Power Capacity of Resistors of Different Values

[RFEverywhere]

Gentleman (& Ladies??),

I require some help.

From what I have read and researched, I am under the impression that the power dissipation capacity of resistors are added regardless of whether they are in parallel or in series.

Here are my questions:

1. Can some one confirm that this is 100% accurate? I had read somewhere that this is only true for resistors in parallel and not in series. What do you think? (Please confirm if your answer is fact or theory)

2. If this is accurate, then does it make a difference if the resistors are of different values and have different capacities? Or is this only the case if you are using identical (same resistance) resistors?
For example:
I need to use a 100 ohm 15W resistor. Can I use the following 2 resistors in parallel?
-1pc 100 ohm 1/2W
-1pc 100K ohm 15W

3. What makes a resistor inductive and non-inductive?

I hope my questions make sense. Thanks to all for your help.

 

[Gryd3]

Here are a couple very handy formulas to help with your answer.

V = IR (Voltage, Current, and Resistance relationships)
P = VI (Power, Voltage, and Current relationships)

What end up happening when you connect devices in parallel is that the electric current will branch and take both paths relative to each path's resistance.
In your case you have two different power rating in parallel... The voltage across both resistors will be the same, the current will depend on what the rating of the resistor is. You need to use the formula above to determine the power required for each resistor to see if they both pass
I am unsure if this is a typo... but putting a 100KΩ Resistor in parallel with a 100Ω will give you an equivalent resistance of 99Ω.
IF the resistors were both just 100Ω, then using two in parallel will result in a value of only 50Ω.

Short answer...
Yes, you can add the power rating of resistor's in parallel, but the resistors in questions should be the same value and same rating.
No, your application will not work. (The lower rated resistor will most likely burn out, which could lead to the brunt of the current redirecting to the remaining resistor and burning that out too... but it's hard to say for sure because your numbers dont appear to be clear)

 

[Externet]

That is my impression. Resistor power ratings are added if the resistors are either in parallel or in series, for equal resistance values.
Your number 2 does not have equal value resistors, should not apply.
Some resistors are wirewound or of helical construction. Those contribute also with inductance much more than carbon type or short ones.
And physical dimensions of the same wattage rating resistor differ for orders of magnitudes in resistance. As a 10 ohm 1 watt resistor is not the same size of a 1 Mohm resistor of 1 watt.
Your question does make sense.

 

[KrisBlueNZ]

Gentleman (& Ladies??)

There is one female who has just recently introduced herself. Otherwise it's the proverbial sausage-fest here

From what I have read and researched, I am under the impression that the power dissipation capacity of resistors are added regardless of whether they are in parallel or in series.

That's right, as long as the resistors are all the same resistance. That's a fact AND a theory.

does it make a difference if the resistors are of different values and have different capacities?

Yes it is. Gryd3 has shown you how to calculate the dissipation in a resistor - it's just current (in amps) flowing through the resistor, multiplied by voltage (in volts) across the resistor.

For resistors in parallel, the voltage is the same for all of them but the current is only the same if they are all the same resistance; if the resistances are different, the total incoming current is split unevenly between them, and therefore, so is the power dissipation.

For resistors in series, the current is the same for all of them but the voltage is only the same if they are all the same resistance; if the resistances are different, the total applied voltage is split unevenly between them, and therefore, so is the power dissipation.

I need to use a 100 ohm 15W resistor. Can I use the following 2 resistors in parallel?
-1pc 100 ohm 1/2W
-1pc 100K ohm 15W

No, that won't work. If you connect a 100Ω resistor and a 100 kΩ resistor in parallel and apply a voltage across them, the 100 kΩ resistor will draw only 1/1000th as much current as the 100Ω resistor will, so it will only dissipate 1/1000th as much of the power. Therefore, for any given voltage connected across those two resistors in parallel, 99.9% of the total power will be dissipated in the 100Ω resistor, and 0.1% of the total power will be dissipated in the 100 kΩ resistor. So your total allowable power dissipation will be limited by the 100Ω resistor, and if that's only rated for 0.5W, that is the maximum allowable dissipation for the pair.

The fact that the 100 kΩ resistor is rated to dissipate up to 15W is irrelevant because it won't be dissipating much power at all, because the other resistor will dissipate the lion's share of the total power. The only way to make a 100 kΩ resistor dissipate 15W is to apply 1,225 volts across it, which will cause a current of 0.01225 amps to flow, giving a power dissipation of 1225 × 0.01225 = 15 watts.

If the two resistors are in parallel, and you apply 1,225V across one of them, you will be applying 1,225V across the other one as well. If the other one is a 100Ω resistor, it will draw a current of 12.25A. With a voltage of 1,225V and a current of 12.25A, it will try to dissipate 15,000 watts! It will disintegrate into a puff of smoke within about 0.00000001 seconds!

A similar situation occurs with resistors in series, except in that case, the most power is dissipated by the one with the highest resistance, not the lowest resistance, because that one will have the most voltage across it, and the current is the same for all of them.

What makes a resistor inductive and non-inductive?

The construction. Wirewound resistors are normally wound helically, like an inductor or an electromagnet. This makes them inductive, unless they are deliberately wound in a special way where one turn cancels the next. Other types of resistors have a very small amount of inductance too, but that's the main issue.