leds in series
rr
Posts: 63
·i know that this may be a simple question however... if i wire 4 leds in series i know that i should multiply the voltage by the amount of ledsie. (4 leds @3.0 volts in series· would be 12 volts) but do i also want to multiply the current by the amount of leds ie.( 4 leds @20ma would be 80ma) i thought that it stayed at 20 ma. but the leds are very dim when i do this(20ma.) and seem to be right on (bright)with 80ma. i dont want to burn up my leds. thanks for the help...·
Comments
Most LEDs prefer about 1.5 - 1.6V at 20mA.
Have you measured the current?
And the voltage?
When the circuit was connected, did you measure the voltage across one LED, what was the voltage?
What is the total supply voltage?
What value series resistor are you using?
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Chris Savage
Parallax Tech Support
csavage@parallax.com
red:
Forward Voltage (V) : 1.8 ~ 2.2·
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
yellow:
Forward Voltage (V) : 1.8 ~ 2.2
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
i have four in series with along with a resistor in series with them. i am powering them with 12 volts figuring that they can drop the extra voltage....··· At 80 ma. they are really bright and they dont seem to be heating up too much but i have only had them on for·a couple of minutes or so. i am guessing that by your responses i am drastically cutting down there life span but they are definitely not burning out right away... at 30 ma they are bright but not as bright as 80 ma.
·
And yes, you're cutting down their lifespan...
(How much depends upon how far ut of recommended range you go.)
4 LED in series with a voltage drop of ~2V means a total voltage drop of 8V, supplying the string with 12V means there is 4V overhead, the current through the·LEDs is set by this excess voltage being disipated by your current limiting resistor. If you are using a 420Ω resistor, the current flowing through it is I=V/R or 4/420= 9.5 mA. I just can't see how you are getting near 30mA from this configuration. If you reverse calculate the voltage drop across the resistor (V=IR) or .03x420=12.6V this is larger than your power supply, something isn't correct, are you sure your using a 420Ω resistor?
Do not operate the LEDs at 80mA, this is beyond thier max rating and you will severly limit the life of them. For maximum lifetime, run them at 30mA or less, or you can PWM them at 75mA 50% duty cycle. Because of the way human eyes work, the PWMed LEDs will appear brighter.
Post Edited (Paul Baker) : 4/18/2005 11:49:10 AM GMT
http://members.misty.com/don/ledx.html
·· With the information you have supplied, the closest standard value resistor would be 150 ohms.· That should give you almost 27mA.
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Chris Savage
Parallax Tech Support
csavage@parallax.com
Is that what you are doing?
If you put 4 LEDs in series using your specifications of 1.8 volts each, you will have 7.2 volts dropped across the LEDs, which leaves only 5.1 volts across the resistor. (12.3 - 7.2)
Using Ohm's Law for the resistor value gives the necessary value of 175 ohms (R = V / I :·· 5.1V / 29.1 mA)
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Name: Bruce Clemens
Work:· Clemensb@otc.edu
Good Stuff on my Blog: http://theDeadBug.journalspace.com
You break open the circuit someplace, does not matter where.....insert your meter.
The meter must be set to current, mA in your case.
It is very common for the current fuse to be blown especially if your meter is used by others in a shop.
Don't worry if you meter reads "minus" current. Ignore the minus sign (which indicates the meter leads need to be reversed).
Post Edited (KenM) : 4/19/2005 2:15:36 PM GMT
but really all my leds were seeing was about 10ma.
i have always put resistors before the leds(on the positive side)...
i thought that it was weird that when i tryed to read current· the leds would shut off, but i am a beginner.
i really appreciate the help from everybody, i think i have a much better understanding of leds....
We've all been beginners, so...
Anyone interested in a Philips 2050 something, experimenter kit?
(50 experiments, components could be mounted with springs. Most of the components are now rather... empty of blue smoke... )
For circuit protection the resistor placement can and should be considered.
There are two basic ways to configure your LEDs.....they can be "sinked" or "sourced."
Sinking is simply the controlling circuit provides the ground to turn the LEDs on (hard wired to positive voltage).
Sourcing is simply the controling circuit provides the +voltage to turn the LEDs on (hard wired to ground).
For a sourcing circuit, the best protection is to put the LED first, then LED(s) to ground.
That way, if there is a short to ground in the middle of the series circuit, the current being drawn from the controlling circuit is limited by the resistor, which is usually a safe level.
The other way around (source LED to ground with no resistor) the current drawn from the controlling circuit will be very high, and likely damage your basic stamp, and maybe the LED.
For a sinking circuit the same can be said, resistor connected first from positive voltage.
However, for a·sinking·circuit, some may say the opposite, resistor "last" because if there is a short to power someplace in the middle of the circuit, too much current will be put through the device·controlling the ground.
A short to power is·probably a lot·less likely to·occur than a short to ground.
So.....stick·with the resistor as the first component from positive·supply voltage.
.....end of long winded·potentially confusing explanation....·