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led, surface mount resistors.


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sallc5 
Member - Posts: 45
Member spacespace
Joined: November 27, 2009
Location: West Virginia, United States
Posted: January 20, 2010 at 11:03 PM / IP Logged  
I have a small LED project in the works for my HVAC controls. Just had a few questions and wanted to run them by you guys here first. Make sure I am on the right track.
Here is a picture of the circuit board I will be working with:
led, surface mount resistors. -- posted image.
led, surface mount resistors. -- posted image.
I am looking to change all the White 5mm LEDs to Blue. The 5mm LED I will be putting in are rated at 3.2v-3.4v and 20mA. So, I will use a 560 ohm resistor at .25 Watts. Correct so far?
If you notice the current LEDs have surface mount resistors. They are currently rated at 147 ohms. It has a label of 1470 on it.
Would it be easier in a sense to remove the 147 ohm resistor and solder in a new 560 ohm surface mount resistor? Or to add an additional 430 ohm resistor to the led itself.
If using the surface mount LED's rated at 560 ohm will the ones linked below work for what I need?
[url]http://cgi./Resistor-560-Ohm-Surface-Mount-SMD-1206-pack-of-100_W0QQitemZ250380857065QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3a4bdcaee9[/url]
TIA
Sall
i am an idiot 
Platinum - Posts: 13,598
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: January 20, 2010 at 11:13 PM / IP Logged  
Buy a couple extra LEdDs and try one with the 147 ohm resistor.  Power it up and check voltae across the LED.  The resistor value is not as critical s most people think. 
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 21, 2010 at 2:12 AM / IP Logged  
i am an idiot wrote:
The resistor value is not as critical as most people think. 
G'dam - you must be an idiot like me! I agree - resistor values are not that critical.
(Alas I made a similar statement on another site (an MPEG-1 layer) only to get a reply "begging to differ". It ended with the begging smartR begging me to leave.... after I had posted my last reply (of course!). The funniest thing - we were discussing PWM to dim LEDs - the switching FET etc can act as a resistor if it were that critical! At least I killed smartR's "thermal runaway" argument. (He was the type that claimed one parallel "string" would rob current from another parallel string.
Apols - I'm FIGJAMming again.)
sallc5 
Member - Posts: 45
Member spacespace
Joined: November 27, 2009
Location: West Virginia, United States
Posted: January 21, 2010 at 9:30 AM / IP Logged  
My concern is that this is an integral part of the automatic climate control system. These boards are valued above $200 and cost over a $1000 at the dealer.
I don't want to be making a dumb mistake.
KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: January 21, 2010 at 11:03 AM / IP Logged  

You need to do your research on the LED BEFORE putting it in the circuit.

What is the voltage driving the LED?  Most likely it's battery voltage, putting you up around 14.4vdc.  Your 560 ohm resistor would put out ~26mA.  Over the max value but the LED will probably still work for a long time.

However, at this current the LED is going to be VERY bright, possibly too bright for your application. 

I would recomend running the LED through a pot and hook it to your battery.  Dial the pot in until you like the brightness, then measure the resistance of the pot and go with that resistance.  I wouldn't be surprised if you ended up going with over 2,000 ohms of resistance to get them to a level they arn't painful to look at!

I would remove the surface mount LED and replace it with the resistor you want.

Kevin Pierson
sallc5 
Member - Posts: 45
Member spacespace
Joined: November 27, 2009
Location: West Virginia, United States
Posted: January 21, 2010 at 11:30 AM / IP Logged  
Well, my charging system usually runs right about 14.9 volts which is optimal for my vehicle.
I see where you are coming from though. Just because that is the optimal range, that does not mean I want them that bright. I will just go down to radio shack and buy some resistors and test them out once I get the leds.
And when you said replace surface mount leds, I am thinking you meant surface mount resisitors, correct.
KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: January 21, 2010 at 1:01 PM / IP Logged  
Yes, I meant resistors, sorry!
Kevin Pierson
sallc5 
Member - Posts: 45
Member spacespace
Joined: November 27, 2009
Location: West Virginia, United States
Posted: January 21, 2010 at 2:45 PM / IP Logged  
That's okay,I appreciate the help!
I finally under stood what you meant by "pot". I always just referred to it as a variable resistor. So, instead of switching resistors in and out, it much easier to use a "pot" and dial in the resistance/brightness to my liking as you said. Then use the desired value for the surface mount resistors.
I will keep you updated on the progress. China drop shipment will take a few days...
ckeeler 
Gold - Posts: 1,461
Gold spacespace
Joined: June 20, 2008
Location: New Mexico, United States
Posted: January 21, 2010 at 7:41 PM / IP Logged  

you might look into these,

http://dkc1.digikey.com/us/en/ph/ONSemi/45vccr.html

never worry about resistor values again. so easy, a cave man could do it!

oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 21, 2010 at 7:55 PM / IP Logged  
DOD - more mpeg1L3 suggestions (constant current devices).
The main risk with this operation (other than driving a short or low resistance) is probably the soldering.
Rather than "replacing" the SMD resistors, you may want to add leaded resistors to the new LEDs. These may include the SMD value, else with a shorting link across the SMD/SMR.
Or just add a dimmer led, surface mount resistors. -- posted image.... (LOL)
PS - I wonder what the patent pending "self-biased transistor technology" is - constant current devices are often made with a pair of transistors, a 3-terminal LM317 voltage regulator, or other nifty combinations.
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