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Author Topic: Placement of resistors.  (Read 2180 times)
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Hello, So I have this image



The supply voltage is 3.8v, the led forward voltage is 3.3v, the current is 20mA each and each resistor shown is 27 ohms.
Would I be able to put a resistor before any of the branches?  Like immedietly after the battery so that it would properly limit the current for each led afterwards so I don't need a billion resistors?

Would it be the same value?  (My brain and everything says yes)
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No, you cannot do that.

Every branch needs its own resistor.  Otherwise, slight differences in the Vf of the LEDs will result in differences in brightness, and some LEDs not lighting up at all.
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Also that each additional led added will dim all the others too, that's why the resistors all million of them are there...

Doc
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I always did one resistor for multiple LEDs. Usually, it isn't much of a problem for me.
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You need a separate resistor for each LED.

A very simplified way of thinking about this is like this...

If you had 10 LED-resistor combinations, you would take 200 mA from the battery.

If you took one of these resistors and connected it to 10 LED's in parallel you would only take the same 20 mA that the one LED would consume, but this would cause all the LED's to be very dim because they would only have an average of 2 mA each ( but as a previous member mentioned this would not be shared equally due to tiny differences in the Vf of each LED ).

If you swapped the resistor for one that would take 200 mA from the battery, then in theory you would have an " average " of 20 mA for each LED, but this also would not be shared equally.
The LED with the lowest Vf ( needs to only be a couple of milli-volts lower ) will take more than its share of current and will burn out long before its rated lifespan; this could be as short as a few minutes ( seconds if you are using cheap LED's with a large difference in Vf ) or may last days or even weeks.
Your 200 mA is now shared between 9 LED's, so the LED with the next lowest Vf gets more punishment than its predecessor and burns out even quicker.
Repeat this until all of your LED's are cooked !

Resistors will only cost you pennies, and leaving them out is not worth the risk.
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This is exactly why I created my LED breadboarding modules with integral resistors (see my eBay shop), so you don't have to faff around with all those resistors.
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I had a techie trainee do just that once... he asked me what I thought of that and I replied that he hadn't given it much thought, why should I?. He did as his thoughts directed and powered it up... it took about a minute for it to be over, was interesting to watch... much like popcorn they went off each brighter than the one before randomly (they were in a circle), I had his check ready for him when he went on break... because he plugged in 10 more led's and tried it again. I asked him why and he replied that he thought he had some defective led's... the first time, he hand't an answer for the second attempt.
Insanity can be defined as doing the same thing repeatedly, expecting different results.
I see a lot of that here too... LOL

Doc
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The only way to save on resistors would to raise the supply voltage and put some leds in series and one resistor for them
say 12v supply, put 3 in series for a total 9.9v led vf
then limit it to 20 ma by 12-9.9 =2.1v/.02=105 ohms
so you would have 5 strings of 1 resistor and 3 leds and one string with one resistor and one led(need to recalculate for the new supply voltage, so about 400 ohms)
so you save 10 resistors at the cost of using a 12v battery/supply

And btw I've got cheap leds from ebay (white) that vf ranges from 2.9 to 3.3 so especially on cheap ones its common to have off vf
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Resistors are like the cheapest component you can buy.  Don't be lazy and cheap.
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Okay! I will use my 330 ohm resistors that came with the Sparkfun Starter Kit with every LED! It is just annoying for me to take many resistors out of the bag, bend them, etc. I used to think that one resistor is okay, because I thought that it doesn't matter where in the circuit I put the resistance. I now know more about electricity. I experience that LEDs not lighting up properly before. I was wondering why I had to put 3 resistors on a common cathod RGB LED. I tried using one resistor on the cathode only, turned on all the LED pins, and only the red light came on. I switch back to 3 resistors, and it worked properly again (showing white).
« Last Edit: June 16, 2012, 09:24:01 pm by dkl65 » Logged


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Okay good thing I asked

Quote
slight differences in the Vf of the LEDs will result in differences in brightness, and some LEDs not lighting up at all.

Had not thought about that at all.

Quote
Don't be lazy and cheap.

Its not about laziness or cost, I have limited space on my board.
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Don't be lazy and cheap.

Its not about laziness or cost, I have limited space on my board.

You can get a whole whack of resistors in various DIP package sizes. So, if you have room for one or two more ICs, you have room for all the resistors.
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yes and did you know that an 805 size resistor solders neatly between 2 .1 in Ctr pads or traces???, even with pins in the holes???.

Doc
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This brings an interesting point, if making a project with lot of LEDs finding tons of resistors of a specific resistance can be quite hard.

Would pencil led work well as a resistor, or is it not resistive enough?
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1% resistors are cheap, and more than good enough

that being said, yea you can get away with just using a single resistor, do it all the time, though it will look like crap (at the least of your problems) but if you just want to barf something on a breadboard and dont care about astethics then it will work
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