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Author Topic: Correct resistors running hot?  (Read 355 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.


--------
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 »

Offline FatalCheese

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Re: Correct resistors running hot?
« Reply #2 on: August 16, 2018, 06:16:22 pm »
I have no idea what happened to my post, but I am more amazed that you managed to figure out what I had intended to ask, and managed to pinpoint what I wasn't considering - watts.  Very informative, thanks for taking the time to write all that on a "hunch".

I am using HDAmodelworx - a white LED, with a 470 Ohm, which is what HDA suggests for a 12V circuit.  It's a 12V/1A supply.

In your value for Amps in your formulas - that's the amp power being supplied by the Power Supply (and not just the LED amp rating), correct?  Using the formula you suggested W=(Vsource-VLed)*Amps - I only hooked up one bulb - so it sounds like I was pushing 8W through my resistor.  No idea what the resistor wattage rating actually is, there are no codes to indicate it, but it's probably 1/4 or 1/2 (whatever HDAmodelworx includes).  Certainly not 8W, so that's probably why my resistors are too hot to touch after a few seconds.

So - I'm not going to be lighting just one bulb in the model - probably 30 or 40 individual LEDs, plus a ~3ft of LED tape.  Setting the LED tape aside - if I hook up 30 LEDs (each with it's own resistor) - this would be effectively pushing ~33mA (1A/30) to each resistor instead of the full Amp to one?  So that would be (12-3.3V)*.033 = .2904Watts, thus a 1/2W resistor on each LED should work fine and not be hot?  I will take your suggested .01A target into consideration - but I just want to make sure I am understanding all this correctly.


Thanks,
Brian

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
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Offline Tshark

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Re: Correct resistors running hot?
« Reply #3 on: August 17, 2018, 08:06:17 am »
You might also consider going with a 5V power supply.  Unless you need a full 12V for something else in the model, such as motors, you are just converting more than half of your supplied voltage into heat that must be dissipated inside your model.  You can get 5VDC LED tape and it is a simple matter of using a lower Ohm resistor for individual LEDs.  FWIW I have used HDA's SMD LEDs as is with the 5V power supply and found them bright enough.

Here is a discussion on 5VDC lighting on this board:

http://scifimodelaction.com/sfmaforum/index.php?topic=7377.0

Hope this helps

Offline Markeno

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Re: Correct resistors running hot?
« Reply #4 on: August 27, 2018, 08:38:10 pm »
Wattage = Volts * Amps

That is where the Volts is the Volts in that particular part of the circuit.  So if you have a single White LED let us say 3.2Volts spec at 20mA (.02Amps) and a 12 Volt supply like you are using.

W = (12V - 3.2V)*.02

W = 8.8*.02

W = .176Watts

So a 1/4th Watt Resistor (.25Watt rating) "won't burn up".  It will get hot though.  A 1/2Watt resistor will run cooler to the touch. 

The Wattage isn't marked on the resistors, but looking at some 1/8 1/4 1/2 1Watt resistors you will "see" a size difference. The bigger the resistor the higher it's wattage rating.  When you buy them they should say if they are 1/8 1/4 1/2 Watt resistors, then you can compare their size to others you may have and get a good idea their rating.  Over time they may have gotten a little smaller than the old days, but mostly they are pretty close even if they are 20 years old (unless they are a different manufacturing type).


For a 12 volt supply and a 3.2 rated white LED..  so at 20mA you would be looking around a 440 Ohm resistor.

R  = (12-3.2)/.02

R = 8.8/.02

R = 440

For a 12 volt supply the same 3.2 rated white LED and running it at 10mA you would be looking for around a 880 Ohm resistor.

R = 8.8/.01

R = 880

W = 8.8*.01

W = .088Watts (half the Current = half the Wattage)

Because your Wattage in the circuit is half the heat generated is half and the resistors don't get nearly as warm.

In reality a 1k Resistor would be fine and a bit lower yet in both Current and Wattage although yet a bit dimmer.


----------

It gets more complex when you are then wiring several LEDs in parallel to a Single Resistor.  While the values all calculate the same, that doubles then triples then quadruples and so on the Wattage through the resistor


W = V*A

W = 8.8*(.02 + .02 + .02 + .02)

W = 8.8*.08

W = .704Watts  This will cook a 1/2 Watt resistor easily enough.  If you ran it at 10mA instead it would be half that, which wouldn't be as bad.   You can get away with doing it with a few LEDs if your resistors are high enough Wattage, and the LEDs are all identical.  You can't mix and match Whites and Reds and Greens, but you could wire 3 Reds or 3 Greens or 3 Whites in parallel to a single resistor.


--------------

The as was suggested to save heat in the resistors you can simply use a lower voltage source.  9 or 5 volts would reduce the resistor Ohms rating you need and the excess wasted voltage.

W = (9-3.2)*.02
W = 5.8*.02
W = .116

W = (5-3.2)*.02
W = 1.8*.02
W = .036

If you lower the voltage source you need a different Ohms value on your resistor to get the same brightness from the LED, but you can get the same brightness just fine.

R = (5-3.2)/.02
R = 1.8/.02
R = 90 Ohms for example

See less heat will then be at the resistors.  Resistors have to turn that excess "voltage" into heat, that is what they do essentially.

-----------

Another solution is to wire some of the LEDs in Series with a single resistor.

W = (12-3.2-3.2-3.2)*.02

W = (12-9.6)*.02

W = 2.4*.02

W = .048 Watts

R = (12-3.2-3.2-3.2)/.02

R = 2.4/.02

R = 125 Ohms  (round up to the next higher resistor value you have available)

This is three 3.2Volt 20mA rated LEDs wired in Series with a single series wired 125 Ohm resistor to the same 12 Volt supply.  There will be very little wasted energy to convert to heat.  If you do it a 10mA then you will find it is again half that Wattage and again only half the heat created at the resistor.

