What type and size of resistor would be best

Discussion in 'The Projects Forum' started by hillelectric, Dec 10, 2015.

  1. hillelectric

    Thread Starter New Member

    Dec 10, 2015
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    Looking for a resistor that will match the resistance of a 40 watt incandescent light bulb. I have a smart home lighting system (Lite Touch) I am working on for a client. Lite Touch has recently gone out of business so there is no longer tech support available. I am replacing many of the light fixtures with LED lamps and have come across a problem. This system uses triac type dimmers that are located in dimmer modules that are remote from the low voltage switches located in each room. This is a $50K system that my company installed around 2006. The problem is that the triac dimmers leak enough power to get the LED lamps to fire so that when the switch is pressed to turn the lights off they go to a dim level and never fully turn off. I have explored many options and due to the manufacturer being out of business and parts no longer available I have narrowed it down to one solution. When I leave the equivalent of a 40 watt incandescent fixture in the lighting circuit that is enough power draw to allow the LED lights to turn completely off. In some areas I simply leave one of the less efficient lamps in place. But in some area this is not possible as recessed lighting wont match and the owner will not go for that. So I need to install a resistor in the lighting control panel the is the same draw or more of a 40 watt incandescent lamp. I figure a wire would resistor with an aluminum heat sink will work best as this load may be on for hours and maybe even days if they leave town and forget to turn the lights off. I just need help sizing. Simple ohms law can decide the amount of resistance needed but I am not savvy enough with electronics to safely know what wattage and size resistor to look for. The system is 120 Volt AC. Thank you anyone for any help with this.
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    40 Watts / 120 Volts = 0.3333 Amps

    120 Volts / 0.3333 A = 360 Ohms.

    That's the easy part, the thing to consider is how to wire this safely and easily, buying a bunch of 100 Watt resistors and a fan will work, but it's going to become big fabrication project and rather expensive. (just try to FIND a 360 ohm 100 Watt resistor?)
    Big power resistors like this are becoming rare, and hence very expensive.

    Just wire a bunch of lamp sockets and use the incandescent lamps.
     
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  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    A 40 watt bulb represents a a resistance of 360 ohms. You will need a resistor that is rated for 60 to 80 watts. But...... You probably don't need a 40 watt equivalent. A 20 watt or less could do the job. Have you tried a lower wattage bulb??
     
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  4. hillelectric

    Thread Starter New Member

    Dec 10, 2015
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    Both of you, Thanks for the help!

    Right now I have only two circuits that I retrofitted LED lights in to that would take quite a bit of dollars worth of repairing sheetrock to change back. Now that I realize the problem I will make sure to figure out a way to leave at least one incandescent lamp in the circuit and if not then I will not upgrade circuit to LED at all. So if I can find a solution for these two circuits we are only dealing with the heat from two resistors.

    I did consider installing an incandescent lamp in the attic to solve the problem but when the lamp burns out then the problem comes back and their attic is not very easily accessible.

    I happened to have this problem happen the last day that technical support was available for Lite Touch. They sent me some resistors and when I tried one wired between the load output on dimmer module and other end wired in to neutral buss it only made the lights slightly dimmer. They are resistors in an aluminum heat sink about an inch long by half inch tall and wide. The marking is Dale RH-25 25W 1.5K Ohm 1% M1516. I contacted the LED fixture manufacturer and they informed me that Lite Touch probably did not take in to consideration the very low levels of power to make these LED lights fire. I tried wiring two then three of the resistors they sent in parallel and it cut the brightness in half and then in third as expected but still glowing enough to be annoying! So I did try 500 ohms of resistance (three 1.5 K resistors in parallel) which would be the equivalent of 28.8 watts, correct? In one of the other circuits a 35 watt lamp left in the circuit solved the problem but it is a 12 volt lamp connected to an inductive transformer and I'm wondering if the impedance of the transformer is helping kill the LEDs on that circuit?? The homeowner is starting to question my ability. They wanted more light and LED is a great way to increase light without increasing amount of fixtures or causing safety issues from the heat produced by brighter lamps....at least on homes without expensive lighting systems.

    Talking through this is helping me remember my schooling and I will continue experimenting with different wattage of lamps without a transformer ahead of them and look in to larger resistors. Do you see any adverse effects from using multiple resistors in parallel instead of trying to find a 300 or 400 ohm resistor? And do you recommend a good place to buy resistors online? Thank you very much!
     
  5. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Use two incandescent lamps in series- they will last an eternity under this condition.

    Additionally, it could be argued that the lamps would would be a more energy efficient solution: Tungsten lamp filaments have lower resistance when cold, offering a lower resistance for better "ghost loading" of the line, while burning less power when hot.

    Use this rhetoric to convince your clients that you are a technical superstar.

    Use cheap house wiring stuff from a hardware store- problem solved- cheap and cheerful.

    Resistors, while looking cooler and more "resolved" as a solution, will be a giant money-pit over the dumb lamp solution.
     
    Last edited: Dec 11, 2015
  6. Lestraveled

    Well-Known Member

    May 19, 2014
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    Digi-key is always good if they have it in stock. Newark Electronics, Mouser Electronics are good also.

    Three 1000 ohm resistors in parallel equals 333 ohms. 1000 ohm resistors are easy to find. Search EBay. Look for "wire wound". The resistors that are rectangular and look like they are made of ceramic are good bang for the buck. The aluminum ones with fins are not worth the money.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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  8. hillelectric

    Thread Starter New Member

    Dec 10, 2015
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    Thank you again for your help! I ordered the resistors off ebay you recommended and I will try that and hope the heat from the resistors will not be a problem. If that does not work I will crawl in the nasty insulation filled attic and install the light bulbs in series to remedy the problem.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    I am sure your local electronics store will stock them but will be more expensive.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    745
    It wont be the wattage, the triac circuit will need a minimum current, probably around 300 to 400mA to work.
     
  11. BReeves

    Member

    Nov 24, 2012
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    Just for grins try a 100 watt bulb with a series diode like a 1N4004. If it works the bulb will last at least 10 years. Have a couple 100 watt bulbs in my bathroom that are 12 years old and still going.
     
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