Hi recently I started a thread where I got great help coming up with a method of replacing the Atlas # 200 snap relay. This was great!

Now I have another issue where someone out there may be able to help me again. I have put this to a model railroading forum and have not received any replies yet, so here goes here:

The following information is on the PL-11 side mounted turnout motor:
The turnout motor is for low voltage use only. 16v AC at 2 amps is recommended. The motor is to be energized only momentarily. Never allow the current to remain on. Use Peco lever switch PL-26 or Probe and Studs PL-17 & PL-18. The motor will work more efficiently with a capacitor discharge unit (CDU) PL-35, (only one unit per layout needed).

What exactly does a CDU do? Is it necessary? Can someone help me build one maybe? What kind of a toggle switch can I instead of the PL -26?

I was originally going to just hook it up to an extra Atlas sliding switch used to energize the Atlas turnouts. This did not work when I tried it.

I am attaching a wiring diagram for the PL-11 switch motor.

 

It seems to me that this capacitive unit can only work on a DC supply, though you've drawn it as a box which could have a rectifier in it. As I understand it, these things work by charging a large capacitor via a resistor (hence the current draw isn't very large--2 amps is way more than it needs) and then when you want to operate a turnout that has a low-resistance solenoid, it dumps all the energy through the coil. That means there's no pulse of current from the supply, and it's safer for the solenoid too because if the current is accidentally left to flow continuously, it's limited by the resistor and so the solenoid won't overheat. The reason that you can use "only one unit per layout" is that it can recharge between uses, and the same capacitor can serve any number of turnouts, as long as you only operate them one at a time.

But I don't think you can use a toggle switch for this. You need something that provides momentary contact, like a pair of pushbuttons for the two settings of a turnout, or a spring-loaded switch with a center-off position.

 

Hi maassmi,

I am not a model railroader, but I'm a good Googler. Have a look at this link.

http://talkingelectronics.com/projects/CDU-2/CDU-2.html

I think you could build this kit one easily.

Regards,
Ifixit

 

Cool web link! That definately helps me build one if I choose to. Thank you.

 

John P, I was thinking that a toggle switch that operates momentary on - off - momentary on fashion with a spring that moves the toggle back to the center position automatically.

 

Not sure what the CDU is. Control Display Unit, perhaps?
For aircraft, CDU means Cockpit Display Unit.
[eta]
Ahh, yes - capacitive discharge unit.

Your existing AC supply is 19VAC, and it is not capable of supplying 2A of current. You need a transformer rated for 16VAC @ 32VA or 32 Watts.
[eta]
Your low power unit can be used with a CDU though.

For the switch, you can use a SPDT (on)-off-(on) switch. The parenthesis means that the switch is momentary in that position; it will spring away when released.

The Peco PL-26 is a bat-handled switch that contacts in passing; the switch stays in one position or the other so you can see by glancing at it which way the switch points are thrown. You can't do that with a normal SPDT (on)-off-(on) bat-handled switch, but the latter is much less expensive.

Your Atlas #56 switch control boxes might work, but I don't know what current the contacts are rated for. If 2A is excessive, the switch won't last long.

Your wiring diagram isn't right. Have a look at the attached.
[eta]
I omitted the CDU from my diagram, but it would go between the AC power supply and the Atlas #56.

 

Hello Again SgtWookie,
I wired the Atlas switch to the Peco motor just as you had drawn it. It did work, but there did not seem to be enough power in the motor to switch the turnout from one position to the other. Do I need to remove the spring that holds the turnout in one position or the other that is in the turnout? I'm thinking that removing the spring would work, but when I tested the switch motor off of the turnout, once I removed the power from the motor, the motor's pull bar went back into the original position. I am getting frustrated with this. Any thoughts on this part of it.

 

What would happen if I were to run my Atlas switch motors through the CDU?

 

I don't know if your Atlas #56 switches are rated for the 2A peaks.

Your Atlas switch motors seem to switch just fine without having a CDU, right?

If so, then your Atlas motors should not be put on a CDU circuit; it might burn up the motors.

