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FreeTrack Forum > FreeTrack : English Forum > Support : Tracking System > Resistor Value Question

RSS > Resistor Value Question, Above or below the nominal?

flips	#1 21/12/2007 - 21h15 Top
Class : Apprenti Posts : 6 Registered on : 21/12/2007 Off line	According to the LED calculator tool it says I should use 18 ohm resistors, my local Radio Shack only had 10 and 15. My question is should I use 2 of the 10's in series, or just the 15. The 15 are .5 watt, and the 10 are .25 watts. Per the calculator my circuit is .18 watts, so that shouldn't matter. Here are my specs: Power supply: 3V (2 AA batteries) LED: Forward Voltage: 1.2V Forward Current: 100 mA Max forward voltage: 1.6V Max forward current 1.2A Also, it says in the calculator it says that the 4 point model is old, and shouldn't be used. Most of the places around in the forum and the handbook refer to the 4 point model. Is this just inferring that there isn't much of an advantage when you go from 3 to 4 points? Thanks for the help. I'm looking forward to getting this working!!! flips
didja	#2 21/12/2007 - 21h29 Top
Class : Webmaster (admin) Posts : 1043 Registered on : 09/07/2007 Off line Www	Hello Flips and welcome ! About the 4 points model, there is no need to use it now, the 3 Point (cap or clip) is absolutely equivalent then we can consider the 4 led one as being obsolete... And about the resistors, I can't really help you because of my mild skills in electronic.... Have a good day ! Rémy
flips	#3 21/12/2007 - 22h17 Top
Class : Apprenti Posts : 6 Registered on : 21/12/2007 Off line	didja, Thanks for the answer. In the handbook it says that the dimensions must be exactly what is shown. Is that not the case anymore? Is it best to have a triangle of LEDs or is a straight line just as good? flips
Deimos	#4 21/12/2007 - 23h52 Top
Class : Beta Tester Posts : 120 Registered on : 07/11/2007 Off line	You'd better use 2x10R in series for each LED - using 18 ohms resistors limits current flowing trough LEDs to 100mA, which is maximum of what they can handle for long term usage. Lowering resistor value will increase the current. That won't necessary damage the LEDs right away, but will cause them to heat up more than they should and in long term that can damage them. Also it will increase battery drain. Personally, i'd first try even bigger resistors option and see how they work. Maybe LEDs will still be bright enough, and additionally you'll have a bit longer battery life. If the LEDs are too dark, only then decrease resistor value. And if you use IF-modified webcam, 60mA should be more than enough to make the LEDs clearly visible, and you could use resistors around 33ohm, farther enlongating battery life.
flips	#5 27/12/2007 - 23h41 Top
Class : Apprenti Posts : 6 Registered on : 21/12/2007 Off line	Deimos, Thanks exactly the info that I was looking for. I just picked up some 33 ohm resistors. One more question. I would like to put in a visible light LED so I know when it it's on, and I don't forget to shut it off. If I do that, can I pick up an LED with a little different characteristics and size a resistor for that or will that throw off the current flow to the rest of the LEDs? I picked up a 3V 15mA red LED. Is that going to be an issue of the 3V is close to or equal to the source voltage? I was thinking of putting in a 10 ohm resistor, which is the lowest that I have. If that won't work can I go pick up a 1.7 V LED and then put whatever the appropiate resitor would be when I put that in the calculator? Thanks again, flips
Kestrel	#6 28/12/2007 - 06h34 Top
Class : Webmaster (admin) Posts : 780 Registered on : 13/07/2007 Off line	flips @ 21/12/2007 - 23h17 a dit: didja, Thanks for the answer. In the handbook it says that the dimensions must be exactly what is shown. Is that not the case anymore? Is it best to have a triangle of LEDs or is a straight line just as good? The dimensions aren't critical since they can be accounted for in the software. However the LEDs need to be in a triangle configuration for 3 point tracking to work. picked up a 3V 15mA red LED. Is that going to be an issue of the 3V is close to or equal to the source voltage? I was thinking of putting in a 10 ohm resistor, which is the lowest that I have. If that won't work can I go pick up a 1.7 V LED and then put whatever the appropiate resitor would be when I put that in the calculator? Try the 10 ohm resistor with the 3V LED, if it's not bright enough use the 1.7V LED with a resistor equal to (3 - 1.7) / (LED current) ohms. Support FreeTrack
flips	#7 28/12/2007 - 12h39 Top
Class : Apprenti Posts : 6 Registered on : 21/12/2007 Off line	Kestrel, Thanks. I didn't realize that the formula was that simple. That's really going to save me trips back and forth to Radio Shack. flips

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