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Scion xA/xB 1st-Gen ICE & Interior In-car entertainment and electronics...

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Old 05-11-2005, 10:20 PM   #1
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Default LED Load Resistors getting VERY HOT!!!

In the process of converting my 7440 tail blinker bulbs to LEDS, I encountered that the 6Ω load resistors I'm using were getting waaay to hot for my liking. (for those who dont know, load resistors are used to keep the low-amp leds from blinking too fast) In fact I measured their temp @ 270+ degrees F!!! That temperature was
achived on a nice 75*day with the blinker going for 7 minutes, who knows how high it can get on a 100* day....maybe 350+! That temp is enough to melt thin plastic and definately set that foam insulation found between the tail light / body on fire. Now I can mount the load resistors on the tail light bolts so that they do not touch anything sensitive, but that still is risky to me. I know that these resitors will be running for only small amounts of time while turning or what not but I have to factor in the worst case scenario. For instance if I leave the hazards on for like 2 days (by mistake or whatever), it would get mighty hot in there and platic could start to melt,drip, catch fire and bye bye tc.
The only way I can think of fixing this issue is by changing the resitance of the load.

Now comes the question......Which load resistor would generate the least heat?

LINK
Click the image to open in full size.
3Ω ?
6Ω?
or
15-25Ω ?

Im hoping that the 3Ω resistor would run cooler since it is a lower resitance, but then again im not an electrician

So what do you think guys? go with a different load resistor? or any other bright ideas? lol
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Old 05-11-2005, 11:05 PM   #2
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your issue isnt resistance, you need to get a larger wattage resistor to deal with the amount of current you are pulling...

how big (watts) is the one you are using?
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Old 05-11-2005, 11:20 PM   #3
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you need to draw this curcuit out for me I am an Electrician If I see the formula I can tell you exactly what to do!! but flecom is right its a wattage issue tomuch DRAW
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Old 05-11-2005, 11:28 PM   #4
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well, guessing from the wire splice thingies (i always forget their name) im scared to think that he may be putting it in parallel, turning it into a giant heater... but hopefully its in series with the LED lights... then the size of the resistor would depend on the current draw of the LED's
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Old 05-12-2005, 12:01 AM   #5
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Click the image to open in full size.

I beleive it is supposed to be in parallel like the picture states.


6 OHM RESISTOR
According to my math It is pulling 24 watts

I got that by 12 volts / 6 Ohms = 2 Amps

2 Amps x 12 volts = 24 watts


So I am assuming that using the 15 / 25 ohm resistor would lower the wattage draw:

12v / 20 ohm = .6 amps .6amps x 12 volts = 7.2 watts

If my math and logic is right, then the higher Ohm resistor will cause a lower power draw......around 7 watts give or take. Hopefully this means that the risistor will not heat up too much.

Felpro you have a good point, but wouldnt the resistor still heat up?
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Old 05-12-2005, 03:38 AM   #6
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That is correct, LED-Maniac. (All of it.)

One should use 15V when figuring out how much power (watts) a load will have to handle in a 12V vehicle, because the normal charging voltage if around 14.8V...

So, it works out like this:

(voltage squared) divided by (resistance) equals (power in watts)

(15 x 15) / 3 = 75

3 ohm

225 / 3 = 75 watts

6 ohm

225 / 6 = 37.5 watts

12 ohm

225 / 12 = 18.75 watts

20 ohm

225 / 20 = 11.25 watts.

One needs a resistor that can handle AT LEAST that many watts to prevent the resistor from burning out, and most resistors in those sizes depend, to at least some extent, on transferring the heat they generate to the surface they are mounted on.

In any case, the resistors will put out that much heat regardless of their wattage rating and the installation needs to be able to handle that heat because it will, it MUST transfer to the surroundings. The resistor can't just "absorb" it.

If the resistor is not mounted to a heat absorbing structure (as it should be - complete with heat sink grease, etc.) it still has to shed that heat. It will get much hotter because the heat is not being conducted away from it, and if it is insulated or has restricted air flow it will be even worse.

Those power resistors need to be mounted with as much direct contact as possible to some sort of heat sink (frame, body, actual heatsink, whatever)...

Good luck!
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Old 05-12-2005, 05:28 AM   #7
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i would personally recommend instead of using a load resistor use a different (solid state?) blinker with some sort of adjustable timing... that really would be the safest thing
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Old 05-12-2005, 06:16 AM   #8
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Thanks for the awesome response Tomas. Apparently heat is the nature of the beast here and wont be able to get rid of it without mounting the resistor to a piece of the fender. Some testing I have done here shows minimall reduced temperatures when mounted correctly.

Im starting to become wary about this install project, i dont want a burnt up tc!


I was considering though, installing a cooling fan over the resistor, but that is just a little too much i guess.
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Old 05-12-2005, 12:27 PM   #9
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seriously your best/safest bet is to find some sort of solid state blinker that you can adjust the timing on it... putting a resistor from +12 to 0 in a car isnt the greatest idea anyway
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Old 05-12-2005, 05:47 PM   #10
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I'd have to agree with FLECOM.

