Dispatch board

5-cent diodes can be paired to effect a ~3/4 Volt drop.  And can be cascaded one after another to increase the drop.  So as shown above, 3 pairs of diode can step down your hot wire on its way to the dispatch board.

If your cupboard (stash of electronic parts) is bare, you will spend much more on shipping than on the diodes!  I hear Radio Shack is attempting a comeback.

Do these diodes just come in one size?

If not, what should I ask for?

Do the diodes come with leads attached to which I just solder on the hot wire?

Thanks Stan, you are always very informative.

There are thousands of different diode part numbers that would work in this application.  If you go in person to an electronics parts store (even a Radio Shack if you have one), ask for a 1N4001 diode; you can also use a 1N4002, 1N4003, etc. up to 1N4007.  They should be the same price.  You need 6 to make 3 pairs.  At retail in onesy-twosy quantities, I'd think no more than 50 cents each.

If you do mail-order, here's an example from eBay and Amazon shipped from within U.S. 

They have relatively long wire leads on them.

If you have a handful of wire-nuts you can assemble it without soldering.  They key is to note that each "pair" should have the silver band on the opposite side.  Hope this makes sense.

Very helpful, and educational.

Thanks Stan.

I ordered IN4007 diodes.

Should I start with 2 pair of these and then add a third pair if necessary?

Actually, I was thinking you should start with 3 pairs...and then add a 4th pair if necessary!

That is, while I don't have the Dispatch Board I would have thunk it was designed to operate well with 14V AC which I figure is the most common Accessory AC voltage in O-gauge.  3 pairs drops the 18V AC to about 15V AC...and 4 pairs drops it to about 14V AC.  I guess it's a question of how much exercise the man needs!

Hey Stan....

Does this circuit generate any heat?

Yes.  The heat it generates is proportional to the voltage being dropped.  Using specific examples presented earlier, say the original voltage is 18V and the desired "final" voltage is 15V.  So there's a 3V drop in the diodes.

15/18 of the power (83%) goes into the Dispatch Board to light the sign and move the man. 3/18 of the power (17%) is converted to "pure" heat.  Of course, in the end, the power into the Dispatch Board becomes heat too as the bulb filaments glow generating heat and the moving figure mechanism warms up from friction and such.

I couldn't find info on the web with actual power numbers (in Watts) for the Dispatch Board.  Again, I don't have one of these widgets, but my guess is it might require, say, 10 Watts of power.  Skipping some tedious math, this means the diodes would generate about 2 Watts of heat...spread over the 6 diodes (or however many are required).  That means each diode is like a 1/3rd Watt heater.  I don't know if there's an "official" definition of warm vs. hot, but I'd say the diodes would be warm to the touch but should not burn you finger.  My understanding is the Dispatch Board operates intermittently rather than 24/7.   In which case I don't think you'd even notice the heat from the circuit.