs was in the 10's of hours

Whatever route you choose, start with basic calculations: Find out the Vf
(forward voltage rating) for your LED's (which will be connected in series
for this calculation) and don't mix LED types if you want this to be easy.

Rs=(VS-Vf)/If

Rs is series resistance, VS is DC supply voltage. Vf is total Vf of string,
and If is forward current in string. Calculate power dissipated in a series
resistor, or a regulator, by (VS-Vf)*If, to avoid a fire risk. Try the LM317
regulator as a constant current source (see its datasheet for this). LM317's
are cheap, and have safety limiting built in so you get good protection.

Lastly, if you're using a lot of LED's the voltage grows too high to be safe.
It depends what you're making but if this is to be safe around children as a
toy or display, you really need low voltage and be sure that currents are
low, so maybe use a 15V supply, buy a few of the smaller 500 mA version LM317
regulators and keep total LED Vf within 12V. This way you also get energy
efficiency.
Click to expand...
??? crap.
snipped the rest I can't be bothered.
 
David Eather <eather@tpg.com.au> wrote in news:v-
mdnaMwisf_UWDXnZ2dnUVZ_t6dnZ2d@supernews.com:

??? crap.
snipped the rest I can't be bothered.
Click to expand...
Snivelling little asswipe! If you'd read the rest you'd KNOW how gormless
your first point was! Troll away, if it makes you feel better. Worked for me.
 
David Eather <eather@tpg.com.au> wrote in news:v-
mdnaMwisf_UWDXnZ2dnUVZ_t6dnZ2d@supernews.com:

45 ma? as long as her life expectancy for the LEDs was in the 10's of hours
Click to expand...
Rubbish. Should still be in thousands if it's not too hot. Heat has a far
stronger effect on lifetime, so if you ignore that (which you evidently did),
you could run them in an enclosed space at recommended 20 mA and they'd still
die young(ish). Most makers over the last 20 years specified a max current of
50 mA, recommended 20. 45 mA is NOT out of line with maker's intentions, no
matter what you might think about that.
 
David Eather <eather@tpg.com.au> wrote in news:v-
mdnaMwisf_UWDXnZ2dnUVZ_t6dnZ2d@supernews.com:

45 ma? as long as her life expectancy for the LEDs was in the 10's of hours
Click to expand...
You also ignore that I was stating how wide a margin you get, I was NOT
suggesting that they be driven at the high end of that margin. Idiot!
 
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