Bipolar power supply

[ccg]

In this months  model railroader mag (august 2011) pg 60. The building a bipolar power supply article, diagram shows two AC power supplies (T1) and (T2). Can 1 AC power supply be used to supply the two full wave rectifiers, (D1) and (D2).

Your thoughts please  CCG

[gregc]

no.   if you want two dc supplies, even if they share a common ground, you'll need 2 AC transformers or one  with 3 output leads.

[cacole]

You could achieve the exact same results by using a 24 Volt, 4 Amp center-tapped transformer.  The ones listed in the Materials list are actually center-tapped transformers but they're not using the center tap.  Cost was probably their main concern.

Using All Electronics as a parts source, the transformers listed, TX-122, 12 Volt @ 2 Amps, are priced at $8.75 each.  A 24 Volt, 4 Amp transformer, TX-244, is priced at $25.00 each; so using two smaller transformer is cheaper than a higher-capacity single transformer.

It's also possible that All Electronics didn't have the 24 Volt transformer when they wrote the article, since All deals in surplus products.

[retsignalmtr]

The Idea of using two DC power supplies is for reversing the polarities of the wires to the DC switch machines. The same thing can be accomplished by using DPDT toggles wired as reversing switches for the stall type switch machines with a single DC power supply.

[rrinker]

 There's a third way. On my friend's layout, he had a 12V AC power bus running around to power accessories. For each Tortoise, a SPDT toggle (actually, for many of them he used pushbuttons, so the Tortoises weren;t always on) and a pair of diodes were used. Since the Tortoise draws such low current, the small glass-cased 1N914/1N4148 tyoe you can get in a 50 pack at Radio Shack work fine.

           --Randy

 

[lonewoof]

I haven't seen the circuit in MR, but the advantage of having two separate transformers is to be able to keep the two DC supplies totally isolated from each other, or to tie outputs together (you could have a common negative or positive,  or positive and negative outputs with a common ground). You can't do all that with a center-tapped transformer.

 

/Lone