Friday, January 05, 2007

Basic Physics

If you insert a 121 ohm, 1/4 watt (terminating) resistor into the 24 volt section of a DeviceNet network, instead of landing it on CANH and CANL of said DeviceNet network, you will find that it gets hot.

Really, really fast.

Fortunately, as your holding the resistor by its body to tighten the terminal screw on the lead, your fingers will give you fairly rapid feedback on this phenomenon, much to the apparent delight of others in the room.

24VDC/121ohms = 198 ma (milliamperes).
24VDC x .198 amperes = 4.76watts
4.76 watts, dissipated over the surface area of a 1/4watt wire-wound resistor = OW! OW! OW!

Hilarity, and darn near a blister or two, ensue...

Stoopid immutable laws of physics!