# COMMUNITY : FORUMS : TECH

## 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:

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!

sort by: active | newest | oldest
 1-10 of 25 Next »
110100101109 years ago
use higher power resistors

you 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

9 years ago
Part A is correct, while part B isn't. You can not multiply the wattage, as ALL of the electrons are flowing through ALL of them. Your thinking of the (theoretical) properties of them both in series and parallel. But they can't be in both at the same time. Theoretically, it is possible to do it in series and then put four sets of those in parallel, but it is impossible in the modern day world. if the resistors where created with exactly the same resistance., it would work. But it will not work, because modern-era technology isn't capable of that technologically advanced processing, especially in a mass-market way.
9 years ago
if your resulting resistor (from series connection) is the same ohm (in sum) as the original then it must dissipate the same power

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
9 years ago
Parallel resistors distribute the current across each resistor, series resistors do not.

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 more current at the end?
9 years ago
there cannot be more current the existing current divides to the number of parallel paths. in series there is one path so the current is the same. in parallel there are 4 paths so the current in each is 1/4 the sum (if the paths are the same resistance) in the parallel part where i wrote current is the same i mean the sum current thru 4 resistors
9 years ago
The paths aren't the same resistance. ever. There is always a very small difference in each resistor. Which is why it won't work.
9 years ago
Padlock it does not need to be perfect to work. its not like if all the current goes thru the path that is little less resistance to see imagine some inexact resistors (with small different error in each) and try one of the examples i shown above. you should get results fairly close to what you get with exact resistors. the errors are small and not significant to an extent where it does not work gmoon the error is given in % of the resistance and not in plain ohms. with small resistors the error is small too. most resistors (last line is gold) have accuracy of +- 5 % their value and this is very accurate for allmost any purpose. say for 0.25 ohm the error is tiny unnoticable 0.0125 ohm a problem you really get with small resistors is that the resistors are of the same magnitude as everything else in the circuit. every wire and joint counts and adds resistance. you may end up with much higher resistance than what you'd expect from the resistors only
9 years ago
It's a fact. Electricity will find the path with the least resistance to ground. Even if the difference is in nano-ohms, it will find the one with the lesser resistance. Which is why it won't work.
9 years ago
then explain why if you connect 2 devices to power using a power strip both work ?
9 years ago
Because a device can only use so much power. If there is more power to start with, then it will flow through both of them.
 1-10 of 25 Next »