M&K Junction Railroad

M&K Junction Railroad
Another train of eastbound coal crosses the Cheat River

Thursday, May 31, 2012

Update on PC Power Supplies

Text and Photos © 2011-2012 by Terry Terrance

If you haven't already done so, please read the original post on How to Convert Used PC Power Supplies for Your Model Railroad.

Since making that post I have been unhappy about the fact that these power supplies cannot be connected in series to produce sufficient voltage to run a DCC system. There are circuits that can convert DC at one voltage to DC at another voltage, either up-convert (boost) or down-convert (buck). These are called DC-to-DC Converters. DC-DC down-converters are fairly common but, even when you find them on the electronic surplus market, they are not inexpensive; and they usually do not have capacities in the multi-amp range. DC-DC up-converters are harder still to find and suffer from the same high cost and low capacity just like the down-converters.

There are many integrated circuits (IC) on the market that can be used to easily design an up-converter. However, doing so and building the circuit is beyond the abilities of some model railroaders.

A few weeks ago, I came across this circuit board being offered on e-bay from a vendor in China. Before the red flags go up (pun intended), I've ordered from Chinese vendors before and all of the transactions have gone smoothly by using Pay Pal.

This board is exactly what I was looking for. It will boost DC input voltages from 10-32 volts and produce an output voltage between 12-35V DC. The maximum output current is 6 amps. This is item number 090421 from a vendor whose handle is "jennyear".  Cost is $13 with free (albeit slow) shipping.

The four screw-terminals on the end is for the inputs and outputs.  Connecting the wires is straightforward.

I proceeded to test the circuit. I connected 12VDC to the inputs and a voltmeter to the outputs. I adjusted the potentiometer (the blue component with the screw on top) on the board to get an output of 19V, as you can see in the next photo.

Notice that right behind the terminal strip there is an LED which is illuminated when the board is powered up.  19V is sufficient to run most DCC systems; besides if you keep the input voltage to your DCC system as low as practical, you'll get the maximum DCC current output, plus you'll minimize the generation of heat in this circuit. So far, so good.

Next I connected a power resistor (measured 20.2 ohms) to the output and put the meter in current mode to test for the output current. With an expected output of an amp at 19V, you cannot use an ordinary resistor for this, it will go up in smoke.

The power resistor is the device at the center. The 0.94 amps shown on the meter is the proper output for the voltage and the value of the resistor. After a few seconds of this output, the resistor became uncomfortable to handle, however the circuit board was still cool; very promising.

The next photo shows the trade-off for getting a higher voltage output from a lower voltage input.  The power supply providing the 12V input is supplying 1.62 amps to get the 0.94 amp at 19V out.

This will not generally be a problem as your PC power supply can likely provide more than enough amps at 12V to get a 5 amp output at the 16-22V required to run a DCC system.

Next I took two 10 ohm power resistors and connected them in parallel to produce an approx 5 ohm resistance (I measured this at 5.5 ohms but this cheap multimeter is known unreliable at measuring small resistance values).  This time I connected it to a PC power supply as the bench supply that I used in the first test would not supply enough current. As expected, the circuit output more current into this lower resistance; 3.6 amps as shown in the next photo.

At 19V/3.6 amps output the heatsinks on the circuit were warm and not excessively hot. I do not have any resistors of high wattage and low  resistance to test this circuit further.  However, I believe that an output of 19V/5 amps (95 watts) can be achieved.

Without further testing, here are some suggestions for finishing this circuit for use with a DCC system.  First and foremost you must fuse the output.  Even a momentary short circuit will fry the ICs that comprise this boost converter.  I'd use a fast-acting 6 amp glass fuse (GMA-6A) and matching fuse holder.  That may be hard to come by locally (although Home Depot does list these fuses on their website), so the fall back would be to use a 5 amp automotive blade fuse and fuse holder which you can find at your local auto parts store.

Use heavy wiring when wiring this circuit to your PC power supply and to your DCC system.  You'll be carrying heavy current out of this circuit and even heavier currents will be feeding into it (probably in excess of 12 amps).  14 gauge would be a minimum.  Be sure to heed my admonition in the article on converting PC power supplies to aggregate several distinct black (ground) and yellow (12V) wires from the power supply for the inputs to this circuit.

House this circuit in a metal project box; and a ventilated one would be much better as the circuit will produce heat during operation.

