Froggy Dilemma on DCC

Model Railroader
1

Another request from an aged novice with my first layout build.

I have installed an area of yard and industry track on my modest layout with Walthers Code 83 track and turnouts. The turnouts are powered using the Rapido Railcrew switch machines.
I want to power all the frogs which the switch machines allow for, but I was puzzled when I tried to check the frog polarity and continuity on the working circuit, the power being supplied by an NCE SB5 Power Booster. I could not get any polarity or continuity readings on the frogs no matter what power reversals I made nor the checks I made to all of the wiring and connections.

Finally, I decided to set up a test turnout on a box that I knocked together complete with working switch machine and frog wiring. This time I used a plain 12 DC transformer so that I could check the results without a DCC circuit. Everything worked perfectly!

My question: how can I test for frog polarity on the working DCC circuit?

At the moment I can disconnect the SB5 booster and temporarily use the simple DC 12v transformer to test all the switch frogs as a work-around. But I would like to know how many of you do test frogs on a working DCC circuit. Obviously, something here that is beyond my understanding.

Thanks in advance for any information!
Jim

2

Jim,

DCC puts a square-wave AC current on the rails, so you’ll get a reading from the frog to either rail.

With that in mind, you can test your frog wiring by disconnecting one rail from your SB5 and then checking the frogs.

They will be live in one turnout position and dead in the other, so as long as the frog being live matches the turnout position, you’re good to go.

3

Thanks very much for that information MrMe.
I can understand the DCC circuit output configuration but I certainly didn’t think of your solution which to me, at least, is very clever.
I will try this tomorrow.

Much appreciation for your help!
Cheers,
Jim

4

Even though it’s AC you only get a non-zero VAC measurement between the frog and 1 rail.

5

In addition to the other good replies, I do the following:

  1. Ensure Rail A and Rail B have the same wiring pattern. I use RED wire for Rail A and BLACK wire for Rail B. I do this everywhere and that prevents polarily and phase errors. This is very important.
  2. My own pratice: Put insulated rail joiners on each track out of the frog. Put feeders past the insulated joiners. This ensures proper polarity and phase and addtionally does not depend on points to carry track current. Optional, but this has worked for me for many years.
3 Likes
6

Thanks everyone for your comments and suggestions.
I’ll try it out this afternoon (Pacific time zone).

Jim

7

You are of course correct. I obviously didn’t have my brain fully engaged when I typed that reply.

Thanks for the correction!

8

Gentlemen, the confusing term for me is “non-zero”.
Do you mean anything other than a zero reading?

9

Well, I guess my ages old knowledge of electronics is long past its 'best before date"!

I have tried testing three turnouts that are wired for frog polarity and continuity and I can’t make sense of the readings that I’m getting.

What is the best setting on a digital multimeter to use in order to take meaningful readings of the frog/rail configurations? What kind of readings should I expect to see?

Is it still advisable to disconnect one of the bus/rail leads from the SB5 booster for testing the frogs? I have tried without one rail feed and with both rails powered. It didn’t help my confusion.

Again, thanks in advance.
Jim

10

Part of what is perplexing you is that there are two separate ‘modulations’ in DCC track power, both of which are significant.

The track power itself is like a kind of square-wave version of AC; it is like a DC ‘carrier wave’ that periodically reverses polarity.

This DC is then periodically interrupted at high frequency to produce a kind of digitally-coded PWM. This is not AC as there is no real ‘zero crossing’ – it is pulsed DC with as short a risetime as practical. This is how the control signals to the decoder are passed.

Note that the applied track power for nominal 15V standard does not require 30V rail-to-rail – which I found surprising.

You’ve already found the best ‘troubleshooting’ for continuity… which is to apply straight DC to the track and eliminate any shorts.

11

Thank you Woke_Hoagland for your explanation.

I have decided to do just what you have confirmed for me; apply a simple DC power source to the circuits to make it easier for me to test for correct frog configurations.

Back to basics!

12

AC volts (~), not DC (-) and typically 20V. Just measure the voltage across the rails to see a typical reading and what you should measure between the frog and one of the rails

DCC should measure ~14V AC. you’ll often read ~ 1V when the polarity is the same (i.e. no volts, zero volts) meaning both probes are at the same polarity

when was this ever advisable? how can you measure voltage if one booster connections is disconnected?

13

the polarity is simply reversed using an H-Bridge

The NMRA baseline Digital Command Control signal consists of a stream of transitions between two equal voltage levels that have opposite polarity, a bipolar square wave.

of course it’s AC. Reversing the polarity reverses the current – hence Alternating Current

(RMS) average as commonly measured is ~14V AC, peak-to-peak voltage is therefore ~28V

14

I faced this same dilemma when i tried to figure out the proper wiring to juice a dbl crossover. I ended up just completely disconnecting the tracks from my power supply and removed all the locos from the tracks and just did a continuity test using the lowest ohms setting on my multi meter.

@user1234 After i got the frog juicer working i still occasionally have problems with some engines stalling briefly then proceeding did your setup with the isolated portion of rail joined up with the points help with that?

15

I put feeders near the points, and feeders directly after the insulated joiners on both frog tracks. No issue with engine stalling over the turnout. Peco turnouts switch the current at the points to the correct frog track and that helps a lot - no stalling.

FWIW - I had one Locomotive CP F7 A Unit - that stalled. I nailed it down to a defective truck. I sent it back for repair and got it back fully fixed. No stalling after that. This was a locomitive issue; not a track issue.