So, as I mentioned in another thread, my modular club is looking into a new portable DC power solution. I've come up with a discreet solution using the Aristocraft wireless set, but the club's main electrical guy is still interested in a more elegant solution. Using a G scale decoder (to handle older openframe motors) and running the entire layout from that decoder. Or rather, a Decoder for each "cab." The issue with this is circuit protection. DCC decoders were never designed to handle shorts across the DC leads to the motor. So, in a derailment situation, the decoder would be toast. To that end, the club member interested in this has found this circuit that he thinks might be a good basis for a Fuse that is fast enough to protect the decoder Electronic Fuse for DC Short Circuit Protection by BRX46 | Circuit Project Electronic I know I'm not the only electrical guy on this board. So I'm looking for comments. Heck, I haven't worked in circuit design since college. I went into telecoms and didn't look back. So I'm more than willing to play second fiddle and be the soldermonkey for this.
decoders I know Litchfield has decoders for G scale. Pop them a question about your concerns. They do answer. I have been buying all my DCC stuff from them. LitchfieldLLC The trains.com site run by Kalmbach has Garden Railways forums. Rich
Good old Google. g scale decoders - Google Search I have not seen any circuit breakers to install in locos. I use HO scale decoders and they shut down if too much current. They start up when they cool off. Don't know about large scale though. Litchfield and some other online DCC sellers should have an answer. There are garden railroads using DCC now. That is why I recommended Trains.com site. garden trains dcc - Google Search Rich
I too would think that any selfrespecting decoder would have built in protection. I'd check the decoder spec. sheets really carefully and/or ask the manufacturers about it before looking for external protection. If you do go external I'd look at using a close tolerance fast blow fuse (not an 'ordinary' one) - ideally one designed for semiconductor protection. (They used to exist, but demand may have dropped for them now that: a) many circuits self protect, and b) a lot of electronic stuff is so cheap it is classed as consumable - not worth the cost of protection.)
I will double check, I've been relying on what I've been told in this regard about circuit protection. I'm actually kind of surprised nobody has thought of doing this already. Or I should say, none of the DCC companies have marketed this concept. It has 2 distinct markets. #1 transitioning from DC to DCC and more importantly, markets just like mine. clubs and such that want/need to support both. And need to support vintage equipment.
I would look at using a variation of the light bulb trick. Many people use a light bulb wired in series to limit current in DCC sections, but in the case you would want the light bulb between the deocder output and the track. The light bulb would have tobe selected to have a low enough resistance to not effect the operation of the trains but a high enough resistance when lit by a short to protect the decoder.
I'd be inclined to use a circuit designed for protection. Hence why I posted such a circuit looking for comment. The lightbulb trick might work, but man, no circuit designer would respect me ever and I'd never feel comfortable about it.
I see one problem with the circuit in the link. It uses relays which being mechanical may not respond fast enough. What you might want to consider trying is connecting a DCC Specialties PSX breaker to the input of a Zimo MX69V. The Zimo decoders are supposed to have over current and over heat protection on their outputs. The PSX can be configured for different trip currents so you may be able to configure the setup so the PSX trips fast enough to keep the Zimo from being destroyed. I would set the PSX for manual reset (J7 pins 1&2). Please keep in mind I have never done this and I'm not convinced it will work in every possible situation.
Let us know how this works out, YoHo. My club is interested in this too. You might want to look at PTC's. They should be fast enough to protect the decoder. Especially, if you you can mount a bigger heat sink on the decoder. http://www.optifuse.com/resettableptcs.php
A PTC looks like it would be a much much simpler circuit as well. I would think we could mount as big a heat sink as we could find. We're mounting this whole shebang in a roadie case, so a huge heat sink wouldn't worry me. Might want fans running too, but again, that's pretty low cost.
Decoders have protection but it can happen if overloade too many time. Too much current and they shut down until they cool off. It has happened to me. Most burnout issues are caused by mis-wiring or a lead coming contact with something it should not touch. That is why the program track option is so important when installing a decoder. Many modelers moving into DCC have little or no knowledge of electronics. Many have no idea of how much current a motor requires or a decoder can supply. Guessing can lead to errors. Rich
Sorry to drag this back up, but I have a question, would a Digitrax decoder with FX3 motor Isolation protection eliminate or curtail the need for the circuit protection I'm asking about here? I assume in this case a non-isolated motor is the same as a dead short? How much protection time does such a decoder provide? I'm curious, because I'm struggling to find a PTC with values I like. Digitrax DG583S might work, though at 5amp, 10amp max its way way more than we need, but I can match it to a 5amp Hold, 8amp trip 16VDC PTC which might work. That's still more than even the most ornery old Athearn open frame should need though. Also, riddle me this? If the DCC system is sized for HO, presumably I wouldn't even be able to send that much current out the system. Would I be able to even cause a problem here with a dead short on the motor leads?
