I placed a yellow LED in a building to illuminate it. Used a 470 ohm resistor but it get too hot to touch, replaced with 220 ohm resistor, but it gets even hotter. What am I doin wrong. Many thanks Andy
What’s your power supply voltage? If it’s too hot use a larger value, not smaller. Try a 1K 1/4 w resistor.
The resistor value is probably fine, you just need a higher wattage rating. Those tiny resistors you get free with lights from China are usually under rated (1/16 - 1/8 watt). You should use at least 1/4 watt for general use, and if space permits, even 1/2 watt.
Mark.
reducing the resistor value from 470 to 220 results in higher current (see Ohms Law). using a 1k 1/8W resistor would be more acceptable. the table below shows the current and power (mW) for various resistor value and the required wattage rating of the resistor
Vcc Vd R ma mW
12.00 1.40 1000 10 112 < 1/8W
12.00 1.40 560 18 200 < 1/4W
12.00 1.40 470 22 239 < 1/4W
12.00 1.40 220 48 510 < 1/2W
1 K, is fine. I have used 1 K, 1/8 watt. I had many 1/8 watt in my supply. Many used 1/4 watt. There are calculators online.
Rich
thanks for your kind reply. Is the “R” valuse the same as Ohms? I bought a box of various resistors over the internet. They are labeled in R values, which I assumed was ohms. Also, the physical size of the resistors is the same, but their value can vary by powers of 10. Is this right? I would have assumed much higher resistance rating needs larger physical size, Many thanks, Andy
The physical size is determined only by their wattage rating, not their resistance value.
Mark.
One labeled 100R would be 100 ohms. But just numbers works differently. A resistor marked 102 is 1K - 10 plus 2 zeroes. This is normally only on suface mount resistors - leaded resistors are color coded. Those light blue bodies on newer ones, combined with very light printign of the colors, means I measure all of them, all of the time, despite knowing the color code by heart.
Size more likely denotes power than value. A larger value resistor generally is no larger than a lower one of the same power. but a 1 watt reistor is bigger than a 1/2 watt resistor is bigger than a 1/4 watt resistor is bigger than a 1/8 watt resistor. The way a resistor works, the size of the overall package doesn;t have to change for a change in value in ohms. But they do have to get bigger to dissipate more power.
–Randy
This is from a Google search.
https://www.futurlec.com/Resistors.shtml
I have used 1/8 watt from a Radio Shack kit I was given some years ago.
I started working with LED’s whn I worked for NASA in 1972.
Rich
Hi Andy,
In the case of a length of wire you would be absolutely correct. Doubling the length would double the resistance etc, etc.
Typically resistors of different values are made with different combinations of conductive compounds. That allows them to be the same size for a given maximum heat dissipation.
Andy
Many thanks All reply have been quite helpful and kind. One last thing. I have found 7 decade variable resistance testers. I was thinking of using them to test appropriate resistance for LED’s I intend for signals. Are these useful? Many thanks
That would work. I used for some years a potentiometer and meter.
Below is how to do that with two meters like I do sometimes. One meter to set the power supply. One to read current. Some years ago I got the meters for free or a couple dollars on sale.
http://www.trainelectronics.com/Meter_HF/index.htm
Rich
They might, but the main thing is to get a firm grip on the relationship between current and voltage, and a couple of inexpensive meters might be one way to do that.
A good analogy is to think of voltage as the pressure in a water pipe. Current is analogous to the volume of water flowing. A resistor is like a restriction in a pipe. That’s pretty much all there is to it 
Many thanks to all who answered my questions. You are very helpful. Blessings,