Problem with resistor, it is very hot

whitstand kent

I just built a led project.
I use 15ohm 5 watt resistor.
When I measure it, the resistor consumes 5V only.
From the V = I x R, the current must be 0,34A. ( = 1,7 watt)

But the resistor is very hot. Is Resistor really hot ?


Budiman

Resistor is hot ?

Yes. Try applying 5 watts and it will get really really hot (burn finger).
Don't put it into a tiny enclosed space. It needs free air circulation.
Refer to its data sheet to be sure you aren't exceeding any ratings, such as thermal derating.

Resistor is hot ?

Firstly is the resistor intended to be mounted on a heatsink? If it is then do you have ba big enough heatsink and the resistor mounted properly.

Second "hot" is a subjective temp. 60degC can feel hot to a human finger but a wirewound resistor may be rated to run at 140degC case temperature.

Re: Resistor is hot ?

First, if you are driving a led the current is so small, because led has a voltage drop. Second: What type of Led are you using that need this current? Third: remember of Joule, than are you emiting heat power for the air (≈1.7W).

leomecma

Re: Resistor is hot ?

To leomecma :
I don't really understand this :
"if you are driving a led the current is so small, because led has a voltage drop."
Do you mean that power supply current is not enough? or the led voltage
is not enough ? BTW I find out that one led only consume about 1.75V, it should be 3 - 3,5V. And what is voltage drop means ? How can I measure it ?
This resistor is connected to 12 lines in parallel. And each line has
two leds and one R (82,5 ogm) in series. All Led are same type.
That's why it take 0,34A. And I don't want to emit heat like this, but I don't know
how to avoid it.

I have turn on the led for 5 hours, and there is no problem, except the heat.

Adding heatsink, is the last option, because heatsink is expensive and take a lot
of place.


Budiman

Added after 2 hours 29 minutes:


To leomecma :
I don't really understand this :
"if you are driving a led the current is so small, because led has a voltage drop."
Do you mean that power supply current is not enough? or the led voltage
is not enough ? BTW I find out that one led only consume about 1.75V, it should be 3 - 3,5V. And what is voltage drop means ? How can I measure it ?
This resistor is connected to 12 lines in parallel. And each line has
two leds and one R (82,5 ogm) in series. All Led are same type.
That's why it take 0,34A. And I don't want to emit heat like this, but I don't know
how to avoid it.

I have turn on the led for 5 hours, and there is no problem, except the heat.

Adding heatsink, is the last option, because heatsink is expensive and take a lot
of place.


Budiman

Added after 1 minutes:


To leomecma :
I don't really understand this :
"if you are driving a led the current is so small, because led has a voltage drop."
Do you mean that power supply current is not enough? or the led voltage
is not enough ? BTW I find out that one led only consume about 1.75V, it should be 3 - 3,5V. And what is voltage drop means ? How can I measure it ?
This resistor is connected to 12 lines in parallel. And each line has
two leds and one R (82,5 ogm) in series. All Led are same type.
That's why it take 0,34A. And I don't want to emit heat like this, but I don't know
how to avoid it.

I have turn on the led for 5 hours, and there is no problem, except the heat.

Adding heatsink, is the last option, because heatsink is expensive and take a lot
of place.


Budiman

Re: Resistor is hot ?

Sorry, but you just explain your configuration now, and I was thinking that you are driving the led directly on 5V using a 15R resistor.

How you are using 1.7W , that represent only 34% of maximum power of component it hasn't problems. Why did you heatsink it? If your equipament hasn't air circulation you just have to measure the real temperature on component and look in datasheet if it support this temperature (put your equipament in worse operational case) if it don't exceed the maximum temperature supported, no problems. Some resistors has best heat conduct packages, you can look for one. But if you use a 5W resistor or a 10W resistor with same package, the temperature will be the same, because power lost will be the same ...

**broken link removed**

leomecma

Resistor is hot ?

Resistors consume power, and convert it to heat.
IT IS NORMAL.

Re: Resistor is hot ?

You must lower supply voltage for 5V. In this case you will not need 15ohm resistor.

Resistor 15ohm 5W can whitstand dissipation of 1.7W with no problems.

Resistor is hot ?

Well, if you dont like to have power dissipated unnecessary - drive Leds through pulsed supply . Can experiment with switch on transistor controlled with pulses where the duty is choosen to accomplish necessary voltage or current . If diodes can take short time load - you can connect them through low ohmic resistor , otherwise - through choke to get voltage filtered and protect leds from high short time current . One simple HC series chip and BJT transistor will replace your hot resistor.
so, if you want cold resistor - you have to pay for that. Though in some northern countries this resistor can extend temperature range for product you are designing (if you gonna use it as heating element).))

Re: Resistor is hot ?

I have turn on my led project for 10 hours, and everything
still ok.

BTW I have one , more question :

If I have a Led 2,5V 20mA, can
I connect it directly to 5V 10ma power supply (eg. if output from TTL/Cmos = 5V 10mA) WITHOUT any resistor ? Is there any risk doing this ?


Budiman

Resistor is hot ?

TTL or CMOS ICs have output equvalent resistor.
and it reduced output current.
Thus with regards to IC output specification you can connect
this LED to that output.