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Author Topic: simple IR leds in series resistor question  (Read 1600 times)
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germany
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hi there!

i just want to power 4 infrared leds in series, that have the following specs:
forward voltage typ. 1,25 V
forward voltage max. 1,6 V
reverse voltage 6 V
forward current 60 mA

i'm using a simple usb cable to power them with my notebook

1. is it okay to just connect them like this?: 5v --- LED --- LED --- LED --- LED --- GND?
    i mean i guess i should give them 1,25 V, 4 LEDs in series makes 5V
    if i use a resistor there, the voltage goes down too much, maybe i could use a 1 ohm resistor, but i dont have that one and i
    dont know if its necessary
2. as i put them in series, its okay with the 100 mA the usb port can deliver, isn't it?
3. is there something dangerous about using the usb connection? can i blow my leds or damage my usb port with this setting?
4. how can i measure which sub-cables (the ones inside the usb cable i'm pluggin into my notebook) give 5v and GND?
    i just touched the red and the black ones with my voltage meter as the cable was plugged into the notebook, but couldnt read anything,
    having the voltage meter set to "20 V". why is that?

thanks!
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Maybe add 20ohm resistor in series just for safety. The diodes should turn on before reaching typical operating voltage so even if you have a resistor in series, they should still turn on.

Do you have a picture of the cable you were talking about?
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It depends how much you love and trust your computer. It *should* be OK in theory to just connect them the way you say...but if something goes wrong, one LED shorts....maybe it takes out the USB port too?

There shouldn't be a need to measure voltage on the cable, the pinout of the USB cable is well known. Just see what pins on the connector it was hooked up to.

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germany
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but if an led shorts, won't the usb port turn off automatically?
can i build in a safety thing somehow? maybe a zener?

and how can i measure where the 5v are coming from? why doesn't it work if i just measure between black and red cable?

this is the cable, im pretty sure the red cable supplys 5v and the black one is GND (i already cut the green and white ones because and - i didnt really know what i did - i measured ampere too. putting the amperemeter between red and black one, the usb port immediately shut down and mac os said it was drawing too much power):
http://i56.tinypic.com/1566zyg.jpg
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but if an led shorts, won't the usb port turn off automatically?

It should...in theory...that's why I asked if you trust your laptop smiley   The high-quality ones will detect the overcurrent and shut it down but I've seen more than one "less-than-name-brand" computer fry its motherboard because of a bad USB plug-in.

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can i build in a safety thing somehow? maybe a zener?

You can add a resettable (PTC) fuse in series, one rated for 100mA or so:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MF-R010-ND

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putting the amperemeter between red and black one, the usb port immediately shut down and mac os said it was drawing too much power

OK! Red and black are the right wires smiley And you have a "name brand" computer that you know will protect you if an LED shorts out.

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I'm just making a guess. Is this related to Nintendo Wii remote control sensor bar?
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germany
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yep
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It *should* be OK in theory to just connect them the way you say
Only simple theory.
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that have the following specs:
forward voltage typ. 1,25 V
You see that 'typ' that is typically, it doesn't mean that is what you get. That value will change depending on lots of things, temperature and LED age being the biggest two.
At best you will have varying light outputs, an inconsistent level of illumination. At worst you will have damaged LEDs.
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germany
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okay, i didnt know that "typically" meant that the voltage the leds draw could change so drastically over time + temperature

can i do anything prevent the leds being damaged? i don't have much space because the leds are inserted into a small tube.
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You could drive them with a constant current source. Or get a higher voltage and approximate a constant current source with a series resistor.
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germany
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i dont get it... is usb such an unconstant power source? i mean... if it gives me less than 5v, its okay, nothing is damaged. can it vary and give more than 5v?

should i think about putting a 3-4 ohm resistor in series, giving each LED only about 1,2 V instead of 1,25 V to be on a safer side?
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is usb such an unconstant power source?
No
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if it gives me less than 5v, its okay, nothing is damaged
NO NO NO NO please read what I said. The 1.25v is not a fixed value, it changes, it will change. The only way you can guarantee the correct amount of current is with a regulator.

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should i think about putting a 3-4 ohm resistor in series,
Like putting your head inside a paper bag for a nuclear explosion. It makes little impact in turning a constant voltage into a constant current.

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giving each LED only about 1,2 V instead of 1,25 V to be on a safer side?
That is not how it works. The LED will drop what it drops, the current voltage curve is a sharp knee.
See:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html
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The USB is a 5V constant voltage source with a current limit, beyond which its voltage drops to 0V if the protection circuit works. A constant current source maintains its current so its voltage will have a limit. I think with what you're doing, just lighting up a few leds without actually controlling them, you're ok with current-limiting resistor, say 50ohm in series. Leds actually work at voltage lower than working voltage. Their current is just very small, with current= (const)*exp(voltage).
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germany
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sorry, call me an idiot, i'm fine with that, but please could you check if this would work for me?


i'm giving each line 5V, each LED draws about 1,25V, so we have 2.5V left. if i put in like 43 ohm, the LEDs gets ~58 mA:
                           ---- ca. 43 R  ------ LED 1 ------- LED 2 ------
5V(USB)------|                                                                                      |--------- GND
                            ---- ca. 43 R  ------ LED 3 ------- LED 4 ------



or should i put in 47 ohm (also easier, i just have to use one resistor) in each line?
i mean its just like 53 mA for each LED then, but if an LED draws lets say 1,1V due to a change in temperature the LEDs get 59,5 mA.
they're not damaged then. if the LEDs draw more than 1,25V... lets say 1,4V, the current is just 47 mA.
                           ---- 47 R  ------ LED 1 ------- LED 2 ------
5V(USB)------|                                                                                  |--------- GND
                           ---- 47 R  ------ LED 3 ------- LED 4 ------



1. LED: typ voltage 1,25. max voltage 1,6. 60 mA forward current. is 53 mA enough for them to give me a good intensity of infrared light?
  should i use 43 or 47 ohm here? and if the voltage the LEDs draw is higher than the typ voltage, like as i said 1,4V, is 47 way too less
  current?
2. with 43 R, the overall circuit is 116 mA, with 47 R, its 106 mA. USB is giving me 100mA. do i have to set something in mac os X to have
   it deliver more than 100 mA? or will it just work fine?
3. do i need to use a ptc safety if i build it like this with the resistors? or is it working fine like this?

thank you so much
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is it working fine like this?
Yes either of those configurations is fine.
However I don't know if 60mA is the recommended working current of the LED or the maximum, if the latter I would back off on the current by 10 to 20% to improve reliability.
« Last Edit: May 30, 2011, 03:38:30 am by Grumpy_Mike » Logged

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