resistor selection

[majuba]

Hi guys

I am having difficulty in understanding basics.

I need to use a resistor in a circuit. The voltage and current ratings concepts I understand.

The confusion arises when looking at the power rating ( watts). How do I detemine the power rating required? What factors should I consider?

Should any consideration be given to wheter a DC or Ac supply is used? If so how does the type of supply (AC/DC) affect resistance or the choice of resistor needed?

Thank you in advance.

 

[jbartos]

Suggestion: Visit

http://www.ee.unb.ca/Courses/EE1713/CPD/lapp1.PDF

for:
equations: 1.10, 1.11, 1.12
The resistor maximum power rating Pmax must not be exceeded. A reasonable Power design margin is recommended. It is not unusual to see the resistor power rating to be 1/2 of Pmax.

 

[rbulsara]

Power rating of a ressitor is always tied to a time period. A given resistor can withstand a large current through it for a short period of time or a small current for a long period of time. It depends on the ability the resistor, and its surroundings ,to dissipate heat (all power in R is turned in to heat).

Manufacturers can provide related information. I beleive (I*I*R*t) should hold good for some range of conditions where heat can be properly dissipated.

AC rms value is equivalent of a DC value for a resistor, producing equal heat. In fact, this relationship is the basis of the understanding of the rms value of AC.

 

[buzzp]

The nominal steady state wattage for the resistor is calculated using I^2R. I design a safety factor of at least three when dealing with resistors, so if you calculate 0.5 watts then 3*0.5=1.5W. So I would likely use a 2W resistor (1.5W not common and cost more). Also, 2W parts come in different packages so they will have some differences as far as steady state temperature. This may have some affect on parts in the vicinity (heating them up). You need to consider which type you need to based on TC (temp coefficient) of the resistor. If you dont want a lot of drift then do not use a ceramic film resistor but maybe a metal film. Tolerance is also critical in some cases. There are lots to consider depending on what your doing. Check mfg's websites for more detailed info.
As someone mentioned, in most cases there is no difference in resistor considerations between AC/DC unless your talking about higher frequencies of AC, then the overall impedance of the resistor should be considered when selecting a resistor. Although, in most cases in will have negligible affect.

 

[Warpspeed]

Calculating the power dissipated is fairly straightforward, but choosing a suitable resistor is another matter altogether.

The key here is allowable temperature rise of the actual resistor. A lot of high power wire wound resistors are rated for 250C temperature rise at full rated power. This is very hot indeed, and will often result in smoke and hot smells, the cooking nearby wiring and so on. The resistor itself might be quite happy, but such high operating temperatures might not be desirable in many cases.

So you de-rate the resistor. If a resistor is rate at 25 watts dissipation with a 250C temperature rise, the same resistor will have 60C rise at six watts, or 90C rise at nine watts.

If you know the desired operating power, and you know how hot you want it to be, then a suitably higher rated resistor can be selected.

 

[jbartos]

Suggestion: The selection of the higher power rated resistor will just increase its heat dissipation area, reduce its surface temperature; however, the engineered and designed amount of power dissipation or thermal energy will stay the same.