# LED Troubles

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#### gp3000000

##### New Member
Hi, I'm new to the forums.

I'm trying to wire up some LEDs for a circuit, using a 9V supply, really basic stuff, however I've bought **broken link removed** and did this calculation:

6V/0.02A = 300ohms, so I've used 360ohms, as it was all I've got at the moment so in testing the circuit used these, however I noticed they were getting burning hot!, in a very short time. So I measured the voltage across the resistor and the current through, and using P = IV: P = 0.0185A X 6.82V = 0.126W.

Using 0.25W resistors so I don't see why they are getting hot? Any ideas? I bought some white LEDs from the same seller and they work as expected, no hot resistors there.

#### Boncuk

##### New Member
You've probably purchased a bunch of low current LEDs. Connect them to a constant current source of 2mA or use a current limiting resistor of 3.3K and see what happens.

Boncuk

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#### Roff

##### Well-Known Member
You've probably purchased a bunch of low current LEDs. Connect them to a constant current source of 2mA or use a current limiting resistor of 3.3K and see what happens.

Boncuk
He said that the resistors were getting hot.

Gp3000000, are you sure they aren't 1/8W resistors?

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#### gp3000000

##### New Member
I've never seen an 1/8W resistor, so don't know what one looks like, **broken link removed** are what I'm trying to use, they're supposed to be 0.25W but I suppose it isn't unthinkable that the wrong ones may have been sent?

#### audioguru

##### Well-Known Member
A resistor at its max allowed power dissipation is extremely hot. Even if its dissipation is only half its max rating it will burn you.
The wires on a little resistor cool it if the wires are short.

#### gp3000000

##### New Member
audioguru: I did consider this, however my previous experience of electronics leads me to struggle to believe it. I never remember the resistor getting burning hot, even warm anytime I've used LEDs in electronic projects in the past. The LEDs would get warm if overdriven for extra brightness, but never the resistors!

Just tried a new 0.25W 360ohm resistor from Maplins and it gets hot exactly the same - this is annoying as I've never worried about fire so much before when wiring LEDs up!

#### gp3000000

##### New Member
audioguru: I did consider this, however my previous experience of electronics leads me to struggle to believe it. I never remember the resistor getting burning hot, even warm anytime I've used LEDs in electronic projects in the past. The LEDs would get warm if overdriven for extra brightness, but never the resistors!

Just tried a new 0.25W 360ohm resistor from Maplins and it gets hot exactly the same - this is annoying as I've never worried about fire so much before when wiring LEDs up!

P.S. Why does it take hours for my posts to appear on this forum? When I try to repost it tells me I've already posted, but redirects me to this thread and there's nothing there?!

#### audioguru

##### Well-Known Member
I can't find the datasheet for a 1/4W resistor but I think its surface temperature is about 200 degrees C at 1/4W. At 125mW its surface temperature is still well over 100 degrees C. You are burnt at 70 degrees C and more.

Your first few posts are reviewed by a moderator (maybe who is asleep in a different time zone than you) to ban SPAMMERS. After a few posts then your postings will be immediate.

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#### gp3000000

##### New Member
So I'd be best to go for 0.6W metal film resistors or similar? And write off the 200 1/4 watt ones I've bought. Hmm. Lesson learned.

You'd have thought I'd have encountered this before having done A-Level Electronics

#### audioguru

##### Well-Known Member
Your 1/4W resistors operate at only 126mW. They are not too hot. They are fine if you don't touch them.

#### gp3000000

##### New Member
Do you think so? They are to go in a car, so obviously safety is paramount.

#### RODALCO

##### Well-Known Member
Did you do an actual ohm reading of the resistor ?

If it was marked wrong a higher current could flow and cause overheating.

A ¼ watt resistor of 390 or 470Ω should be adequate.

#### gp3000000

##### New Member
I have measured them yes, and they are correct.

#### Roff

##### Well-Known Member
So I'd be best to go for 0.6W metal film resistors or similar? And write off the 200 1/4 watt ones I've bought. Hmm. Lesson learned.

You'd have thought I'd have encountered this before having done A-Level Electronics
I don't know what's going on here. I put a 1/4W, 200 ohm resistor across a 5V (measured 5.14V) supply, connected by clip leads at the extreme ends of the resistor leads for a little thermal isolation, and I had to touch the resistor to my lip to detect the warmth.

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#### audioguru

##### Well-Known Member
I wonder if cheap Chinese resistors concentrate the heat into one small spot?

##### Banned
You said you measured the voltage across the resistor but not the current, are you sure you're getting the current you think you are?

#### gp3000000

##### New Member
I measured both, the current was 0.0185A (18.5mA)

##### Banned
I'm gonna side with audioguru at this point, it may just have been a tiny spot that was heating. Even if it was only 1/10th the area of the resistor that was getting hot your finger would think the whole thing was. Human perception is a HORRIBLE method of measuring temperature, because it can be easily fooled and it's relative not absolute. I bought an IR non-contact thermometer for these kinds of things, I found out though that such a small target as a resistor doesn't read properly =( Works great on mosfets or larger packages though.

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#### Boncuk

##### New Member
He said that the resistors were getting hot.

Gp3000000, are you sure they aren't 1/8W resistors?

The sentence is good for any interpretation!

#### Roff

##### Well-Known Member
The sentence is good for any interpretation!
Maybe to a German in Thailand.

I'm just giving you a bad time.

I had to read his post 2 or 3 times before I was sure he was talking about the resistors.

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