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Size Resistor for 3V LED Lamps

David asks:

“I have 10 lamps, each is a 3V LED lamp. My transformer measures 15V AC output on the accessory terminals. I don’t know the output current, but I assume it’s plenty to run these lamps. What size resistors do I need and how do I hook them up with the lamps?”

4 Responses to Size Resistor for 3V LED Lamps

  • Dr. Phool Singh Chauhansays:

    Hi David, First of all you will have to convert AC to DC using a Bridge Rectifier. Then this raw DC will have to be smoothed (filter) and regulated. You have not specified size, color or power of LED. However, I assume you are using white LED of less than 5mm size. In this case, typical operating current for single LED is about 10 mA unless specified. You can use an electrolytic capacitor of minimum 500mF/35V and commonalty available 7812 voltage regulator. You will have to drop 9V (12-3) for each LED. So value of resistor will be (9×1000)/10=900 Ohms. So you may use any value between 1K -1.5 K depending on the intensity of illumination required. Higher the resistor value dimmer the LED.

    Power rating can be calculated as voltage drop x current. It will be 9×10=90 mW for each LED. So 1/4 Watt or higher corban film resistor will be fine for continuous operation with good margin. If you plan to use all LED’s in parallel (which I will not recommend for reason of redundancy) you have to multiply power rating by numbers of LED. For example in present case 90×10=900 mW. It means you have to use single resistor of a higher power rating than 1.5W wire wound type. The resistor can be connected in between power supply and LED. It will be better to connect between +ve (or hot) line of power supply and Anode of LED to keep wiring simple and uniform code of wiring. It will help in diagnosing and rectifying in case of any problem quickly. So please try the best to follow conventions. I hope it will help.

  • mel sloansays:

    I use a separate 12vDC – 2 amp power supply I purchased at Amazon ($.6.99) to supply the power for 15 LEDs and 15 tortes remote turn-outs. Each LED has a 470k resister on one leg. I usually operate one turn-out at a time so the 2 amp power supply is plenty power.

  • Joesays:

    Easier way – buy a AA battery holder for 2 AA batteries (that = 3 Volts DC). Wire it into your layout and go from there. Put a S.P.S.T. switch in and you’ve got it!
    I put velcro on mine and stuck it in my control panel and a “pigtail” so it is easily removed to use as “test power” when I am wiring buildings, etc. on the workbench.
    I’ve lost count but the number of LEDs I have on this one circuit is well over 50! there is no drop in brightness from the time I started with 5-8 LEDs and now.

  • Dansays:

    Connect a 1K (1000ohm x 1/8watt) carbon or similar resistor between one terminal of power supply to one leg of led and connect the other leg of the led to the orher terminal of power supply. Repeat for every led.

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