Resistor types and power ratings

[Terry Gould]

Hi,
Im new to this forum so I hope I've posted in the right place.
Im trying to make the 3 first circuits in this pdf: http://www.micrel.com/_PDF/App-Notes/an-5.pdf

How ever how do I choose what power ratings for the resistors and on the resistor (Rs) connected to the load they state it should be a Vishay Dale CPSL-3 but these are hard to get hold of in the resistance I need. Will any 3W resistor be ok?

Also how is the resistor capable of the current the load will be drawing is say 12v at 30amp how can this be powered by a 3w resistor?

I know this is probably really obvious but Im self taught with electronics making micro controller circuits, simple things like timers and control modules for various things.

Thanks

 

[KrisBlueNZ]

Hi Terry and welcome to Electronics Point

The Vishay web site (http://www.vishay.com) has the following information about the CPSL series resistors:

• They are low-inductance wirewound resistors. Standard wirewound resistors have significant inductance because they are wound like an inductor. Low-inductance wirewound resistors need to be wound specially.
• They are low-value. We can also guess this from the fact that the one in the app note has a value of 22 mΩ.
• They are four-terminal "Kelvin connected" resistors. See https://en.wikipedia.org/wiki/Four-terminal_sensing
  
• They are designed for current sensing.

At least some of these features are important for the application, otherwise the Micrel engineers wouldn't have specified them. You may be able to find similar components from manufacturers other than Vishay.

A couple of formulas may be useful in helping you calculate the required power ratings for resistors. These formulas are the result of merging Ohm's Law, I = V / R, with the Power Law, P = V × I. They are:

P = V² / R
and
P = I² × R
where
P is the power dissipated in the resistor, in watts;
V is the voltage across the resistor, in volts;
I is the current through the resistor, in amps;
R is the resistance, in ohms.

These formulas apply for DC circuits where voltages and currents are steady, and in AC circuits if RMS values are used for currents and voltages.

For your example of a load drawing 30A at 12V (= 360W) with a 22 mΩ current shunt resistor in series with it, you need to realise that although the resistor has the full 30A flowing through it, it doesn't have 12V across it. That voltage appears across the load. The voltage across a current shunt resistor is normally much less than a volt, so the power dissipation in the shunt will be much less than 360W.

You can calculate the power dissipation for that resistor using the second of the formulas above:
P = I² × R
= 30² × 0.022
= 900 × 0.022
= 19.8 watts.

So if you're using a 22 mΩ shunt resistor with a load current up to 30A, the shunt resistor will dissipate up to 20W and should be rated for at least 30W.