scubaterry wrote: Jeff (By the way, Terry, the idea on using one resistor per building isn't awful.) Great response! Are you a politician? Sorry that was lowI have three other computer pwr supplies yet to be converted. You mentioned you would use 5v. would that be a better voltage than 3.3 because it gives you more control over the LED current? IF so then I need not mess with the 3.3 on the remaining units. Tks again.Terry in FLorida
Jeff
(By the way, Terry, the idea on using one resistor per building isn't awful.)
Great response! Are you a politician? Sorry that was low
I have three other computer pwr supplies yet to be converted. You mentioned you would use 5v. would that be a better voltage than 3.3 because it gives you more control over the LED current? IF so then I need not mess with the 3.3 on the remaining units. Tks again.
Terry in FLorida
Ouch!
I worked with a guy who always said, "I don't disagree with you." Of course that usually meant he did!
The one resistor per building makes sense the same way the one resistor on the sign does, the 'thing' is a unit, and within that you are not going to add or take away.
In my opinion the 5V output would be better, since it looks like the forward voltage on white LEDs is 3.5V or so (at least the ones I looked at). So with the 3.3V supply you really don't know how much current is being drawn, and the whole thing becomes a guessing game (at least that's how I see it). And I don't like that kind of guessing game. Unless you are building a really big city one 5V PC power supply output is going to drive every LED you can imagine using. If you wanted to you could use the 3.3V output for red and green ones, since their forward voltage is below that.
Jeff But it's a dry heat!
Jeff - I think I will go with the 5v. I wanted several pwr supplies so I would not have to run so much wire around the layout. Excellent point on the red/green LEDs. I had totally forgotten about the couple dozen signals that will need juice so I can still use the 3.3v supply with appropriate resistance. You are a great source of info. I appreciate it.
There is another solution.
If the supply is putting out 3.3 volts, simply put a SINGLE rectifier diode in series with the supply output before the paralleled LEDS. Make this diode large enough to handle 2 x the total current (5 amp perhaps).
Voila! 3.3 volts becomes about 2.7volts regardless of the number of LEDs in use. No mucking about with either resistors or one of anything per LED. Most power rectifiers "drop" nominally 0.6 volts. For larger voltage supplies use a Zener.
Simple, elegant and effective.
Karl
The mind is like a parachute. It works better when it's open. www.stremy.net
Yuppers...that is how I do it...love voltage clamping diodes :)
Brian
Wrong wrong wrong....
You cannot forget the voltage dropping resistors!!
A 3.3 volt power supply is not a good one to use to drive white LEDs. The voltage too low for safety. (Doesn't make sense ??). Leds do not have a fixed forward voltage drops - the drop can vary within a batch of led even...
The white smt LEDs that I use have a rated voltage drop of anything between 2.8 and 3.8 volts. So at 3.3v some leds may not even light up. Even with a dropping resistor, the difference in current that you will get through the led could be a problem. What I mean is that the smaller the drop across the resistor, the greater the change in current flow will be between different LEDS.
You are far safer using the 5V supply with a dropping resistor that will provide the LED with the Typical Vdrop at no more than 50% of the rated current.
Safer yet is to use a current regulator (A LM317 (~$.60) and a single resistor). What I would then do is use the 12V supply and feed 3 leds in series from one current regulator.
The other thing to consider is that LEDs are very inconsistant in their light output from batch to batch (and even within batches) The white ones that I use can vary over a range of 4 to 1 (at the same current). When you drive an LED beyond it's rated current, more and more of the excess power gets turned into heat - not light.
Also eyes are not linearly responsive - twice the light output doesn't look twice as bright. I reduced the current draw on the LEDs in one of my passenger coaches from 10 to 1 ma and all it did was make it look more realistic (ok - I do have about 10 LEDs in there - hard to light all of the compartments and corriders otherwise)..
LM317 as a current source : Connect a resistor between the output and the adjustment pin and connect your LED string to the adjustment pin. (The current is still being supplied by the output pin - the LM317 adjusts the output voltage so the drop across the resistor is 1.25 volts).
The value of the resistor is R = 1.25/A where a is the current required in amperes. So a 1 ma current regulator would need a 1250 ohm resistor. 10 ma would need a 125 ohm resistor etc.
I use the little plastic package version of the LM317 - costs about $0.70 each. It has built in over-power control - if it gets too hot, it throttles back the power.
Villy
LM317 Sources:
www.allelectronics.com
Fry's Electronics
Radioshack
They are easy to find!
also Digikey
They come in all shapes and sizes (and current handling capability)
Brian and Villy - I downloaded several data sheets and PDF files on the LM317. I'll read up on it and then I can ask some intelligent questions. Tks to everyone for their help.
Terry
only thing to be careful about is that not all of the manufacturers make all of the different sizes...
The one that I use small a small plastic transistor like case - I believe that it was made by Texas Instruments..