« Last Edit: August 27, 2018, 08:41:10 pm by Markeno »

Offline Markeno

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Re: Correct resistors running hot?
« Reply #5 on: August 27, 2018, 08:57:51 pm »
Rough Diagram Example.  The top being the not so great 4 of the same type LED wired in parrallel to a single resistor.  The second being 3 wired in series a much more efficient setup that generates far less heat.  If you wired up 4 of the same LEDs in parallel with their own individual Resistors, you have the same thing as the top, but the heat is spread out across 4 resistors instead of one.




Yes if you use series, there are draw backs.  A break in the circuit will mean 3 LEDs go out not just 1.  If you happen to see many LED strips with "sections"  not working but other sections working that would be the kind of thing that happened.  If you put to many in series and don't leave enough "headroom" and are using batteries, you may find the LEDs will quit working effectively before the batteries are drained very much.   Series is always mixed with Parallel anyways if you have more than a couple LEDs as each group of series wired LEDs are then wired in Parallel with the next group.

Offline FatalCheese

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Re: Correct resistors running hot?
« Reply #6 on: September 09, 2018, 08:56:44 pm »
Markeno, this was immensely helpful.  Thank you!

I figured out my resistors are 1/4 watt, and with what you've explained, this makes perfectly clear why at 9V, they aren't warm, but increasing to 12V made them too hot to touch.

Obviously have to make a call here on whether change my wiring and/or resistors (which will be a lot of re-soldering) or dropping the voltage, which impacts the brightness of the LED tape (which I can't change the resistors on), or maybe replacing with 5V tape as Tshark suggested. Things to chew on. 

Thanks again!

Brian
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Offline Markeno

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Re: Correct resistors running hot?
« Reply #7 on: September 12, 2018, 08:32:38 pm »
You are welcome.

If you have added a resistor on some 12Volt tape, you really don't need it unless it was too bright to start at 12Volts for your needs.  I think though you just mean you can't change the little surface mount resistors on the tape (which certainly is not a very practical thing to do).

If you want to run the 12Volt tape at 12 Volts, but possibly run the rest at 9 Volts (or lower). You can get a little buck converter.  Hook the 12Volt tape to the 12Volt Supply.  Get a Buck Converter and hook the 12 Volt supply to it, and set it to 9Volts or whatever you want even 5Volts..  Then hook the rest of the LEDs to the Buck Converter Output.    They are often rated at 3Amps but I don't think you should use them much past 1Amp, but you are unlikely to get anywhere near that with LEDs in a single model doing at most .02Amps per LED (1Amp=50 LEDs at .02Amps) Then all of your resistors are based on the Buck Converter output Voltage, letting you use lower value resistors if you like and have them generate less heat.

Here is a link to some similar to the ones I have.  There are alot of different designs, some bigger some smaller, some larger ones with Voltage readouts..
https://www.amazon.com/UPZHIJI-LM2596-Converter-3-0-40V-1-5-35V/dp/B07BLRQQK7/ref=sr_1_8?ie=UTF8&qid=1536799727&sr=8-8&keywords=buck+converter

You could also use fixed output 7805 (5Volt) or 7809 (9Volt) regulator, which is cheaper yet typically.  The difference is they are "Linear Regulators", and the Buck Converter is a "Switching Regulator". 

Linear Regulators work the same as a resistor sort of, they will produce Heat with the excess Energy that they need to "burn" off just like a Resistor.  At low current they won't make much heat, but a 7805 will be dropping over half the 12Volts supplied to it as heat (dropping 7Volts converting to heat and 5Volts out)  The 780x Regulators come in a few Amp ratings, the cheaper models are 1Amp I believe max, and then they generate enough heat to generally require a heatsink.  At lower current levels, they will generate far less heat, and that is the Current passing through, so if you hook up an LED and it is using .02Amps that is all that is at the Regulator, but at 30 LEDs at .02Amps you then have .6Amps at the Regulator at 7Volts (for a 12Volt supply and a 5Volt regulator) 4.2Watts of heat if I am doing the math right.   

The Buck Converter on the other hand, is actually turning on and off very fast, letting through just enough "averaged" voltage to output 9 Volts or 5 Volts or whatever it is set to (adjustable with the little trimmer potentiometer on it), so they create a lot less heat and waste a lot less energy.  There are negatives to them if you really want to look at the details, but I won't get into that at this time as that isn't very relevant here.  The very minimal wasted energy and therefore very little heat produced by the Buck Converter makes it a great candidate for model lighting.  Plus they can be very small (although there are larger ones with Output voltage displays etc, which if you didn't have a variable power supply could be used as a cheap one).

There are other fixed voltage switching regulator options that are kind of like the 780x series regulators in how you hook them up, and other designs.  So there are a lot of options. 

A few things to keep in mind though with Buck Converters, as they are variable output, you do need to adjust the voltage before attaching something to it, they are likely going to be set to a rather "High" output voltage at the factory.  You also generally need to use a multimeter (or voltmeter) attached to the output to be able to see what it is set to so you can get it adjusted to the voltage you want.  Once you set it just don't turn the trimmer again and it will stay on that output voltage, possibly put on a dab of glue to "lock it" from turning, maybe a type you can remove if you need to (and nothing too liquid that it will run in side causing it to short internally).  I went through the ones I purchased and preset them down to 5Volts so I wouldn't forget and hook something to them I didn't want to get 12Volts or something.. 

 




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