Don't remove the spring from the switch. I'm re-drawing that Talking Electronics schematic; it doesn't have to be quite as complex as he has it. I think you'll be able to get everything to make one from Radio Shack.

Have a look at the attached. I simulated the circuit in LTSpice; that's what the plots are on the bottom. The spike in the center is when the switch is pressed for 1/10 second, and a couple Amperes of current blasts through the motor for a moment.

 

Here, I've updated the schematic I posted earlier so you can see how to make your Atlas switch motors and the Peco switch motors work together with a CDU. See the attached.

 

I am trying to find printed circuit boards for the CDUs and I am not able to. On Radio Shacks website, they only have one type of board but a bunch of connectors. Not having much luck from my searches. Probably because I really don't know what I am looking for... Any help on this?

 

You really don't have to buy a custom circuit board. You could use a generic board, and use bus wire or component leads to connect the parts, and solder it all together.

 

SgtWookie, do you think you draw your CDU for me much in the same fashion you drew the curcuit for me in the other thread. That drawing is easy for me to understand. Any help is appriciated.

Also, the basic difference that I am able to see between your design and the one that was posted earlier by ifixit is that your design lacks four resisters. This is correct, or am I missing something?

Wish me luck. If I actually get it built, I will post a photo of it...

 

Also, the basic difference that I am able to see between your design and the one that was posted earlier by ifixit is that your design lacks four resisters. This is correct, or am I missing something?

The 1K resistor in SgtWookie's version does the same job as the 4 resistors in the other version (which together act the same as a single 550 ohm resistor).
The reason there were 4 was to spread the power between them, but because SgtWookies uses a higher value, the power in the resistor is reduced so 1 should be fine.

 

Cool! Good to know. Thank you.

 

Okay, I am preparing a shopping list for the construction of this CDU. I happen to have some resisters I purchased on eBay for another lighting project. I was wondering whether I can substitute these anywhere in this project? I also have several different sizes and colors of LEDs along with these resisters. Does it matter what size or color of LED I use. For this I was going to use my smallest one or one of the smaller ones that have the colored lens (this package is already open). I am attaching a photo of the resisters I have since I don't really have a clue.

 

OK, here you go; I made it more simple yet:

Fewer parts to assemble. I eliminated the LED & resistor, D3, and combined the two 1,000uF caps into one 2200uF cap.

The 2200uF capacitor won't look quite like the one shown in the layout; it has axial leads instead of radial.
Axial leads means that there is one connecting wire on each end of the cylindrical cap.
Radial leads means that the connecting wires come out of the same end of the cylinder.

You may have to solder a wire on to either or both wires to make it/them long enough; make sure that the wires can't short out against anything.

[eta]
Updated board layout; I'd forgotten a wire.
[eta]
Added schematic & simulation graphic

 

Now I am scared! I was just finishing this sketch when yours arrived.
I understand how I can replace the one larger capacitor for the two. But it also appears that I may have one of the diodes backwards. Please take a look.

 

Now I am scared! I was just finishing this sketch when yours arrived.
I understand how I can replace the one larger capacitor for the two. But it also appears that I may have one of the diodes backwards. Please take a look.

1) Diodes are all backwards.
2) TIP120 connected incorrectly; with pins down and lettering facing you they are 1:base 2:collector 3:emitter
-- you have the base and collector swapped.
3) LED is connected backwards.
4) The polarity of electrolytic capacitors is usually shown by a black stripe, which is closest to the negative lead.
5) The output of the CDU is DC, not AC.
6) Wiring of switch not shown correctly.

Besides that, it's pretty good.

 

Scared - Not scared of the project, it was just scarry that you SgtWookie was thinking and doing the same thing for me. Trying to draw it out for me so that I might understand it better. I would love to keep an LED so I know that power is flowing through properly. Also in your new sketch, it appears that the diodes are pointed in different directions from your first diagram you sent. My drawing was based on your diagram. I used long and short legs on my drawings of the LED and the capacitors to illustrate what I see in the photos I am finding on the internet. I know you are putting a great deal of time into helping me and I would like to thank you for your time, knowledge and assistance.