RailPro > RailPro Specific Help & Discussion

ScaleTrains LM-3S Question

<< < (2/3) > >>

Alan:
Tim, your circuit looks fine to me. You don't give yourself enough credit.

Optocouplers use light from a LED to saturate (turn on) a phototransistor. Optoisolators use radio waves from a transmitter to trigger a receiver which in turn saturates a transistor. Their fundamental purpose is the same - join together two circuits without using an electrical connection. Optoisolators use CMOS technology so they are less expensive, more reliable, and operate at much lower power. However, due to the use of CMOS construction optoisolators are static sensitive so stick with optocouplers for model train uses. Here is a whitepaper if you want to delve into the detail: https://www.mouser.com/pdfDocs/siliconlabswhitepaper_isolator_vs_optocoupler_technology_updated.pdf

G8B4Life:
Well thanks Alan, but there's not a whole lot of credit to give myself on this one, I just played around in the simulator for hours (lots of fun actually) until I made something that worked for what was needed and then refined it (the first version had a big no-no until I remembered not to do something a particular way).  I expected there to be a mistake as just because the simulator said it worked doesn't mean that it would, and I got so thoroughly confused trying to understand the datasheet for an opto coupler that I didn't really know if it should be used in the way I've done it, even though the simulator said it would work.

Also, thank you for the very clear and concise simple explanation of the difference between an opto coupler and opto isolator. Why can't electronics sites have the same clear, concise, simple explanations as you?  :)

Oh, for Smoke I should have pointed out in my circuit diagram post that the resistors for the opto couplers will need to be sized for the opto couplers that one picks.

This is the one I based my diagram off: https://www.digikey.com/product-detail/en/broadcom-limited/HCPL-181-00BE/516-1787-1-ND/1966493.

- Tim

Smoke:
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?

G8B4Life:
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

Alan:

--- 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?

--- End quote ---

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.

Navigation

[0] Message Index

[#] Next page

[*] Previous page