How to Connect LEDs
First and most important is: LEDs should be thought of as current (ampere) devices. Where an incandescent lamp is rated in volts, LEDs are rated in milliamps.
LEDs are best operated using a circuit that supplies constant current. When current is flowing through an LED, a voltage of about 1.5 to 3 volts will be across the LED terminals (this will be stated on the data sheet as forward voltage – Vf). Once the LED has enough current to illuminate, changes in current will result in changes in brightness, but little change in the voltage across the LED will result. If you attempt to connect LEDs to a voltage source (no current limiting device) the led will most likely burn out from excess current. Another important property is: LEDs are polarized, that is polarity (+ and -) must be connected correctly.
Fortunately there is a simple circuit that approximates a constant current source. Connecting a resistor in series with the LED will do the job. The way this works: Excess voltage is dropped across the resistor while the voltage across the LED is relatively constant and set by the LED properties. This assumes that the supply voltage is significantly higher that the 3 volts or so need across the LED. The current is limited by the resistor. The amount of current flowing in this series circuit is: Amps = (Supply Voltage – LED voltage) / resistance.
Here is an exercise showing how to calculate the resistor value. The typical maximum current for a common LED is about 30 milliamp. Assuming our supply voltage is 12v, the desired current is 10 milliamp and the LED will drop 2volts, we need the resistor to drop 10v. The voltage across a resistor the resistance times the current (V = A x R). To find the desired resistance (R = V / A) or 1000 ohms = 10 V / 0.010 A. Resistors are available in standard values so you should pick one on the higher side of the calculated value. Also resistors have a power rating in watts. The power dissipated by the resistor is the current times the voltage (Watts = V x A). In this case, 0.10 W. Resistors are available is standard power ratings, 1/8, ¼, ½, … Again, you should pick one on the higher side the calculated value.
To control the LED brightness use higher current for high brightness. Take care to not exceed the LED’s maximum current rating. If you don’t know the rating, it is usually safe to assume 20 milliamp. Also keep in mind that a 12V car or boat battery can go as high as 14.3 volts when being charged.
This has happened to me on numerous ocean yacht deliveries, where even though we've taken on filtered fuel, we'd start plugging filters once we got out in the ocean into the agitate cycle.
Upon inspection, the tanks and sumps were clean. We'd pipette out a mid-column fuel sample and it would be unremarkable as well. Yet, there would be sludge in the bottom of the Racor bowls on the 30's and on the 10's, eventually causing us to lose the plant. This is the reason that we insist on having many fuel filters. Mysteriously, this never seems to happen to the owners of these yachts, either because they don't take them very far, or they don't sail them very hard. Whatever the reason, it is a problem.
- If you don’t know which terminal on the LED is + or -, just try it one way then the other, using the resistor in series with the LED.
- Use the resistor as a substitute for a fuse. Place the resistor near the power source, as you would do with a fuse. This will limit the current low enough to prevent damage.
- LED Christmas lights are an inexpensive source for LEDs.
- Use your volt/Ohm meter to verify resistor value if you are weak on the color code.
- If the resistor gets too hot to touch then its power rating is probably too low.
The common resistor where the body is about ½ inch long is rated at ¼ watt. Here are some handy formulas for resistors:
To increase resistance, place resistors in series ( R = R1 + R2 … + Rn)
To decrease resistance place resistors in parallel (R = (R1 * R2)/(R1 + R2)
To increase total power rating use equal value resistors in series or in parallel.
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