Resistor Wattage and Detection

[dmidkiff]

I plan on hardwiring in a resistor to the feeders of a manual thrown switch to have feedback if it is left open after a crew has left the location.  The frog polarity will change using a micro slide switch so the detection unit will only see the resistance if the frog is powered a certain way.  My question is what wattage of resistor would be best?  It seems that most wheelsets out there are 10kohm for detection, and I have 1/8 and 1/2 watt resistors with the same resistance.

Would W = V^2 be appropriate?  So, W = 15^2/10000 = 0.0225, an 1/8 watt resistor would be sufficient?

Layout is DCC with an SB5 booster.  I plan on using RR CirKits BOD-8 boards with the coils around feeders to the stock rail of the switch.

Thanks, Doug

[peteski]

Yes Doug, a 1/8 (0.125W) resistor  will work well. The chosen wattage should always be higher than the calculated maximum power dissipated by the resistor.  I try to use a resistor rated at lest double of the actual dissipated power. In this case you have way more than double (which is fine).

This is really about how much heat will be generated by the resistor and how much the resistor can dissipate before getting too hot (which also depends on its surroundings).  But in your case you have large safety buffer.

[dmidkiff]

Thanks, peteski.  Just wanted to make sure I was thinking correctly.

Doug

[Ed Kapuscinski]

I also love this idea.