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Topic: what are the limits of leds in series? and are their any hazards to be aware of? (Read 1 time) previous topic - next topic

arduinonooby2014

Aug 25, 2014, 08:29 am Last Edit: Aug 25, 2014, 08:32 am by arduinonooby2014 Reason: 1
Im thinking the more LED's you would place in series with another would simply DIM the brightness of the LEDs as the only result.
I have a more experienced guy on youtube telling me I could do damage to my board if I dont match up resistors and led arrangement properly.

I would like to set in series 4 LEDS to blink when called upon, and hope I can get away with a single resistor..Can I?
I do not mind if they arent as bright, I am mostly concerned with any possible damage i could do.
Is that youtuber full of crap or is there really something to worry about.

Thanks in advance

raschemmel

If you follow proper electronics practices, there is no particular limit to how many leds you can put in series.
Obviously this implies the following:

1- The voltage powering the leds does not come from the arduino
2-The only thing in common with the arduino is the ground connection
3-The current must be sinked with a transistor or Mosfet , (Not sourced) (this means the + end of the led string goes to the Vcc for the leds (not the Vcc of the arduino)

4-The switching device (transistor or mosfet) is controlled by an arduino output (transistor requires a base resistor, mosfet does not)
5-The current is limited by a single resistor (since the current is the same through all the leds)

Example:
Someone tells you that you cannot put 100 leds in series.
Pick a led.
read the datasheet and get the forward voltage
Let Vforward = 2.2V dc for this example
Multiply this voltage by 100: (100*2.2V dc = 220Vdc
If you could get a power supply that could supply 220V dc and you put 100 of these leds in series with 220 ohm resistor
and used a switch or push button to connect  the resistor at the end of string at the cathode end, there is no reason they wouldn't work
because you have not violated any rules. I wouldn't do this if I were you because 220V dc is lethal and could kill you.

To answer your question, you need to find out the forward voltage, use a 9V battery with the + lead connected to the ANODE end of the string of 4 leds, and a 220 ohm resistor at the CATHODE end of the string and connect the resistor to the collector of a 2N2222 transistor with the emitter connected to ground and the base connected to an arduino output with a 1 k resistor and the NEG lead of the 9V battery connected to the arduino ground.
The reason you can't use multiple leds in series powered by the arduino output is that the forward voltages add up and two resistors with a forward voltage of 2.2V each would put you at 4.4V dc. and the voltage drop across the resistor would normally be about 3V,
which means that if Vcc = 5V,  the two leds would be 4.4 volts and that would only leave 0.6 V drop across the resistor which, by Ohms Law means that the current through the resistor would be 0.6V dc/220 ohms = 0.00272 A (2.7 mA) so yeah, I would say that would be dim to say the least.
If you use Ohms Law
V = I * R
P = I * V
and the datasheet for the led to get the forward voltage and current, you can calculate all this for yourself.

JimboZA

And as corroboration of what raschemmel said above, have a look here, about halfway down
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

Paul__B


If you could get a power supply that could supply 220V DC and you put 100 of these LEDs in series with 220 ohm resistor

fungus


I would like to set in series 4 LEDS to blink when called upon, and hope I can get away with a single resistor..Can I?


Yes.


Is that youtuber full of crap


I don't know exactly what he said ... but putting LEDs in series is no problem.

(Although they might be very dim if you only have 5 Volts available)

Advanced Arduino

raschemmel

@Paul_B,
OOPs what ?
What's your point ? Can you find anything wrong with my calculations ? Are you suggesting it wouldn't work ?
What 's your point ?

Quote
there is no reason they wouldn't work because you have not violated any rules.

I wouldn't do this if I were you because 220V dc is lethal and could kill you.



[EDIT]
(See Crossroads comment in Reply#6)

CrossRoads

You can have two Red in series from Arduino, just change the resistor:
(5V - (2.2V + 2.2V))/.02A = 30 ohm.

Other colors typically have a higher Vf, from 3.2 to 3.5 to 3.7V, so two in series can not be driven. Check yours out to be sure.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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