Having problem with one resistor getting really hot ... HELP !

Thread Starter

TuLaLiT

Joined Jul 27, 2009
7
Hey guys,

First of all thank you for reading this and I'm a total newbie to this so please be patient while I'm still learning :p

Anyway I'm currently confused as hell with the problem Ive got at the moment.

Long story short I'm currently designing my own LED Tail lights for my car.
I've got everything working perfectly fine on the car except there is one resistor ( the 39ohms ) it gets very very hot even after a short while of turning the tail light on.

The resistors I used for my project are 0.5 watts. I went to picked up 5watts 39ohms resistor but even that one still gets a bit too warm for my liking.

I have included the schematic hopefully it makes sense to you guys. I didn't actually use any diodes even though it shows on my schematic ( not gonna make any difference right ? )

I used the 2.2ohms and 39ohms resistors to adjust the brightness of my LEDs

What did I do wrong here ?
 

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crutschow

Joined Mar 14, 2008
25,269
I didn't actually use any diodes even though it shows on my schematic ( not gonna make any difference right ? )
If you meant the two right after the resistors, yes it's gonna make a difference, otherwise why would they be there? o_O
Without the diodes, the tail light circuit will supply current to the stop light circuit, greatly increasing the current through the 39 ohm resistor and likely causing the overheating you are seeing.
 

Thread Starter

TuLaLiT

Joined Jul 27, 2009
7
Hi crutschow,

Being all honest the reason that I didn't use it because I didn't actually know what diodes actually for.

So you reckon if use diodes it will solve the problem I have ?

Thanks mate !
 

tranzz4md

Joined Apr 10, 2015
310
- Both the 2.2 and 39 resistors carry the entire array when energized, so their ratings and power dissipation should be evident.
- measuring the voltages across points, and currents actually carried are useful in troubleshooting. Arrays give many opportunities for poor connections, which result in substantial imbalances and resultant heating and additional failures.
 

crutschow

Joined Mar 14, 2008
25,269
Being all honest the reason that I didn't use it because I didn't actually know what diodes actually for.
Well, that's not a very good reason not to use them. :rolleyes: Do you disconnect and remove all the parts on your car that you don't know what they do?

Diodes only conduct current in one direction so they prevent the brake light current from getting to the tail light circuit and vice versa.
So you reckon if use diodes it will solve the problem I have ?
I would expect so, unless there's some other error you made. ;)
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
4,925
With any single LED having a forward voltage of 3 volts, four of them in series would equal 12 volts. Meaning you have to dissipate the energy of the remaining voltage through the resistors. The additional diodes will also have some forward voltage drop as well, which must also be calculated in.

Looking at your calculation sheet, I'm wondering if you used it properly. It would seem you told it you have 12 LEDs when in fact you have three sets of four, each with their own resistors. And I'm assuming that 50 mA is rather high for LED's. I'd not expect them to live long at that kind of current.

Many times sitting at a red light behind a car with LED tail lights, sweeping my eye across the back of the car I could see the LED's strobing. Meaning they're not always on. Your system seems to have them on at all times, which increases the heat buildup. Since I'm not familiar with how OEM LED tail lights are designed (OEM = Original Equipment Manufacturer) I couldn't make any satisfactory contributions to how you should design your tail lights.

Another thing about exterior lights on a vehicle, they must be DOT (Department of Transportation) approved. Meaning you can't just hang any kind of lights on your vehicle. Even some after market kits are not DOT approved. Before you continue I think you should research those two details, how OEM LED tail lights are designed and what the local DOT requirements are for your area before you hang something on your car and get ticketed. Or worse, have them fail and then you get rear-ended only to have it discovered that you did not have DOT approved lights on your car. You'll likely be held liable for the damages, and I'd imagine your insurance might even be able to deny coverage for such an accident; meaning you could expose yourself to some serious financial troubles.

Long story short - be careful.
 

ElectricSpidey

Joined Dec 2, 2017
1,196
You need to make a proper schematic.

What is connected to the other ends of the 2.2 & 39 ohm resistors…the switches that connect to positive?

If the positive is connected as the diagram shows the LEDs would never turn off.

And if both resistors are connected to positive through switches (brake pedal, dashboard) then those diodes are irrelevant.
 

Thread Starter

TuLaLiT

Joined Jul 27, 2009
7
Diodes only conduct current in one direction so they prevent the brake light current from getting to the tail light circuit and vice versa.
I would expect so, unless there's some other error you made. ;)
Hi, Crutschow !

Just wanna say thank you for the input. Once I installed the diodes everything working flawlessly :)
Who would've thought they played a major role in my circuit.
I definitely learned something new :)
Also thank you for all the other members with their inputs

So here are the finished products :)
 

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MisterBill2

Joined Jan 23, 2018
6,755
The purpose of the diodes was to prevent current from flowing back out of the connection and into the rest of the system, so of course without them it would not function correctly. But as I look at the pictures it seems that the diodes may be a bit more powerful that the 50mA 3volt description states. This is an interesting application and it would be interesting to see what it looks like after a few months of use. So please post a description of how it is looking then, about in August, so that we can understand how well it keeps working..
 

