led, surface mount resistors.

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: 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

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. 

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.)

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.

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.

Yes, I meant resistors, sorry!Kevin Pierson

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...

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 ... (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.