When driving LEDs with RC4 dimmers, is a current-limiting resistor required? Or can the LEDs be connected directly to the dimmer outputs?
If you are using LED tape, or other LED products that are not “raw components,” the resistors are probably already there. If the product has a voltage that is higher than 3V or 4V, then all the work has been done for you already. Use 12V and 24V LED tape with RC4 dimmers directly, just like you would connect lamps, solenoids, relays, etc.
But if you’re using actual LED components, like you’d purchase from www.digikey.com or www.mouser.com, then a resistor must be used. In this case, the voltage rating is called “forward voltage.”
Start with a relatively low voltage battery. Six volts works well, either lead-acid, or NiMH. Then, use an online calculator to determine the best resistor value for the LEDs being used. A couple of good calculators are here:
http://home.cogeco.ca/~rpaisley4/LEDcalc.html
http://led.linear1.org/1led.wiz
You will note that you need LED specifications to do this:
The battery voltage,
The “forward voltage” of the LED, and
The “forward current” of the LED.
Forward voltage is the voltage at which the LED begins to conduct (and draw current, and emit light). Below this voltage, no current flows and the circuit behaves like an open circuit.
Forward current is the maximum current you can safely apply to the LED without damaging it. Common small LEDs have forward currents in the ballpark of 10 – 50mA. High-power LEDs that replace lamps can withstand much more — 350mA or even higher.
The correct values for the LED you are using can be obtained from the manufacturer’s datasheet. There is no other way — there are no “standard” values.
Using a resistor is the easiest and most effective method to limit current. Nonetheless, critics point out that they waste power. This is true, but that waste is reduced when you use a battery voltage that is closer to the forward voltage of the LED. With a 6V supply, it is my opinion that the ease of using a resistor outweighs the drawbacks.
Jim
RC4