Here's how I replaced the balky disc rectifier in a recently-acquired RW transformer with a venerable (but operable) zener diode!
DISCLAIMER 1: Don't try this at home. Ever. I'm a trained and federally certified aircraft and powerplant technician, as well as a professional electronics tech. I know, fear and respect house mains current.
DISCLAIMER 2: See Disclaimer 1.
DISCLAIMER 3: If you INSIST upon delving into transformer guts, PLEASE PLEASE PLEASE unplug the dratted thing first. We much prefer talking TO you than ABOUT you. Even if this IS the Internet.
Ahem.
To start with, a nice diagram and schematic of the RW transformer may be found here:
http://ogrforum.ogaugerr.com/topic/need-help-on-a-rw-110-transformer
The second PDF page gives a good summary of the operating principles behind the whistle switch. Basically, there are two approaches to replacing the disc rectifier:
1) Use a regular diode. In this case, one keeps the booster circuit (between wires 9 and 12 in the diagram).
2) What I did, which is to junk the whole assembly and just put a nice, big, juicy ol' zener diode I had knocking about the house* between wire 12 and contact A. In this case, wire 9 is removed with the booster circuit. While this does remove the "2 stage" functionality of the whistle switch...with Brer Zener in there, ya honestly don't need it.
*If your house is not overrun with nice big juicy ol' zener diodes, move to a house which is. Or pick a diode up online.
So! On with the purty pitchers.
Here is the stock rectifier disc assembly. Note the cruel, close set eyes and criminal demeanor. Obviously up to no good.
Back of the disc rectifier, showing the brass lead going to Contact A. You will need to loosen the tabs up top and lift up the deck in order to get the rectifier bracket loose and remove the stud.
The chassis with the rectifier assembly and wire 9 removed. Note the big yellow wire (wire 12).
View of the back of the rectifier bracket, showing the brass path to Contact A.
New lug made from a 6-32 screw. Note that the screw head has been taped to prevent unfortunate incidents involving the unwanted transfer of electrons to/from various spots.
The zenon diode (a cherished family heirloom*) installed. Note that Wire 12 goes to the base of the diode and that a small jumper soldered to the contact of the diode goes to the new Contact A lug.
*True story. I found it in a fishing tackle box full of old electronics parts I inherited from my late father circa 1982.
You may, if you wish, fabricate a small bracket to hold the diode in place. Or, if you're like me, you can just wrap the little beggar up in electrical tape and call it good. ;-)
So! The RW is now assembled and hums happily. I tried the whistle switch on two different whistle tenders and they whistle up a storm now!
Hope this helps!
Mitch
DISCLAIMER 4: House current. Voltage. Be careful with that stuff. Leave it to the experts if you're not sure of what you're doing.
I think it would be helpful to those wanting to try this to put out a little information about the zener diode's specifications, like:
o What the heck is a zener diode?
o What should the zener voltage be?
o What power rating should the zener diode have?
I'll try to answer these questions:
A regular diode conducts current only in one direction, sort of like an electrical check valve. A zener diode conducts current in either direction. In one direction, it's just like a regular diode, having perhaps only fraction of a volt drop. In the other direction it conducts only when the voltage is higher than a specified value--the zener voltage. If you go to buy one, the seller will want to know what zener voltage rating you want. (They were discovered by the late Clarence Melvin Zener.)
As long as the DC voltage for operating the whistle relay is small compared to the overall track voltage, that DC voltage will be about half the difference between the zener voltage and the diode's forward voltage, which is about .7 volts. Lionel said that the relay should operate at 1.1 volts; so the zener voltage should be about 1.1 V * 2 + .7 V = 2.9 V. That's the bare minimum for a relay in its prime. A little more wouldn't hurt.
A regular diode heats up a little because of the product of the forward voltage and the average forward current. A zener also heats up because of this but also quite a bit more because of the product of the reverse (zener) voltage and the average reverse current. The average current is approximately 90 percent of the rms current. Half that is the average current during each half of the cycle, so the power heating up the zener diode is the sum of the forward (.7 volt) voltage and the reverse (zener) voltage, multiplied by 45 percent of the rms current. If the train draws an rms current of 5 amperes and the zener voltage selected is 4 volts, the power is 5 * .45 * (.7 + 4) = 10.575 watts. So the zener diode should have at least this power rating.
Bob Nelson
Good points all, Bob! Thanks for mentioning that! :)
The Lionel service manual states that internal transformer whistle controls provide a "pick up" voltage of 4-5 volts. Does that factor into the calculations above?
It is something to consider. That 4 to 5 volts seems to me to include quite a large margin of error, compared to the 1.1 volts that Lionel says the relay needs. I also think it may be more than modern phase-control power supplies put out. My guess is that it is just the consequence of switching in a regular diode, as the old Lionel circuit did, before shunting it with the resistor. The 10-volt zener diode needed to generate that much DC will gobble up quite a bit of the overall rms voltage.
I would want to experiment to see what zener voltage just reliably operates whatever kind of whistle I had, whether relay-operated or electronic, and not go above that value. Perhaps those who have used zeners successfully can advise us about their diodes' ratings and what sort of whistle circuits are in their locomotives.
Welp, I can't speak as to specific voltages and such, but a field test using my home layout, A-U to the lockon, with a 2025, 671W tender* and a small consist of MPC coaches shows that, when the whistle button is engaged, the lights on the loco and cars do dim a little bit, as opposed to brightening as happens with the original circuit...Other than that, the whistle operates reliably across the range of voltages.
Incidentally, if you have a whistle-operating transformer that isn't delivering power to the track, check the whistle switch. I had to tinker a bit with the contacts of this RW in order to get it to work properly, and there's still a bit of slop in the wiper which will need tightening at some point... :)
*This means that there's a S-1 turbine out there somewheres with a 2466 tender, I suppose. The hazards of picking up stray locomotives.
Mitch, is there a number that is legible on the diode?
Didn't notice a number, just the polarity diagram. :)
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