Digitrax PM-4

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Does anyone have one of the new Digitrax PM-4 Power Management modules yet? If so have you any experience with using it where you have an automotive light bulb (type 1156 as recommended by Allan Gartner) in series with the track.

Let me explain the reason for my question. I have two pieces of track work that are short circuit prone. I isolated them and put a light bulb in series with each one. This worked OK when I was feeding the track directly from my Chief DCS100. I then purchased 2 PSII Power Shield electronic circuit breakers from Tony's Train Exchange and used them to separate my layout into two power districts. Unfortunately, the PSII does not have any delay on the short circuit protection (confirmed with the designer) and sees the cold light bulb as a short, thus shutting down immediately. This defeats the light bulbs and gives me back my original problem.

The DCS100 has a 1/8 second delay (alterable by OPSW to 1/2 second) so that it does not see the cold bulb as a short. So my question is, Does the PM-4 have a delay such that it will not see the bulb as a short but still operate before the DSC100 delay times out?

-- Dale Gloer (dgloer@ca.ibm.com), November 09, 1999


If you use the light bulb trick for circuit protection you don't need the PSII circuit breakers. The light bulbs will automatically limit the current but not shut down the layout. Sounds like a cost savings to me.

-- Don Vollrath (dvollrath@magnetek.com), November 10, 1999.


I can not say with 100% certainty but I doubt if there is any delay in the PM-4's. But I think you might be missing a point on the use of the current limiting lamps and that of the PM4's or PSII's.

As you have already noted, a lamp is a veriable resistance, that is one with a cold filiment versing that of one that is hot, and verious degrees inbetween. Thus is not a good choice for large area protection. But is ideal for small areas, such as turnouts where the load will only be that of a single loco.

Where the PSII's and PM-4 are constant and will hold tight up to their rating. Ideal for large area protection, such as districts, blocks, etc. Areas where the load will vary within the protection.

Both are all designed for the same reason, and use. That is to offer protection but not shut the entire layout down. Same thing with multiple boosters.

But keep the lamps to areas that are small such as frog/points on turnouts. And use the PSII's or PM-4's for the larger areas. Let the boosters cover the over all. But do not series the local protection of lamps with the PSII's or PM-4's. And do not try to use the lamps for large are protection, as you might find entering the area with say 3 to 5 loco's, you might not be able to get out. But at the least you will notice the slow down as the resistance builds in the lamp. You could easily use the PSII's or PM-4 to cover the turnouts but that is not as cost effective as the lamps.

So if you need to district the layout, then use either multiple boosters, if the current demand is there. Or sub out with the PSII's or PM-4. And if you still want to break it down farther, then use the lamps on the turnout frog/point power, but come back to the booster, not in series with the PSII's or PM-4's.

But if there is no real need to sub out the layout, and your basic problem is at the turnouts, then the lamps should be all you need.

As you can see, it all depends on your layout and your needs, there is no real set answer. But if you do need both lamps and sub districts, just make sure that the lamps come back to the booster and not down line from the other circuit protection.

Remember Always Have Fun and Enjoy!, Don Crano Akron, Oh NMRA #096211 mailto:donc@sssnet.com Visit Model Railroading with DCC at: http://pages.sssnet.com/donc/

-- Don Crano (donc@sssnet.com), November 19, 1999.

Don, I had considered all your points when I installed the lamps and in the end I have wired them back directly to the booster. The reason for having them downstream of the PSII was convenience and avoiding a bigger rewiring job.

In the end though, lamps are still only a bandaid for the following reason. My layout probably will not require more than one booster for quite a while, but I can and sometimes do have enough locomotives on it to draw two or three amps. This means that even a hot bulb can run the current up over the booster capacity. The real fix is to correct the track work problems.

Fixing the track work is a long term proposition and I may never be able to overcome all the problems (can you say Shinohara double crossover) so I may just buy an Easy DCC booster just for these two troublesome sections.

One of the attractions of the PM4 is the reversing feature. I have two turntables and thought about using one section of the PM4 for each turntable. Then I never have to bother with track polarity as the turntable turns.

None of this has dimmed my enthusiasm for DCC. It is just that I didn't expect a few of the surprises that have come along.

Dale Gloer

-- Dale Gloer (dgloer@ca.ibm.com), November 22, 1999.

The PM4-2 has four reaction time settings. For your problem slowest might just work. However, you should the track problem. When I started modell railroading over thirty years ago, my father and I were working together on the layout. His golden rule was that every engine and car has to go smoothly and without problems through every piece of trackwork, otherwise either the rolling stock or the the track needs to be fixed. This might even mean replacing that (in)famous double crossover. When I built my own layout, I follwoed the same rule, and I am not encountering any operating problems.

-- Ulrich Albrecht (albreuf@auburn.edu), December 01, 2003.

Dale, There are several hidden problems with the Walthers/Shinohara double crossover. See Allan's notes on how to positively fix it. But I found that most of the problems were caused as steel wheels crossed over the frogs. Even though the frog points are insulated a wide wheel tread can touch both rails leading up to the point as it passes by. A second problem occured when steel wheel flanges touched the bottom of the flange-way at the frog. The molding is made of plastic, but there is a conductive strap buried in it to pass track voltage to rails beyond the frog. The strap is bare and therefore energized on two of my frogs. Run your loco through in a darkened room and Look for sparks.

Both of these problems are resolved if you simply put all 4 track switches in the same position... N or R. Easy to do if you use 4 Tortoise motors wired in parallel. 1 toggle sw throws all 4 switches. I have several D-Crossovers and zero problems with them since I did that.

Drop down a couple of extra rail feeders to the fixed polarity stock rails and let the electrical contacts at the points do their thing. If you still have problems, check the wheel gage on offending rolling stock.


-- Don Vollrath (dvollrath@magnetek.com), December 03, 2003.

re - Using lamps Vs circuit breakers Vs boosters

Lamps work well to allow full booster current (and more) to flow through a track short circuit for a short time. The peak current is simply open circuit track voltage divided by the cold resistance of the lamp filament (plus the resistance of any thing else in the way, like puny wires). Initial current may be higher than 10 amps. In less than a few tenths of a second, the lamp heats up, increasing the electrical resistance and limits current flow to a much smaller value of 2-3 amps. With a 1156 or 1157 lamp most 5 amp boosters will allow the above to happen and not trip out...which is the desired result.

However, if the booster is already suppying several amps to other track loads...locos or lighted passenger cars, etc...the additional current drawn through the light bulb can be enough to cause the booster to trip before the lamp has a chance to heat up. This ends up defeating one of the main reasons you used the bulb...namely to protect from a short without shutting down the rest of the layout.

If you think about it, the booster will, and should, trip out on overcurrent in order to protect itself. Ditto for the PM4 or other brand of electronic CB. But the relative speeds of each protective device must be such that the one closer to the track must be the fastest, and the booster must be the slowest. The booster may also be feeding other loads and be close to tripping because of them. Making the PM4 or other CB act more slowly may cause the booster to trip first.

My take on this is that the light bulb trick works OK on an unloaded booster. If the track supply booster will be loaded to more than 1/2 of capacity, the light bulb trick is not likely to work and should be replaced with a much faster electronic CB.

Of course you could experiment with smaller watt automotive lamps or better yet, fix the problems that are causing the short circuits :>)


-- Don Vollrath (dvollrath@magnetek.com), December 03, 2003.

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