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
Hi,
Thanks for your response. Would a 12v DC power supply work with this particular circuit?
macmack
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.
–Randy
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
Hi,
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
Sorry, 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.
Fred W
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.
–Randy