Resistors in LED Rope Light

[ThaiWay]

Yeah, Randy... that alibaba site is just a china business directory search engine.  No details about Lanhe's products.

Eric... pure DC is what the rope gets from a bridge rectifier dongle.  Compensating for the missing part of the string is exactly what I need to do.  I checked the link, and it's an interesting discussion, but way over my head.

Now I guess I gotta make a trip across town to the electronics neighborhood for some resistors...  there's no "mall" to run to with a Radio Shack )  If you know anything about Bangkok you know about our ubiquitous gridlock, so that's a good half day just for a couple of five cent parts.

Could you also run the numbers for me for 48 (4 resistors/1.33m) and 60 (5 resistors/1.66m) and then I'll have all my lengths covered.  Really appreciate your help with this!

John

[Hauvega]

48 led with 4 resistors needs about 4500 ohms.  60 led with 5 resistors needs about 2100 ohms.
Always go to higher value.  You can always tweak it later.  The resistors will need to be rated at the min of 3 watt. I would test at 5 watt to see how warm it will operate at.

All these numbers are based on the total string voltage and not incoming voltage. 

As far as Radio Shack,  They do not have that much any more.  I prefer Digi-Key, Mouser, Electronic Goldmine, and many other online vendors.

Eric

[ThaiWay]

Many thanks Eric... I'll make the trip to Baan Mor tomorrow morning, and let you know how it goes!

John

[Hauvega]

Take care and good luck,  John   

Eric

[ThaiWay]

Eric...  Testing 48 LED's including 4 x 300 Ohm with a 4.7K 5W 5% resistor.  The brightness of the LED's looks about the same, but the resistor gets quite hot.  Holding it tightly between thumb and forefinger after awhile it's almost too hot to handle.  5.0K appears slightly less bright, slightly less hot.  What would you suggest?

John

[edit]
...and after running the 5.0K for 10 minutes, holding for 10 seconds gets really uncomfortable.

[Hauvega]

If you can get me a voltage reading across the resistor.  Trying to get me a check on power readings.

Probably have to double the wattage since the resistor is adsorbing most of the power.  We can also divide up that resistor into smaller ones to try and dissipate the power.

Eric

[n1ist]

20 mA over 5K is 2W (with a 100V drop).  I'd expect that to be toasty.
/mike

[ThaiWay]

65V across the 5K 5W... I'm starting to think that (as usual) I'm making this too complicated.  Perhaps I should just wrap the unwanted 66cm in black duct tape. 

It's great to get all this input tho!

John

[Hauvega]

Black tape is a good idea.  If the voltage was stepped down before using a transformer then the resistors would not have to take half the load.  Another possibility a light controller operated at a lower dimming level.  The only bad thing if it goes above the safe voltage,  POOF (lights out on the playground).

Eric