RailPro > RailPro Specific Help & Discussion
ScaleTrains LM-3S Question
Smoke:
--- Quote from: G8B4Life on May 14, 2020, 12:12:27 PM ---Yes, Paul Falstads circuit simulator is really cool.
Yep, RP outputs are switched ground, so in my diagram the +14v is the LM blue wire (common) and output 1 is the is the white wire (9 pin side), output 2 is the yellow wire (9 pin side), output 3 the green wire (9 pin side) and so on (http://ringengineering.com/RailPro/Documents/LM-3Instructions.pdf). I've draw in "switches" in the diagram to simulate turning the functions on/off.
Hopefully what I've written allows you to follow the diagram, I can't explain it any better at this time of the morning. I need rest.
- Tim
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It makes perfect sense knowing the LM is used switched ground. I will see if I can find a dual channel optocoupler to design around, otherwise I will plan on using 2 single channel ones.
--- Quote from: Alan on May 14, 2020, 01:34:19 PM ---
--- Quote from: Smoke on May 14, 2020, 11:40:20 AM ---The circuit creater/tester is really cool!
I'm a little confused how I would actually wire the circuit you've drawn. You have the switched outputs being common, with a constant 14VDC supply. I was under the impression that the outputs were positive and common would be negative, but you have it drawn the opposite way. Are the outputs actually switched ground?
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RP modules, along with most electronics, use a transistor to ground as the switch. This is referred to as "sinking" current as opposed to having the switch on the high side (+ v) which is called "sourcing" current. The reason for sinking current being the popular approach has to do with a transistor's physical construction. NPN material transistors sink current. PNP material transistors source current. PNP material must have ≈3x surface area of NPN material for equivalent current carrying capability. This is due to the physics of electron/hole mobility inside the semiconductor wafer material itself. NPNs are therefore less expensive to make, take up less space, and switch on/off faster. Under the hood we live in a NPN dominated world.
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Thanks for the info Alan. I've dealt with sinking and sourcing before (mostly with DI/O), but never had a clear understanding of what they actually were. Now the concept makes sense on why you would use sinking instead of sourcing.
Alan:
MCT6 is a widely available, low cost, general purpose optocoupler available in SMD or through hole. About $0.50 ea.
http://www.vishay.com/docs/83645/mct6.pdf
Smoke:
Thanks Alan! I sent you a PM with some questions for designing the PCB.
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