Hymie

Joined Mar 30, 2018
848
Note well Tonyr1048’s last paragraph of his post – if you are in the UK, fitting non-type approved vehicle lights will result in an MOT failure. Other European countries will have similar legislation in force – as will most countries around the world.

So although the lights look very nice – they are almost certainly illegal.
 

MisterBill2

Joined Jan 23, 2018
6,755
Note well Tonyr1048’s last paragraph of his post – if you are in the UK, fitting non-type approved vehicle lights will result in an MOT failure. Other European countries will have similar legislation in force – as will most countries around the world.

So although the lights look very nice – they are almost certainly illegal.
This is probably true, but my observation is that if the lights don't attract attention then probably you won't have any problems. That is, if the brightness is similar to other vehicles and your driving does not attract the attention of the law you should be OK.
But if you live in one of those police-state countries where conformance to being just like everybody else is mandatory it may be different. Not naming names, but if you live there then you know what I mean.
 

Tonyr1084

Joined Sep 24, 2015
4,925
my observation is that if the lights don't attract attention then probably you won't have any problems.
Tail lights are supposed to attract attention. Not admiration. They may appear just as bright but the real test is whether they can be seen in full direct sunlight or under some weather conditions - I don't know what the reg's are, but they must be DOT approved. And some cop out to make a name for himself just might spot your fancy lights and say "UUMMmmmm. Looks like I got me a good one!"

Nevertheless, this is your vehicle and it's up to you to do what you believe is right. And there's a reason why unregulated lights are not allowed. They may confuse other drivers. YOU may know what the lights mean, but what about some granny who's never seen lights like yours before.

Hey! Don't let me rain on your parade. It's your project. Do as you choose. And live with whatever outcome may be.
 

Thread Starter

TuLaLiT

Joined Jul 27, 2009
7
Hi guys,

I live in New Zealand and the police down here they are quite relax.
A shop in Japan actually used to make aftermarket tail lights for the car I have but they have been discontinued.
So all I did just copy their design.

They can be seen easily during full on direct sun light.

We do get our car inspected every year to a testing station but I'm not worry about that since I have a friend in the inside "wink wink"
 

Thread Starter

TuLaLiT

Joined Jul 27, 2009
7
The purpose of the diodes was to prevent current from flowing back out of the connection and into the rest of the system, so of course without them it would not function correctly. But as I look at the pictures it seems that the diodes may be a bit more powerful that the 50mA 3volt description states. This is an interesting application and it would be interesting to see what it looks like after a few months of use. So please post a description of how it is looking then, about in August, so that we can understand how well it keeps working..
Hey mate,

Since I installed the diodes the "TAIL" lights were wayyyy too bright so swapped the 39ohm to 270ohm and the "BRAKE" stays at 2.2ohm.

I have tested the lights on the car running on "TAIL" only for 1/2 hour and "TAIL+BRAKE" for 1/2 hour. On both occasions NONE of the LEDS or resistors even get wam let alone hot ( like previously ).

So finger crossed they will last for a long time :)
 

MisterBill2

Joined Jan 23, 2018
6,755
In response to the remark that the diodes are "irrelevant", WRONG!!
The diode in the tail light line keeps the braking signal from also feeding all of the running lights and front lights with the stop light power, and the diode in the brake lights line prevents the tail light feed from activating the center high mounted stop light.So those diodes DO matter. Besides that, very early in this discussion was the comment that a resistor was getting very hot without the diodes in the circuit.
 

ElectricSpidey

Joined Dec 2, 2017
1,196
In response to the remark that the diodes are "irrelevant", WRONG!!
The diode in the tail light line keeps the braking signal from also feeding all of the running lights and front lights with the stop light power, and the diode in the brake lights line prevents the tail light feed from activating the center high mounted stop light.So those diodes DO matter. Besides that, very early in this discussion was the comment that a resistor was getting very hot without the diodes in the circuit.
If you noticed...that was a "qualified" statement, based on a guess.
 

Thread Starter

TuLaLiT

Joined Jul 27, 2009
7
In response to the remark that the diodes are "irrelevant", WRONG!!
The diode in the tail light line keeps the braking signal from also feeding all of the running lights and front lights with the stop light power, and the diode in the brake lights line prevents the tail light feed from activating the center high mounted stop light.So those diodes DO matter. Besides that, very early in this discussion was the comment that a resistor was getting very hot without the diodes in the circuit.
You are correct sir I forgot to mention when I hadn't installed the diodes as soon as I plugged in the tail lights, the high stop light would light up without me pressing the brake pedal and that confused the living hell out of me LOL

But now ever since I installed diodes on the high stop light and tail lights everything works mint ! :)
 

-live wire-

Joined Dec 22, 2017
912
You should consider replacing the glorified shorts (I mean incandescent bulbs) with longer lasting and more efficient LEDs. It'll probably be beneficial in the long run. There is also generally a wider variety of colors and temperature.
 
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