Hi,
I was contemplating building the "Current Blocking Type Capacitor Discharge Switch Machine Power Supply w/ Charged Indicator LED" from this link (the diagram is about 1/3 the way down the page).
http://home.cogeco.ca/~rpaisley4/CDPSU.html
I was wondering if this would be able to power a minimum of 4 twin-coil switch machines (2 Atlas Switch Machines & 2 Snap Relays) in a single push of a button? If not, what would I need to change?
Thanks in advance for any replies,
macmack
The bigger the capacitor, the more power available to throw twin-coil machines. In your situation, I would start off with the 4.7 microfarad capacitor. If that doesn't hack it, just install another capacitor IN PARALLEL with the first.
It's a good idea to use a separate 12VDC power supply for your CD circuit and switch machines. A wall wart from a dead 12V rechargeable tool will work. Mine is built around a cheap 12.6V filament transformer.
Chuck (Modeling Central Japan in September, 1964 - with a bunch of twin-coil switch machines)
The 'charged' LED is just extra parts in my mind. The one withotu the LED is the same circuit I built many years ago from "Practical Electronic Projects for Model Railroaders" which served me quite well on my N scale layout, even with those pesky sticking Atlas control boxes - once the CD supply wa sin, I neve rburned up another coil. I used the old train set power pack to run that, usingmy then-new Tech II to actually run the trains. Actually, i still have it, wonder if it would work (the CD supply - the Tech II I gave to the club about 15 years ago).
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Thanks for your response. Would a 12v DC power supply work with this particular circuit?
How big of one? One rated at 125ma would not work, one rated at 5 amps would be massive overkill. 1-1.5 amps would be about right. With a DC supply you could elminiate the 4 diodes - or just built it as is and hook up a simple 12VAC transformer.
Thanks for your response. I was thinking this one. It says it's rated 12v DC, 1.25 amps, regulated.
http://www.walthers.com/exec/productinfo/514-N3512
If I used this supply, I would not need the 4 diodes at the beginning of the circuit where the power supply comes in? Would it be powerful enough to throw a minimum of 4 switch machines at once?
macmack Hi, Thanks for your response. I was thinking this one. It says it's rated 12v DC, 1.25 amps, regulated. http://www.walthers.com/exec/productinfo/514-N3512 If I used this supply, I would not need the 4 diodes at the beginning of the circuit where the power supply comes in? Would it be powerful enough to throw a minimum of 4 switch machines at once? macmack
The 4 diodes in the front end are there to change AC to DC. You don't need them with a DC power supply. The 4 diodes are usually packaged into what is called a full wave rectifier - 4 diodes linked correctly in a single component with 2 AC leads and 2 DC leads. These are commonly available in 1 amp, 3 amp and larger configurations. Current rating should be bigger than your power supply output. Use at least a 50PIV rating, more won't hurt.
The switch machine throwing power of the CDU is controlled by 2 factors: 1) the size of the capacitor(s), and 2) the input voltage to the capacitor(s). The circuit recommends a capacitor of between 2200ufd and 4700ufd. The higher value should be good for 4 Atlas switch machines. Again, I would use a 50 volt rating on the capacitor to avoid any issues.
I would recommend against the regulated DC power supply, and here's why. A capacitor being used for energy storage - like this circuit - charges to the peak voltage, not the RMS (nominal AC voltage). For sine wave AC, peak is 1.4 times the RMS voltage. Thus, a 16 volt AC input, losing 1.4 volts in the rectifier, is a 14.6 volt RMS DC. The AC ripple will go as high as 1.4 times 14.6 volts = 20.4 volts peak, which is what the capacitor will charge to. This is also why you want more than a 25 volt rating on the capacitor. Your switch machines will operate very happily on 24 or even more volts, and will be much "snappier" than at 12 volts. The regulation of a regulated DC power supply removes the AC ripple, and therefore only provides the stated 12 volts.