Adjust the circuit for a voltage a 2-3 volts above the minimal voltage needed to run your DCC system.  The output of this circuit may sag under load (however, it was a stable at 19V with the 3.6 amp load) and you want it to stay above the minimum for your DCC system.  Do not exceed the maximum input voltage for your DCC system.

Once connected to your DCC system, test that the circuit can supply sufficient output to allow the fast-acting DCC circuit breaker in your system to operate.  Put a dead short on the tracks and test that the DCC output goes to zero instantly.  If not, the circuit cannot handle the current surge and should not be used.

That's about it.  At some point in the future I will use this set-up to power a Lenz Compact DCC system for my bench.  At that point I'll write another blog post with the results.  If you get around to doing something first, let me know the results.

Roster Shot

Some time back, I wrote a post about an old school "box of sticks" boxcar kit that I had nearing completion after starting it more than 20 years previous (see Oldie But Goodie?). Well it's taken a couple of more years to finally get it done but, as you can see, it's on the layout, painted and operational. 

There was a great sense of satisfaction in finally finishing this project.  This is in spite of the fact that I could have purchased an Atlas O Trainman model of essentially the same prototype for about $42, taken it out of the box, put it on the rails and have been done with it.  Arguably my model is inferior to the Trainman RTR.  My model is missing a few details (most notably some of the grabs and the brake staff and wheel, which I will remedy); however, my brake rigging (borrowed from an Intermountain kit) is freestanding rather than the cast on blobs of the Trainman.

Where a kit like this really shines is in confidence building.  It probably took me as long or longer to build this kit as it would have taken to scratch build it.  However, faced with the terra incognita of scratch building this prototype, I might never have started.  The kit gave me a road map, however sketchy, for turning a box of sticks into a boxcar; and the confidence that someone, somewhere at some time thought that this could be done, therefore I should be able to do it as well.

It's a shame that these types of kits are no longer available as I have found that they can be great builders of proficiency.  But wooden kits have long been relegated to ancient history and the more recent cast-resin kits are on their way out as the makers of resin kits retire. 

Speaking of cast resin, I have a bunch of these kits and one is under construction and is next up for completion after I finish a Red Caboose tank car kit.  Stay tuned.

Monday, May 7, 2012

On Grades and Helpers

For starters, here's a quick video that I shot entitled "The Cheat River Grade Helper":

After shooting this video, I realized that I should do something about the bare backdrop on the Cheat River Grade. I cannot make a photo backdrop right now, but maybe I can at least paint the backdrop a sky blue.

 In my previous post, I remarked on the necessity for using helpers (or more head-end power) on the Cheat River Grade. This grade is 2% and nearly matches the prototype. In building my model railroad I have used Charles Roberts' book: "West End" as my "bible". My experience with needing helpers on the grade caused me to review the section in Roberts' book on equipment. Roberts compiled statistics on how many cars the prototype could pull up the grades. He used an interesting metric, number of cars puled per powered axle. Using this as a metric eliminates the differences in wheel diameter, steam vs. diesel, etc. What he found was that over the period of time from 1850 to 1991, the B&O could haul between 2.1 and 2.4 cars per powered axle up the grades. That is essentially the same result that I have obtained after testing with several model steam and diesel locomotives; between 2 and 2.5 cars per powered axle (and the latter number with a lot of wheel slip).

I believe that John Armstrong observed in one of his books on layout design that given needle-point axles model locomotives will pull more on the grades and less on the flats than prototype locomotives. He further observed that to create a helper grade that would seriously challenge model locos, 3% was about right.  I would certainly never dispute John Armstrong, but in my particular case the grades and, especially, for the combination of curves and grades, 2% seems to be about right.

It will help that most of the hoppers going up the Cheat River Grade will be empty (unlike the video above, which was just for fun) and I can arrange that most of those will be plastic models.  But the loaded hoppers coming down the Cheat will have to go up Cranberry Grade, also 2%; and these hoppers will be loaded.

During the period 1949-1952, the B&O used A-B-B-A sets of F7 diesels as pushers to help trains headed by, typically, a 2-8-8-4 articulated up both of these grades.  Assorted 2-8-8-0s and Q4 2-8-2s (as seen in the video above) were also used as pushers.  I intended to use A-A sets of diesels and Q4s as pushers.  The eight powered axles on the diesel pushers would be about 16 cars capacity, that should be OK.  The four powered axles on the Q4 would only be 8 car capacity - that might not be enough.  Matching the pusher to the train length could make the operating game more interesting.