pfenbach I'm late to this discussion, but am interested as I am trying to use Tansey's Option A myself and wanted to find someone who had actually implemented it. The issue I am having is that if I choose position 1 on the rotary switch, for example, then the first resistor that receives power gets very hot - too hot to touch even. And the same thing happens as I move to other positions on the rotary, so it's not just one resistor. Since I can't believe that this is how this setup is supposed to function, I'm wondering if any of you that actually use Tansey's config could provide me with any guidance as to what I might be doing wrong. Thank you.
I'm late to this discussion, but am interested as I am trying to use Tansey's Option A myself and wanted to find someone who had actually implemented it. The issue I am having is that if I choose position 1 on the rotary switch, for example, then the first resistor that receives power gets very hot - too hot to touch even. And the same thing happens as I move to other positions on the rotary, so it's not just one resistor. Since I can't believe that this is how this setup is supposed to function, I'm wondering if any of you that actually use Tansey's config could provide me with any guidance as to what I might be doing wrong. Thank you.
Yes, that first resistor has to dissipate ~0.257 watts (12 x 12 / 560), so I would expect it to get pretty hot. 0.257 watts may not sound like much, but consider that the typical through hole resistor that we often use in model railroading application is often only rated at 1/4 or 1/2 watt.
Thanks for the response. I'm no electronics expert, but I am using the 560 ohms 1 watt resistor that Tansey recommends. Should a 1 watt resistor get that hot dissipating .25 watts?
pfenbach Thanks for the response. I'm no electronics expert, but I am using the 560 ohms 1 watt resistor that Tansey recommends. Should a 1 watt resistor get that hot dissipating .25 watts?
Probably.
It finally occurred to me to get out my multimeter and actually measure the voltage of my power supply. I was just using an old wall wart that stated the output as 12v DC, 400mA. Turns out it was putting out 17 volts. So using your formula: 17 x 17 / 560 is over .5 watts. Once I tried using a PS that actually measure 12v output, I got very little heat on the resistor. Thanks again for your responses.