Loading the flasher with an external resistor that has to get rid of that much heat is a much poorer solution than a flasher that wouldn't need that external load.

The question is: Are solid state flashers available that fit the Scions?

Probably, but I don't know.
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Old 05-12-2005, 06:35 PM   #11
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Guys, I just called Scion and they have stated that the blinker relays for these cars are so new that you cant even get the replacements on the market. Hence it seems that unless I somehow modify the current flasher, then this project is dead Safety really is the biggest issue here and you guys helped me realize that!

I also have a wiring diagram of the tC flasher turn signal circuit, and the relay is a monster! It definately isnt an off the shelf part @ pepboys. Upon closer inspection it seems to be solid state...i.e. no moving parts. It has 27wx2 stamped on it twice. So unless I can find a lower wattage one, this isnt happening grrrr!
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Old 05-12-2005, 06:43 PM   #12
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How about not leaving the blinker on for 7 straight minutes? Under normal conditions, you shouldn't have a problem.
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Old 05-12-2005, 07:32 PM   #13
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The four-ways are usually off the same flasher unit and sometimes they will be on for l o n g times, Max...
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Old 05-13-2005, 04:27 AM   #14
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Quote:
Originally Posted by Max2k
How about not leaving the blinker on for 7 straight minutes? Under normal conditions, you shouldn't have a problem.
I wish it were that easy, but lets say if i have to leave the hazards on for a long time......car broken down on the side of the road, then I dont want to have a problem.
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Old 05-14-2005, 06:31 PM   #15
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ANother option to the load resistors is just putting a standard incandescent (wire filament, like stock) light bulb in parallel with the LED bulb. You still have to find a place to mount it and keep it away from anyhting that may melt, but it does work.
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Old 05-15-2005, 07:03 PM   #16
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xB Nutt, thats an awesome idea!
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Old 05-15-2005, 07:31 PM   #17
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Quote:
Originally Posted by LED-Maniac
Quote:
Originally Posted by Max2k
How about not leaving the blinker on for 7 straight minutes? Under normal conditions, you shouldn't have a problem.
I wish it were that easy, but lets say if i have to leave the hazards on for a long time......car broken down on the side of the road, then I dont want to have a problem.
then just keep your stock bulbs in your glove compartment and swap em out if yer car ever did break down. its not like they are hard to change.
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Old 06-09-2006, 05:16 PM   #18
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how much of a real risk is it to have these resistors in? i just installed them on my led tails... and placed the resistor in the middle of the backing of the lights. the back end of the resistor is up against the back of the taillight..but if youre sayin that it can really cause a fire..im scared. i'm thinkin of takin them out..and just gettin the relay flashers..i assume thats the esasiest and safest. but if i keep those resistors in.. will i really set my car on fire??? im scared ahhhh...
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Old 06-10-2006, 02:38 AM   #19
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I wouldn't be terribly concerned about starting a fire, but definitely about melting any plastic that the resistor might be touching- like the taillights.

Modern flashers are extremely load sensitive, deliberately so. Some will flash very fast with insufficient load, and some won't flash at all. Either way, it's telling you that there's a bulb out...

Which is why a load resistor is required with LED turnsignal replacement bulbs- they hardly draw any current at all.

Use an ohm meter to measure the resistance of the led display- it should be very high. Now measure the resistance of the bulb you're replacing, it'll be pretty low, and is approximately the value of the resistor required to get things to work right. There's a specific formula for doing this, but it's not much of an issue when one of the parallel resistors has a value orders of magnitude lower than the other...

It's important to remember that the parallel load resistor can be placed anywhere, so long as the wire gauge feeding it is adequate. It doesn't need to be by the taillight- it can be under the dash or in the engine compt is you're willing to go to the trouble to wire it up that way. And inexpensive computer processor heatsinks will do a really good job of dissipating the heat from a resistor like the one pictured- find a safe place to mount the heatsink, where some air can circulate, drill some holes for that and to mount the resistors, screw 'em down with some heatsink paste...

If the heatsink comes with a fan, you really don't need it for this application.
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Old 09-12-2006, 06:35 PM   #20
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Quote:
Originally Posted by LED-Maniac
Guys, I just called Scion and they have stated that the blinker relays for these cars are so new that you cant even get the replacements on the market. Hence it seems that unless I somehow modify the current flasher, then this project is dead Safety really is the biggest issue here and you guys helped me realize that!

I also have a wiring diagram of the tC flasher turn signal circuit, and the relay is a monster! It definately isnt an off the shelf part @ pepboys. Upon closer inspection it seems to be solid state...i.e. no moving parts. It has 27wx2 stamped on it twice. So unless I can find a lower wattage one, this isnt happening grrrr!
Hey will this work. ...... http://autolumination.com/equalizers.htm
They have one for NEW TOYOTA'S (will that work)
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Old 09-12-2006, 06:35 PM
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