# Power dissipation across multiple resistors

Discussion in 'General Electronics Chat' started by tom66, Jun 22, 2009.

1. ### tom66 Thread Starter Senior Member

May 9, 2009
2,591
218
If I have a 100 ohm/0.25 watt resistor in my circuit I can dissipate 0.25 watts safely across the resistor $\frac{{V^2}}{R}$. But what if I have two resistors... I now have 50 ohms, but how much power can I dissipate? What about multiple resistors of different values...?

Any help appreciated. This is not homework, this is for a project to make my own oscilloscope.

Tom

2. ### Externet AAC Fanatic!

Nov 29, 2005
1,403
186
Hi.
Two 0.25W resistors in parallel can dissipate 0.5W. Two 0.25W resistors in series can dssipate 0.5W.
Many resistors in parallel, or in series, can dissipate the sum of their rated power.
Miguel

3. ### gerty AAC Fanatic!

Aug 30, 2007
1,248
346
If you put 2 50Ω in series, it's still going to be .25 watt, since the current remains the same in a series circuit. Is this what you had in mind?

Externet, you beat me by 2 min!!

4. ### Externet AAC Fanatic!

Nov 29, 2005
1,403
186
Yes, but not trying to beat... And our responses differ The difference can be in the word 'can'.
Miguel

5. ### tom66 Thread Starter Senior Member

May 9, 2009
2,591
218
Okay, sounds great. I understand. But what about different values for each resistor?

6. ### gerty AAC Fanatic!

Aug 30, 2007
1,248
346
Are you going to put them in series or parallel?

7. ### tom66 Thread Starter Senior Member

May 9, 2009
2,591
218
I'm just curious what each configuration would allow me to do. There is no schematic at the moment.

8. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
It is good practice to use resistors which are rated for double the power expected in the circuit. If you expect 1/4W power dissipation, then use 1/2W resistors.

If you use 1/4W resistors with 1/4W actual power dissipation across them, it may cause the value of the resistors to change over time.

If you expect to dissipate 1/4W across a 100 Ohm resistor rated for 1/4W, then you have a few options:
1) Increase the rating of the resistor to 1/2 Watt.
2) Use two 200 Ohm 1/4W resistors in parallel.
3) Use two 50 Ohm 1/4W resistors in series. The closest standard value is 51 Ohms.

9. ### Audioguru Expert

Dec 20, 2007
11,251
1,350
A resistor operating at its max power rating is extremely hot. It will melt insulation and might char a pcb. It will burn you if you touch it. Don't operate a resistor at its max power rating.

10. ### tom66 Thread Starter Senior Member

May 9, 2009
2,591
218
I know... I have burnt myself on a resistor before.

What I am asking is if it is possible to create a 1/2W resistor from two resistors at 1/4W. This is theoretical, I am not going to run them at 1/2W instead probably 3/4 the total rating (3/8W).

11. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Yes. Double the resistance and connect in parallel, or half the resistance connected in series.
The "rule of thumb" still applies. I strongly suggest that you use resistors rated for at least twice the expected power dissipation.

Resistors are very cheap. No sense in ruining a board or project because you used under-rated resistors.

12. ### Externet AAC Fanatic!

Nov 29, 2005
1,403
186
Yes, either in series or in parallel.