# 3W COB gets too hot ... what to do?



## mikek753 (Nov 15, 2013)

Hi all,

I got 3W COB from ebay
for replace dome and cargo car light.
take a look at g+
This produces enough light, but gets too hot.
Yes, I mount it to heatsink that helps, but not for long, about 2-5 min run time only.
I connected it directly.

How can I manage so much heat?

thanks.


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## bshanahan14rulz (Nov 15, 2013)

Do you have a driver circuit between the 12V and the LED, or at least a high power resistor? The ebay page says that it is best run at ~300mA. So, it would seem that you are in need of a 300mA driver capable of 12V output, and that can handle at least 15V in


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## mikek753 (Nov 15, 2013)

no, I don't have anything, just connected directly
How to calculate what resistor is needed?
This what I found  but I don't know what voltage drop is?


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## bshanahan14rulz (Nov 15, 2013)

Voltage drop can also be phrased as Forward Voltage, Vf, or sometimes just voltage. Unfortunately, the specs listed with your LED in that ebay page simply say that the Vf is between 9 and 12V. 

Now, keep in mind that your LED will be dimmer when not being overdriven. I might recommend a 15 Ohm resistor capable of 2W of dissipation. It will probably be a ceramic, rectangular, wire-wound resistor at that power level, not the usual peanut shaped ones. However, 9-12V is a wide range. I am estimating by using ~3.4V per LED, assuming 3 in series, 3x3.4=10.2V for the Vf.

Using that calc page you linked to, I assume 13.8V for input, 10.2V for voltage drop, and 300mA for current. It returns with a recommendation of a 12 ohm resistance. However, for single resistors, the actual value you will find will be 15 ohms. It also recommends a 2W resistor for that value. 

If you can find an automotive LED driver that outputs 12V @300mA, that would be a more suitable choice, but ain't nothing wrong with the right resistor ;-)


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## mikek753 (Nov 15, 2013)

thanks a lot.
makes sense now
I just have to find from where to buy 4 such 15ohm 2w resistors ...

BTW - how to know expected heat of this COB?


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## DIWdiver (Nov 15, 2013)

If you are in the US, Digikey or Mouser are probably the best places to get the resistors, unless there's a local electronics place that stocks stuff like that. Radio Shack used to, but it's gotten less and less. I just checked, and they are online only. 

I would get a few each of these: 10ZCT-ND, 12ZCT-ND, 15ZCT-ND, and 18ZCT-ND from Digikey. You could get 4 of each for around $6 including shipping. Try the 18 ohm first, measure the voltage across it and calculate the current. If it's low, and/or the heatsink doesn't get hot, try the 15 ohm. If that doesn't satisfy you, try the 12 or the 10 ohm. Keep in mind the readings will change depending on whether the engine and battery are off/cold, off/warm, on/cold or on/warm and on the LED temperature. The highest current will occur when the engine is on and the battery has recovered from the starting load, and the LED is as warm as it will get. 

If you get them working properly at 300 mA, they will be taking in around 3 watts (10V times 0.3A) of electrical power, about 15-20% of which goes out as light, and the rest is turned into heat. That heatsink should be more than enough to take a few watts and stay below 70C, which is cool enough to handle but too hot to hold - if it has airflow around it. You'll have it closed up pretty tight when it's installed, so you might find that it gets hotter than that and your leds don't last long. But how much do you use them anyway?


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## mikek753 (Nov 15, 2013)

thanks a lot for the info
I placed order at digikey for 10,11,12,13,15,16 Ohm pairs

the light in dome should run longer as door can be opened for 10-20 min or worst case it's in ON position and forgotten for overnight (when doors are closed dome light goes off in a min, but when in force ON position it stays ON all time) , but not as bright
the light in cargo should not run longer then 10 min, but should be as bright as it can

I hope that from 10 Ohm to 16 Ohm will cover my range - 60%
Also, when engine is off I expect 12V in dome, while running engine can deliver 14V - this would be another 20% off for the worst case overnight IMHO

Mike


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