hello all simmer,Just finished to hook up the overhead backlite. I am thinking about the Korry indicator now. I will not use the original Korry light bulb. Instead I will use LED light which takes 3V power.For backlite light, I can use an old PC AT power box to supply the 5V power. Now, for 3V, any suggestion of power supply with abandent current, at least above 4 amp.http://alkit.fotopic.net/p15978929.htmlThank you!Alkitwww.alkit.net

:( why not use resistors??taken that you use 1 led, 3V and 20mA, at 5V input, that gives you a value of 100 Ohm for a resistor.No need for an extra supply!

The limiting value here is current. Most LEDs are roughly 1-2 amps. The best thing to do is to seperate your overhead into 3 or 4 sections, each with a seperate power supply. A 2 amp source should be plenty with this technique.Roberthttp://777Simulator.comhttp://HSBRegistry.com

hello twabi2,What type of resistro do i need??Hi Robert,Do you know how many LEDs can I power up by 1 amp (approx.)?Thank you!Alkitwww.alkit.net

any type will do, just buy the cheapest (make sure they're 100 Ohm)and I already said a LED takes about 20mA, so that would be 50 LEDS (give or take some)

>:( why not use resistors??>taken that you use 1 led, 3V and 20mA, at 5V input, that gives>you a value of 100 Ohm for a resistor.>No need for an extra supply!To be exact 470 Ohms resistor for long lasting life on LEDs

Are you sure that your LEDs are 3V/20mA? This sounds like the "absolute maximum ratings", which are stress ratings only, and not "recommended operating conditions". Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.How to calculate the resistor needed:R=V/I, where R is Resistance in Ω, V is resistor drop voltage (5V-VLed)in Volts and I is current in Amps.For V=2V, I=0.02A, R is 100Ω.Because resistors have a typical 10% error in their values, I would suggest 120Ω to be safe. This will limit the current to 17mA-+10%.Power dissipated (as heat) is P=V^2/R=33mW so the cheaper resistors that are a quarter of a watt certified will have no problem.Most LEDs have a 1.7-2.2V dropdown voltage at 5-10mA in -normal- working conditions. With the above simple equations (remember the 10%) you can easilly calculate the value.Remember that if your LEDs are less than 2.5V drop voltage, you can put them in pairs (in series) and use a smaller resistor.

Hmmmmmmmmmmm..............An LED's brightness is a function of the amount of current flowing through it. You'll find that most LED specifications will give you a forward voltage drop in relation to the amount of flowing through it, and it's NOT a linear relationship. So, your 3V 20mA LED maybe 3 volt of forward drop when 20mA is flowing. Check to see how "bright" you want each LED first, but don't exceed the max rated current, that is the key. BTW, running components at max ratings isn't a big deal unless you are concerned with MTBF's, and running your setup 24-7.Gus B

How did you get that value?? That's way too much I think

>How did you get that value?? That's way too much I think470ohm is the most widely used limiting resistor value for 9-12V power.And thus it is widely over-dimensioned for 3V.150ohm should be a suitable value.Depending also on the intrinsic efficency of the led: meaning that some diodes produce more light then others with the same power consumption.