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first time posting here and need some help.   i have an old Lionel black river freight with a dc track and some 1122 switches that i am trying to get working again.   So after viewing several posts i am considering making the switches constant voltage since i have an extra transformer.  The transformer i have powering the track came with the train and has the reverse option by moving the throttle to the left and forward to the right.  When i put it in the reverse the outside rail becomes positive and middle rail is now the ground.  But here is where i am puzzled,  if i use a capacitor discharge and since the negative side of the capacitor is grounded to the outside rail during forward operation,  will the voltage flow backwards into the capacitor and explode it when i put the train in reverse? 

If so what is a work around?

Jeff 

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Hi Jeff,

   Welcome to the forum!

    You can make your own bi-polar cap (photo from the internet):

htthttps://electronics.stackexchange.com/questions/69715/can-two-electrolytic-capacitors-be-made-into-a-bipolarps://electronics.stackexchange.com/questions/69715/can-two-electrolytic-capacitors-be-made-into-a-bipolar

Or just a diode on the cap would protect it from reverse polarity but it would only function for one polarity.

I have asked this question on the MERG form, a UK paid membership group, and they tell me to put diodes across the caps, similar to what is in this photo also from the net:

So if you typically use one 2200 mfd cap, you'll need two 4700 mfd caps here to get about the same capacitance in the bi-polar configuration, and put them in a project box just to be extremely cautious.  Anytime I work with caps, I wear goggles until they are in a project box.  

You'll hear a lot of disagreement about this!

Just study what is on the internet from EEs.

Best wishes!

Take care, Joe.

Last edited by Joe Rampolla

Hi Jeff,

     I use this method for a cap discharge circuit with AC on the track, and it works well. 

My suggestion is to convert your loco to AC with this Lionel board (about $16.50) in the tender:

and run on AC so you can use the cap discharge technique above with AC. The circuit above will also work with DC provided DC+ is on the center rail.   BTW, Lionel's reversing board works on DC too, so you could always keep DC+ on the center rail and still reverse by interrupting power.  You also get a neutral position with the reversing board. 

Hope this helps!  I use bi-polar caps as electronic flywheels, so I reacted too quickly to your question.

Lionel has info on their web site about the AC conversion:

https://www.lionelsupport.com/...AndFAQs/TechTip2.cfm

Take care, Joe.

 

P.S. Link for ordering the board from Lionel:

https://www.lionelsupport.com/...;submitButton=Search

Last edited by Joe Rampolla
Jeffb posted:

... i am considering making the switches constant voltage since i have an extra transformer

I may be misunderstanding the situation, but...

if you have an extra transformer (i.e., plugs into a separate wall-outlet) for switch operation, its output will be electrically isolated from the original track power transformer.  Your existing capacitor-discharge circuit should work as-is with the capacitor "-" tied to the outer-rail and capacitor "+" being charged by the extra transformer's "+". 

The problem is if you want to use the same DC-output transformer for both track power and switch power.  In that case, you could buy a DC-to-DC converter (isolated-variety) module.  Then tie the "-" output of the module (now electrically isolated) to the "-" of the capacitor-discharge circuit.  Again, because of the isolation, the capacitor discharge circuit does not "care" what voltage is on the outer-rail.  DC-to-DC modules (isolated) are thumbnail sized and maybe $5 or so...it can be a relatively low power converter ($ proportional to power) since capacitor-discharge circuits are slow-charge (input), fast-discharge (output).

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