Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Current Divider Assistance

  1. Dec 29, 2009 #1


    User Avatar

    Hello all,
    I'm attempting to create a dimmer circuit for some lights Id like to install in my room. Ive attached the diagram below.

    Heres the problem: When I increase the resistance of potentiometer 2, for example, to decrease the current lamp 2 receives (thus dimming the lamp) the other lamps brighten. I want the other lamps to maintain their luminous output at a constant level dependant upon the amount of resistance of their respective pots.

    Bear with me (this is my first circuit) but I dont understand why this is happening.

    Two Questions
    1) If the resistances of all the other pots remain the same, the same amount of current should flow through them, no? Why are they getting more current?
    2) What are potential solutions to this problem?

    Thanks for your help

    Here are the parts I'm using on the breadboard:
    -12v battery; 12v, 60mA mini indicator lamps; 10k ohm, .5W pot; 300ohm, .5W resistor in series with lamp and pot

    These are the parts Id like to use in the finished circuit:
    -12v transformer (wired to mains); 12v, 35W MR16 halogen lamps, appropriate pots and resistors

    Attached Files:

    Last edited: Dec 29, 2009
  2. jcsd
  3. Dec 29, 2009 #2


    User Avatar

    Staff: Mentor

    If you measure the output voltage of the transformer as you vary a pot, does it vary? That would explain why the other bulbs change when you vary one pot. It would be because the output resistance of the transformer is not negligible in your circuit.
  4. Dec 29, 2009 #3
    Actually his trial version uses a battery, even more likely candidate for internal resistance I'd say, especially if the battery is "tired".

    By the way, what about safety on the transformer version - need to be sure of the transformer rating, and with 35W lamps those "appropriate" pots may run a bit warm.
  5. Dec 30, 2009 #4


    User Avatar

    By run a bit warm, do you mean warm enough to be a fire hazard, or too warm to touch?
    Is there a formula I can use to calculate the heat dissipated by the pot and its accompanied fixed resistor?
  6. Dec 30, 2009 #5
    What I really meant was that resistive dimmers for 35W bulbs have to be fairly substantial, and mounted in suitable housings to deal with the heat to avoid burnt fingers. Resistive dimmers are generally pretty inefficient, and for this reason they are not so often used nowadays.

    Unfortunately, filament lamps undergo a large increase in resistance as they heat up. Thus when you reduce the lamp voltage for dimming, the lamp resistance falls noticeably. Due to this effect, at some settings a dimming resistor may end up dissipating more than half of the lamp's nominal wattage. (If the lamp's resistance were constant, the dimmer could only dissipate up to half power). I'm sorry that I don't have a formula for this, perhaps it's safest to rate the pot for the full wattage.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook