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MR16s will not dim with new LOR DC Board


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While I was out tonight I also stopped by radioshack and bought 2 10ohm resistors. They do fade a lot better and I tested them on a d-light board and a LOR board and they are pretty much identical. I wish the fade curve was a little more like incandescents (LEDs seem really ramp down faster when they are close to turning off) but it is a big improvement.

I will have to get some more resistors and try some different combinations to see what works the best. Green seems to dim faster than the red ones so I will need to try out different ohm resistors and try to find ones so they were be consistent with each other.

I tried only 3 LED floods on one resistor and they were not as bright so it looks like one resistor per flood is the way to go.

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OK, just double checking. Per the attached diagram, mounting the resistors across the wire is putting them 'in series' vs. parallel.

Does the resistors have to be mounted at, or close to the two power prongs on the bottom of the MR-16? Or can they be mounted about 6-8" away where I installed wire connectors? How about further away from the light, as long as it still stays in series?

Also, are resistors non-directional? To put 2 10ohm resistors, do polarity matter (in this case both facing the same way)?

Thanks,

Michael

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skibozo wrote:

OK, just double checking. Per the attached diagram, mounting the resistors across the wire is putting them 'in series' vs. parallel.

Does the resistors have to be mounted at, or close to the two power prongs on the bottom of the MR-16? Or can they be mounted about 6-8" away where I installed wire connectors? How about further away from the light, as long as it still stays in series?

Also, are resistors non-directional? To put 2 10ohm resistors, do polarity matter (in this case both facing the same way)?

Thanks,

Michael

I think you have it backwards there. Your resistor would only be on one of the wires that go to the lamp. You would need a resistor for each LED.

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pshort wrote:

EDIT: I'm thinking that the power rating of the resistors might want to be increased a bit, perhaps to 5W (especially for the red lamps). The lower wattage resistors might get to be a bit toasty after an extended period of full brightness.

Phil

Doing a couple of quick calculations (which may be totally wrong!), using 12V and assuming 2 watt dissipation.The MR-16would bea DC resistance of 72 ohms (P=V^2/R ... 2watts=144volts/R... R=72 ohms). Circuit current would be .167 amps.

Adding a 22 ohm resistor in series will drop the circuit current to .127 amps, reduce the voltage (and the brightness) of the MR-16 to 9.39 volts. So you would end up with 1.2 watt dissipation from the light.

The resistor power would be .31watts (2.61v^2/22).

Someone will have to put a voltmeter across the resistor to confirm. But seems like a lower wattage resistor may be okay? Higher wattage resistors would be better, but I'd hate to spend $4 on radioshack resistors for a $3 light! Going the Mouser route is the way to go at 14 cents, but they only have 165 of them left. Richard may clean out their supply before the rest of us get our order in!

Since I have got ohms law out... Using Richard's example of one 22 ohm resistor feeding 3 MR-16's in parallel would give you:

Resistor: 22 ohms -5.7 volt drop - 1.48 watts

3x MR-16's: 24 ohm equivalent- 6.3 volts across terminals - 1.65 watts

I'll have to confirm all of this with a voltmeter later. :shock:

Phil et. al., thanks for fading fix on these. It is a good thing I am so far behind on my lighting setup asI won't have to re-do my MR-16 setup since they are still in the box. I'll just do them with the resistors the first time.

Thanks,

Jim.

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I'd have to be in a great hurry to pay that amount for RS resistors.

In any case, the math is a bit more complicated than that. The 2W number is highly suspect, the LEDs are highly non-linear, and there is capacitance thrown in to boot. At this point I haven't run the numbers, so I'd go with WireKat's assessment on the resistor power (I used a pair of 10 Ohm, 10W resistors that I had on hand, so their temperature rise is fairly meaningless here).

--

Phil

PS. The DC voltage drops across the lamps that I observed some time back are 5.8V for the red module, 7.8V for the green, and 7.6V for the blue module, all at 150 mA. This means that the drops across the resistor are 6.2V, 4.2V and 4.4V respective, for a power dissipation of .87W, .63W and .66W. The voltage drop across the lamps doesn't change much when the current goes up to .25A, so the power dissipated by the resistors could be as much as 1.5W for the red modules. The reason that I was pushing for higher wattage resistors was to leave some margin, and to take into account that the resistors will get quite hot if they are run at full power continuosly for an extended period of time, and that people may be putting them into restricted enclosures.

--

Phil

PPS.

Mouser 262-20-RC should work fine (20 Ohms, 2W, 2943 in stock, a bit more expensive).

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RichardH wrote:

skibozo wrote:

I think you have it backwards there. Your resistor would only be on one of the wires that go to the lamp. You would need a resistor for each LED.

DOH!!! My brain doesn't function well @ 4:00am!!!!:{Good thing I double-checked! Thanks Richard.

I'm guessing since it's inline location in relationship to the MR-16 doesn't matter.

Brought a couple of 10 ohm, 1w resistors yesterday, no 20 ohms in stock. Just to be safe, I'll order some 20 ohm 2w resistors. Ouch!!! $0.04 more per unit!! Oh well, the budget is shredded anyways.

Jerry, was at San MarcosFrys last night. Registers were down, waited over 40 minutes to check out!!! You would think a high-tech'n company would use better equipment.....hope everything works on Black Friday!!!!:shock::shock:

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Jerry, was at San MarcosFrys last night. Registers were down, waited over 40 minutes to check out!!! You would think a high-tech'n company would use better equipment.....hope everything works on Black Friday!!!!:shock::shock:

so that why there was no 10 ohm

I was there at noon today and pick- up 20, 22 ohm resistors :laughing:

Registers was working :P

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