Any suggestions for wiring 5 or more LED spotlights I'm making to a single 12V DC adapter? I would like to have the spots (film canisters) located in different areas but receiving power from one source. Thanks!
Do you know the current (amps) rating for your spotlights and do they require a current limiting resistor?
If they can be powered directly with 12VDC without a resistor, total up the amp rating of each spot (i.e, 50mA X 5 = 250mA) and use a 12VDC power source rated at an amp rating higher then your total. Remember to observe the correct polarity; "+" to "+", "-" to "-".
Can your LED spots be connected directly to a 12v source? If yes, they have a current limiter built in and I'm kind of betting that the limit is set to about 20mA -- though you should verify this with a meter (using the mA setting, and put the meter in the circuit, not across it as with volts).
Once you know how much current your spots take you can divide that into available current from your wall-wart, though I suggest you back off from maxing out (by at least 20%). And the spots should be wired in parallel; this will give each 12v and allow them to use the current they're actually rated for.
Before you put them in parallel you MUST make sure they have a current limiter. One of my customers had an RGB module and decided to use just one resistor in the common -- the problem is that the voltage across the red LED differed (was lower) from the green and the blue, and the effect was that turning on the red killed the green and blue. The lesson is that each LED should get its own limiter to allow for 12v across the combination (resistor + LED).
a good rule of thumb is for the wall wart to supply a least twice the current that your spots require. As Jonny said you do not want to max out the wall wart it will run very hot and burn out much sooner that designed. The higher the power supply is rated for will just give you greater head room to work with and do not worry about the power supply having too much current the spots will only use the current that they need and no more.
Thanks for the reply's.
I was planning on using Dynoflyer's method of building LED spotlights with one resister for each LED. Or should I use the solution recommended by the LED Calculator which uses one resister per spotlight (5 LED's) and then connect the spotlights in parallel?
I'm not sure which of the DC adapters I have laying around I will use, but I'll make sure it can provide more the enough current.
Yes you can do that you need to take the measured voltage from the power supply (do not trust the labeled voltage 12 volts can be 14 or 15 volts). subtract the voltage drop from each led from the power supply voltage. Use that number in the resistor calculator. for example lets take a voltage of 14 volts and a current of 20ma, and a voltage drop from each led at 1.2V (common drop) some leds may have a higher voltage drop if that is the case then you may have to use a 12 volt supply. with 5 leds the formula is:
e.g 14V - (5 x 1.2V ) = 8Volts so then 8 / .02 = 400 ohms
I would recommend using a lower voltage wall wart in the 7 to 9 volt rating.