## Resistors are too warm

Hello,

I am building a multi-touch surface using the FTIR method and after the surface is powered for about 10 minutes there is a heated smell in the room, I have noticed that the resistors are very warm and that a couple have started to brown or burn. I really need this table to be able to be safely powered at least 18 hours a day.

Here are my specs:

I am using 88 Infrared LED's (T 1 3/4 INFRARED LED, http://www.allelectronics.com/make-a-store/category/340250/LEDs/Infrared/1.html )

--8 LED's per series

--forward voltage of 1.5v

--forward current of 100mA

I am using 2, 10 ohm 1/4 watt resistors parallel (so 5 ohms) with each of the 11 series'

The power supply output's 12 volts DC and 1.2 amps

When I calculate using http://led.linear1.org/led.wiz it says I only need 1ohm 1/4 watt resistors, I haven't tried this yet but it doesn't seem to me that this would rectify the situation. My guess was that I need 1 watt resistors, but the calculator didn't seem to agree.

Any thoughts as to what my problem is? is it safe to go with 1 ohm 1/4 watt resistors? or do I need 1 watt resistors? Any help would be very appreciated, thank you!

I am building a multi-touch surface using the FTIR method and after the surface is powered for about 10 minutes there is a heated smell in the room, I have noticed that the resistors are very warm and that a couple have started to brown or burn. I really need this table to be able to be safely powered at least 18 hours a day.

Here are my specs:

**LED's:**I am using 88 Infrared LED's (T 1 3/4 INFRARED LED, http://www.allelectronics.com/make-a-store/category/340250/LEDs/Infrared/1.html )

--8 LED's per series

--forward voltage of 1.5v

--forward current of 100mA

**Resistors:**I am using 2, 10 ohm 1/4 watt resistors parallel (so 5 ohms) with each of the 11 series'

**Power Supply:**The power supply output's 12 volts DC and 1.2 amps

When I calculate using http://led.linear1.org/led.wiz it says I only need 1ohm 1/4 watt resistors, I haven't tried this yet but it doesn't seem to me that this would rectify the situation. My guess was that I need 1 watt resistors, but the calculator didn't seem to agree.

Any thoughts as to what my problem is? is it safe to go with 1 ohm 1/4 watt resistors? or do I need 1 watt resistors? Any help would be very appreciated, thank you!

active| newest | oldestyou can split the power to multiple resistors like this

say you need 10 ohm 0.8 watt

connect 4 X 2.5 ohm resistors in series

you get 4 X 2.5 ohm = 10 ohm and 4 X 1/4 = 1 watt

if you use different resistance reistors the larger ones (in ohms) get most of the power - it does not spread evenly

to keep resistors cool dont load them above 1/2 of their watt

to keep resistors warm but not hot dont load them above 3/4 of their watt

for example we need 100 ohm resistor to put on 10 V

10 V / 100 ohm = 0.1 A (current thru the resistor)

0.1 A * 10 V = 1 W (power on the resistor)

and we want to use 4 X 1/4 watt resistors instead of 1 X 1 watt

series connection

lets implement it with 4 X 25 ohm 1/4 watt reistors

4 X 25 ohm = 100 ohm

10 V / 100 ohm = 0.1 A (sum resistance remains the same --> current remains the same)

now lets find the power dissipated in one of the 4 resistors

0.1 A (series circuit - current thru one is current thru all) * 25 ohm (one resistor) = 2.5 V

0.1 A * 2.5 V = 0.25 W (on one resistor)

each resistor dissipates 1/4 W

parallel connection

lets implement it with 4 X 400 ohm 1/4 watt reistors

400 ohm / 4 = 100 ohm

10 V / 100 ohm = 0.1 A (sum resistance remains the same --> current remains the same)

now lets find the power dissipated in one of the 4 resistors

10 V (parallel circuit - volt on one is volt on all) / 400 ohm = 0.025 A (crrent thru one)

0.025 A * 10 V = 0.25 W (on one resistor)

each resistor dissipates 1/4 W

2 X 2 net connection

lets implement it with 4 X 100 ohm 1/4 watt reistors

we connect 2 in series to make 200 ohm 1/2 W and then connect the 2 strings in parallel to make 100 ohm 1 W

100 ohm * 2 = 200 ohm (one series string)

200 ohm / 2 = 100 ohm (all)

10 V / 100 ohm = 0.1 A (sum resistance remains the same --> current remains the same)

now lets find the power dissipated in one of the 4 resistors

10 V (series circuit - volt on one string is volt on all) / 200 ohm (one string) = 0.05 A (crrent thru one string)

0.05 A * 100 ohm = 5 V (on one resistor)

0.05 A * 5 V = 0.25 W

each resistor dissipates 1/4 W

the resistors are inexact but the maximum possible oveload due to that is not significant. you can use more resistors and then you dont load them to the maximum

There is only one current path, so the current through each series resistors is the same. If the current at each resistor was measurably less, how could there suddenly be

morecurrent at the end?