# Foo - Resistor wattage

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View Full Version : Resistor wattage

phantomcow2
09-23-07, 09:04 PM
Say one has some 1/4watt resistors floating around. It turns out that 1/4 watt just isn't enough for the application. Is it possible to split the load between several resistors? If I wire a bunch in parallel to get me my desired resistance, can I get away with 1/4 watt?

It turns out that the ideal size for my needs is actually 1/2 watt.

StrangeWill
09-23-07, 09:09 PM
How about you go to the store...

skinnyone
09-23-07, 09:10 PM
abso-frikkin-lutely.. dont do it for diodes though..

G-Whacker
09-23-07, 09:12 PM
UMMM, no.
The power rating part of your proposal is OK , current is constant in a parallel circuit.
But your resistance will drop according to number and the values of the resistors.
So if you calculate that correctly, you should be OK (assuming you use equal value resistors).

BTW, resistors are cheap and easy to get, so just get the right one!

skinnyone
09-23-07, 09:20 PM
UMMM, no.
The power rating part of your proposal is OK , current is constant in a parallel circuit.
But your resistance will drop according to number and the values of the resistors.
So if you calculate that correctly, you should be OK (assuming you use equal value resistors).

BTW, resistors are cheap and easy to get, so just get the right one!

I thought his post read that he was paralleling bigger resistances to get his desired value.. no?

ken cummings
09-23-07, 09:21 PM
Check a basic book on electricity. Two of the 1/4 watt resistors in parallel should do the trick.

G-Whacker
09-23-07, 09:23 PM
I thought his post read that he was paralleling bigger resistances to get his desired value.. no?

You are correct sir!
I hearby take back the UMM NO part of my reply!

skinnyone
09-23-07, 09:24 PM
You are correct sir!
I hearby take back the UMM NO part of my reply!

you are such an emo kid.. :p :D

phantomcow2
09-23-07, 09:36 PM
Okay so because I am going to be putting some 1/4 watt resistors in parallel to get the right resistance, this will do the job of a 1/2 watt resistor? Just want to be certain before it's time for lift off.

daz-o-matic
09-23-07, 09:44 PM
Yep. Two 1/4 watt resistors will each share the load and power dissipation.

twobikes
09-23-07, 09:50 PM
If you use two 1/4 watt resistors in parallel as a replacement for a 1/2 watt resistor, the resistance of each 1/4 watt resistor will need to be double the desired resistance of the 1/2 watt resistor. The parallel configuration of two equal resistors has the effect of dividing the resistance of each resistor by two.

ElJamoquio
09-23-07, 10:57 PM
If you want a 1/2 watt, you wire them in series.

If you want twice the power dissipation, wire them in parallel.

rsd212
09-23-07, 11:53 PM
Series or parallel will work. Just calculate your equivalent resistances right. Remember P=IV, so in parallel you're halving the I, in series you're halving the V. If you're going for 100ohm 1/2W you can wire 2x200ohm 1/4watt in parallel or 2x50ohm 1/4watt in series (for example).

If you're that close to the rating you may even want to go in triplicate or higher, just in case your resistors are at opposite ends of the tolerance window...

x136
09-23-07, 11:56 PM
Crikey, mate, buy some half-watt resistors in the resistance you require. The things are what, five for a penny?

efficiency
09-24-07, 12:10 AM
www.digikey.com

deraltekluge
09-24-07, 01:54 AM
www.digikey.comIf you use two identical resistors, each will dissipate half the power, whether they're connected in series or in parallel. If you use a series connection, use resistors of half the desired final resistance value. If you use a parallel connection, use resistors of twice the desired final value. In either case, the total resistance and thus the total power dissipation will be the same, with the power divided between the two resistors.

But the best solution is just go buy a resistor of the right value and right wattage rating.

twobikes
09-24-07, 07:07 AM
Often it is not easy to find a store that sells 1/2 watt resistors. Radio Shack stores are everywhere, but they carry nothing heavier than 1/4 watt resistors. It is easy to say, "Go buy some 1/2 watt resistors." but not always easy to do so. In addition, the original post indicated he already has some resistors he wants to utilize.

DannoXYZ
09-24-07, 02:06 PM
If the resistance values are correct for the 1/4-watt resistor and you want 1/2-watt power-handling, you'll need four of them. Wire two in series and the other two in series. Then put these pairs in parallel. The result is a 1/2-watt array with the same resistance as the original.

twobikes
09-25-07, 07:05 AM
I have a basic electronics simulation program from Protolab. Radio Shack once sold it. It is handy for setting up problems like this one and testing circuits without picking up a soldering iron or burning up components in the real world.

I set up two 200 ohm 1/4 watt resistors in parallel with a 7 volt DC power supply. Power dissipation was 0.49 watt. I set up two 50 ohm 1/4 watt resistors in series with a 7 volt DC power supply. Power dissipation was also 0.49 watt. Increasing the voltage to 7.1 volts in either circuit blew a resistor.

eubi
09-25-07, 10:48 AM
P = Power in Watts
I = Current in Amperes
E = Voltage in Volts
R = Resistance in Ohms

For power: P = IE = I^2 R

Ohm's Law: E = IR

For resistors in series, R1+R2+... = Rt

For resistors in parallel, 1/Rt = 1/R1 + 1/R2 +...

Go nuts...