So a cheap, unregulated wall wart or AC side of a broken power pack (the variable DC side is normally what breaks) will provide more power from your CDU than the regulated DC supply. If you can get a 12-15V AC wall wart, that's fine, if not the unregulated 12-18 volt DC is the better alternative. The current rating (amps or milliamps or VAC for AC) rating of the power supply determines how fast your CDU will recharge after firing. Something in the 1 amp range (12 VAC) is plenty; I would recommend at least 500 ma (6 VAC).
hope this makes sense and helps
Fred W
Thanks for your response. Actually, I do have an older (12 years old to be exact) Bachmann Power Pack that I used before, for my switch machines (AC terminals without a CDU.) I measured the voltage across the AC terminals at 18.7 volts. I also measured the amps at 1.8, but I'm not 100% sure I did it right (it's been awhile since the last time I did this, I did check the meter's book before doing this though.) The reason I say that is, the power pack itself says it's rated 20v AC, 7 Amps Total output. If I used it with the circuit, would I need to change any components within the circuit to operate a minimum 4 machines at once?
macmackHi, Thanks for your response. Actually, I do have an older (12 years old to be exact) Bachmann Power Pack that I used before, for my switch machines (AC terminals without a CDU.) I measured the voltage across the AC terminals at 18.7 volts. I also measured the amps at 1.8, but I'm not 100% sure I did it right (it's been awhile since the last time I did this, I did check the meter's book before doing this though.) The reason I say that is, the power pack itself says it's rated 20v AC, 7 Amps Total output. If I used it with the circuit, would I need to change any components within the circuit to operate a minimum 4 machines at once? macmack
I'm having a hard time imagining what Bachmann made that would draw 7 amps. And that they would supply a power pack that big with a train set. UL has been reluctant to approve toys with over 100 watt transformers/power packs since the 1980s much to the 3 rail O crowd's dismay. Getting a larger transformer approved has usually meant removing it as an accessory for a toy train set that could be used by kids. That 7 amps seems very high, even for their G (Large Scale) trains. Are you sure it doesn't say 0.7 amps instead of 7? Or 20 VA (volt-amps)?
I doubt the charging circuit of your chosen CDU design is going to hold up switching an inrush current of up to 7 amps when the capacitor begins charging. It would take careful checking of the ratings of the various components, and adapting the circuit to higher-rated components. I would imagine the rectifier and transistors would have to be heat sinked for longevity. One of the beneifts of a CDU is that a smaller power supply than could power the switch machines on its own can be used. The circuit is designed for something on the order of a 1 amp, 16 volt power supply. I'm sure it can handle up to a 2 amp power supply without redesign.
Which brings me full circle. A 20 volt, 7 amp power supply should be able to throw 4 Atlas switch machines without a CDU in the picture, and without breaking a sweat. So again, I question the rating. I assume the voltage reading of 18.7 was obtained without a load attached. Which yet again brings the rating on the power pack into question, because most transformers put out a slightly higher than rated voltage under no load. An 18.7 volt reading would be about right for a 16 volt AC output under no load.
There is no practical way to measure the maximum output current of the power pack. The best you can do is if the power pack has a current limiter instead of a circuit breaker, you can measure the short circuit current. Otherwise, you would have to try smaller and smaller resistance loads until you got the circuit breaker to pop. Then go one resistance size larger for the load and read the current. But that procedure is impractical if the ratings are truly 7 amps at 20 volts. Your load would have to dissipate as much as 140 watts while you experimented.
If the power pack is as big as you state (7 amps), don't bother with the CDU. If it isn't, it will work fine as the power supply for the CDU. Just make sure the capacitors, diodes, and rectifiers are rated at 50 volts.
yours in CDUs
Sorry, my mistake. It actually says it's "Max. Output: 7VA total."
macmackSorry, my mistake. It actually says it's "Max. Output: 7VA total." macmack
It will work just fine for the CDU power supply, but would be marginal for powering switch machines (even Atlas) without a CDU. Output will be about 1/2 an amp on initial charge, so charging may take up to 2 seconds after throwing turnouts. Use the 4700ufd capacitor, rated at 50 volts (or two 2200ufd capacitors in parallel). Diodes and/or rectifier only need 1 amp and 50 PIV ratings - a 3 amp rating will future proof a different power supply.
Enjoy.
Yes, this is about exactly the rating of the old train set power pack I used for my CDU - it was the power pack from an old Aurora Postage Stamp N scale set. Without the CDU, even a single N scale Atlas turnout would make trains noticeably slow. Couple the CDU with using a GOOD pack to run the trains and it was trouble free.