I am in the process of installing about 10 tortoise switch machines that I am poweringwith a MRC slow motion switch power supply. In the process of installing one of theswitch machines I forgot to kill the power and unwittingly created a short circuit.The first indication I had of this was smoke coming out of the MRC unit. I killed thepower and it actually still works. While the MRC unit has internal overload protectionapparently it has no short circuit protection. Does anybody have an idea for adding a circuitbreaker to the outputs? There is always the potential for a short. I am pretty good at wiring but protecting the +12v and -12v is baffling me.
EWT123the +12v and -12v is baffling me.
P.S. I cooked an MRC power pack once. Sure it still worked fine, but always had that nasty fried electronics smell to it, so I junked it out.
Think of it as two 12V batteries. The + of one battery is connected to the - of the other battery. That means one battery has its + terminal free - between there adn that common connection, you get +12V. The other battery has its - terminal free. Between it and that common connection, you have -12V. It's all just relative - if you hooked your meter up backwards, then the first one would be -12 and the second one +12.
I assume this all means you have SPDT toggles. One common wire from that cetner common terminal of the power supply goes to all Tortoises, one wire comes back to the center terminal of each SPDT toggle on the panel. One side of each toggle goes to the +12V, the other side to the -12V.
If you don;t have any LEDs wired in series with the Tortoise for indicators or signals, a 9V power supply would be plenty. Just get a pair of 9V DC wall warts, and hook the + of one to the - of the other. That goes to your common wire. The remaining free + wire goes to the wire that connected to the +12V on the MRC unit, the remaining free - wire goes to the wire connected to the -12V on the MRC. How big a power supply in amps you need depends on how many tortoises. A 500mA supply is good for nearly 30 Tortoises. 25 to leave a margin.
Be warned that most wall wart type supplies have an internal fusible link that will melt if you short them out, and they are not repairable - so don't short them out! Or install a fast blow fuse rated at something less than the power supply but greater than the load of the Tortoises on each leg of the supply. That way if you short anything it will blow the replaceable fuse and not the irreplaceable one in the wall wart.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Thanx. If I wasn't this far in the construction process I would probably junk it also. It does work though. You got a suggestion for a resettable breaker? This thing is rated at 300ma. I am not controlling the switches with my DCC, just using SPDT switches. Just for grins, I i hooked up a 10 amp 125/250v breaker thatI had lying around and it obviously would not trip when I introduced a short.
Texas ZepherEWT123the +12v and -12v is baffling me.Circuit breakers don't care about the polarity. Just put one breaker in each circuit.
but is this protection really needed?
will they ever be needed during normal operation, or only because you forgot to turn off the power during construction?
greg - Philadelphia & Reading / Reading
EWT123 I am in the process of installing about 10 tortoise switch machines that I am poweringwith a MRC slow motion switch power supply. In the process of installing one of theswitch machines I forgot to kill the power and unwittingly created a short circuit.
I am in the process of installing about 10 tortoise switch machines that I am poweringwith a MRC slow motion switch power supply. In the process of installing one of theswitch machines I forgot to kill the power and unwittingly created a short circuit.
Just out of curiosity, how did you unwittingly create a short circuit?
Rich
Alton Junction
I managed to do the same thing...ruined a 1370 powerpack when installing my 20+Tortoises, don't know how. I decided to find the recommended Circuitron 800-7212 power supply, about $13. I just hooked it to a terminal block and then to the Torti DPDTs (with bi-color LEDs). I suppose you can get a cheaper walwort but some are filtered, some regulated, etc, so I decided to get the official version. I did not add any short protection, just try to be more careful.
http://www.modeltrainstuff.com/Circuitron-800-7212-AC-Adapter-p/cir-800-7212.htm
Paul
Modeling HO with a transition era UP bent
A newly wired/soldered spdt was lying on a small roll around table that I use when crawling/working under the layout. I had just finished the connections at the terminal block, picked up a bunch of tools and dumped them on this table and apparently on top of the switch. Turned on the power supply, got distracted and the next thing I knew the thing was smoking. One of the tools had bridged the solder lugs. Stupid and unlucky on my part but it did get me to thinking. I realize that a short won't happen in normal operations and what I did won't happen again but I do want to protect this circuit in the event of some bizarre event.
I've toasted my share of supplies. Now, I simply put a small fuse in the line. Fuse holders and fuses are very cheap.
I also use a 9-volt wall wart for my Tortoises.
It takes an iron man to play with a toy iron horse.
You start out with the best intentions - turn off the power supply while hooking up wires. Works great when you hook something up close to the power supply, just reach down and turn it on after the wires are in place to test. Then you're working on the other side of the room and it's a pain going back and forth testing things, so you end up leaving it turned on.
That's how you accidently create a short and smoke the power supply.
Ater I posed the question, I went back and-read the OP's initial comment.
The MRC power supply shorted, smoked, but did not wind up fried.
I must admit that I have shorted more than one of my MRC 1370s wiring up Tortoises and control panel DPDTs, but I never saw one smoke and I never fried one. Yikes.
The fuse and inline holder work great. Its been a long time since I have seen one of these automobile glass fuses. A five pack of 1 amp fuses and the inline holder cost all of 6 bucks. I added a grain of wheat bulb as a visual indicator. It was actually fun blowing the fuse when I tested it!
Thanx.
gregcTexas ZepherEWT123the +12v and -12v is baffling me.Circuit breakers don't care about the polarity. Just put one breaker in each circuit. but is this protection really needed? will they ever be needed during normal operation, or only because you forgot to turn off the power during construction?