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Author Topic: Correct resistors running hot?  (Read 43 times)

Offline FatalCheese

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Correct resistors running hot?
« on: August 08, 2018, 11:14:33 am »
Hi all,

I
You're in it this far...what's another $20?

Offline Markeno

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Re: Correct resistors running hot?
« Reply #1 on: August 08, 2018, 08:59:23 pm »
I think you are missing something to your post there.

The correct "resistance/ohms" value is only how much they resist and keeps the Current in the circuit where you want it, it isn't how much power they can handle, that is the Wattage.  1/8th 1/4th 1/2 1 Watt etc.  Any resistor heats up, the wattage rating means it can safely dissipate 1/8th 1/4th 1/2 Watt of power without "burning up".  If you hook up a 1/8th Watt in a 12 Volt LED circuit it can burn up, if you hook up a 1/4th watt it will distribute essentially the same heat throughout its larger casing seeming to not get as hot but will still get warm and a 1/2 may not feel very warm.

If you wire a White LED (3.2Volts At 20mA) with a resistor to a 12Volt power supply and have it a 20mA (0.02A).

(12 Volts -3.2 Volts)/.02 Amps=440 Ohms

For that same resistor in that circuit you can use Ohm Law to find the Wattage/Power put through it.

Watts=Volts*Amps
W=(Vsource-VLed)*Amps
Vsource = 12
VLed =3.2

12-3.2 gives us the 8.8 Volts across the Resistor as the LED taking 3.2 Volts of the 12 volts leaving that 8.8 Volts for the Resistor.

.176Watts=(12Volts-3.2Volts)*.02Amps
.176Watts=8.8*.02

Or we can do something that can be easier in theory. 
W=A2*R
 .176Watts=(.02A*.02A)*440 Ohms

So a 1/4th Watt (.250Watt) Resistor won't "burn up", but that doesn't mean it won't get "hot", it actually will get fairly warm or even hot to the touch.  If yo use a 1/8th Watt (.125Watt) resistor it will get too hot.. likely burn you if you touch it, probably start to take on a charred burned look and is well over it's rated wattage.

If you drive that same White LED off of 12 Volts but want to run it at 10mA of current.  Btw, you won't loose half the visible light when running them at 10mA.  You could even run that same White LED with a 1000 Ohm resistor just fine, and I expect it would be plenty bright for a scale model.

(12 Volts -3.2 Volts)/.01 Amps=880 Ohms
8.8 Volts/.01 Amps = 880 Ohms

.088Watts=(12Volts-3.2Volts)*.01Amps
.088Watts=8.8 Volts * .01 Amps

At 10mA (.01A) we have half power at the resistor (.088=.176/2).   So in this 12Volt system the 1/8th Watt would work, although still get warm, it won't burn up.  Also just because it is made to "handle" that Wattage doesn't mean it won't get warm, it just means it won't get so hot that it will damage the resistor (it might burn your hand though and certainly melt your plastic models).  It is typically recommended to overrate the Wattage on a resistor by using the next size up if you don't have a lot of excess dissipation.

Also the Wattage ratings are for "open air".   Sealed in an in closed space.. (like sealed in a model), or wrapped with Electrical Tape or covered in Heat Shrink, they become "derated", they get Hotter than they normally would because they can't dissipate the heat like they would in open air and can burn up or get very hot.

By the way.  If you are working with Red (1.9Volt at 20mA) LEDs with a 12Volt source the Wattage at the resistors is even higher .202Watts if using a 505 Ohm resistor.  Yes I know the values I posted for the Resistors are not typical but this is so the "10 or 20mA" calculations would be accurate.  If you use a 510 (which is one I have), then the Wattage would be fractionally "lower", if you use a 470 the Wattage will be Higher at the resistor (as the current would be higher at both the Resistor and LED about 21.5mA).  Really Resistors do have that 5 or 10 or 20% variance in them, a 510 may very well be a 505..

I don't suggest running LEDs at 20mA.  10mA is often plenty, and even less is often good.


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Using 9Volts the Wattage at the resistors are lower.  At 5Volts they are lower yet.  Wiring 3 White/Blue etc LEDs in Series in a 12Volt supply lowers the Wattage at the Resistors as well (you can do 4 Red LEDs in series with a 12Volt supply).

Wattage at the Resistors means Heat Generated, which means wasted Electrical Energy.  If you are using batteries, it is best to set it up to waste as little power at the resistors as possible/reasonable.  If you are using an AC to DC Adapter, well it is still wasted energy, it is still additional Heat inside your model/item which may be bad if it is a plastic model.
« Last Edit: August 08, 2018, 09:03:57 pm by Markeno »

 




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