# Need help understanding electric current pickup through trucks



## SRV1

I've been thinking about some caboose trucks I have and what I want to install for electrical pickup off the rails. I saw some pickup wipers a guy was making on a website. He doesn't have anything that will work with the trucks I intend to use. (The center of axle is an oddball shape.) So I've been thinking about how to make something of my own that will work well. His use foil sheets with wipers. One covers one side of the truck and then there's an insulating washer. Then the other sheet goes down over the bolt and the wipers run on the other side of the truck. With another insulated washer on top.
This got me to thinking about the metal wheels. From what I understand, if you lay a piece of metal across both rails you'll cause a short circuit. How do two metal wheels on the same axle not cause a short circuit? 

Then, I've got some lights that use a bridge rectifier. It says for these, you can run a small brass wire across both axles on each truck for electrical pickup. How can you do this without causing a short??
What is the bridge rectifier doing? By looking at the diagram, it appears it doesn't matter what's going into the one side of it but it splits to positive and negative on the other side where the wires hook up. 

I ended up buying one of these Digitrax DH163D to run 3 lights off of. Will I still need to use the bridge rectifier with this installed? What would be a good way to get power to the DH163D? 
Example, should I make some crude wipers that rub across one side of a truck and the other side of a truck, on both trucks? Just do this on one? Or should I run a wiper on one side of the front truck and one wiper on the opposite side of the rear truck? I guess I"m not understanding how the power is staying separate when the metal wheels are on the same axle, unless they are insulated from each other?

I've seen pickup shoes around but for what they are, it looks to me like you could just as easily take some copper wire and make something very similar that drags on the rail. If that's a good option.


----------



## D&J Railroad

In your first example, I think one wheel is insulated from the axel. Look real close at the center of each wheel and one of them will have an insulator on the axel then the wheel is mounted on that.
In your second example, diodes are used in the bridge rectifier. A diode will only allow voltage to pass in one direction. I think the end of the diode with the ring around it is the output of the diode, so voltage being introduced on that end will not be present on the other end.


----------



## SRV1

Ok. I re-read the packaging. It says free rolling insulated axles. So wire across these axles wasnt going to work anyway.

So im thinking a custom wiper on one side of front truck and one on opposite side of rear truck.

Sent from my SCH-R950 using Tapatalk 2


----------



## NIMT

Just buy a set of ring trucks and be done with it.

http://www.ringengineering.com/PowerPickUpTruck.htm


----------



## Howard1975

Looking at some of your previous posts (to see what kind of model trains you have), I see you operate regular North American type trains, in 2 rail HO scale. All of these HO scale trains require the wheels to be insulated from each other on an axle, when also equipped with metal axles. That is because when one rail is positive polarity, the other will be negative polarity. That is how the vast majority of HO scale trains work. 

Don't worry about the 3-rail Marklin HO popular in Europe, that is different topic. They don't have any insulation because they don't need it. Both rails in Marklin track are the same polarity. 

But with all equipment designed for 2 rail HO scale trains, all equipment must have isolated wheels. That means all freight cars, all passenger cars, even all locomotives, in order for the electrical system to work properly. The wheel-sets with metal wheels and metal axles, will have a small piece of insulated material in either one, or both wheels. Look very closely and you will see it. On some brands, it's easier to see then other brands -- but they ALL must have insulating material when using a metal axle. Same with all locomotives, there must be something to isolate the metal wheels from each other on an metal axle. Otherwise you will have a direct short circuit. 

Pictures to show what I mean.



















Look closely and you can see the black piece between the axle and the wheel itself. 

Okay regarding the pickup wipers, there are a few different ways that can be done. 

First of all, you can buy trucks already assembled and wired to collect electricity. Just mount on your cabooses and wire it up, for the lights. 

Second choice is to make it yourself. Here are two good articles about adding pickup wipers.

http://www.members.optusnet.com.au/nswmn1/ExtraPickUps.htm

http://www.55n3.org/cars/tender_wipers/


And a nice picture, using the brass centering spring that comes along with a Kadee #5 coupler, which fits inside the coupler box with the coupler, showing the piece modified. 













Hope it helps you, or give you some new ideas.

Howard


----------



## briangcc

I like Howard's suggestion and the only thing I would add is this...it only works if the truck is plastic. Mount that same setup on a metal truck and you'll cause a short circuit.


----------



## SRV1

briangcc said:


> I like Howard's suggestion and the only thing I would add is this...it only works if the truck is plastic. Mount that same setup on a metal truck and you'll cause a short circuit.



Yup. Thanks for the tips. I'm using metal trucks so I'll probably just use a spring like the above pic on one side of one and one side of the other truck. Would this be ok? Would the truck remain isolated from the wheels of the other side?

When I get a chance I'm going to test an LED on a car with these trucks just to see how well or poorly its going to work. The wheels are ribbed but I think I can keep the contacts above the ribs out towards the flange. 
I bought the trucks before I thought I was going to be adding lighting. I would like to try to use the ones I have before buying something else. But I guess I will if I have to.


EDIT: Ah, never mind. Just ordered some plastic caboose trucks. Why risk it.


----------



## timlange3

You could make your own pickup with thin copper. Use the bolster screw to fasten it and wiper(s) to the middle of the axle. That would pickup one rail, on the other truck, flip the wheelsets so the insulator is on the opposite side compared to the other truck, that will pickup the other rail then. Generally, using a bridge rectifier with an led light you are using the diodes in the rectifier as a voltage dropping source. Each diode drops 0.7 V, so with two you get the 1.4 V drop which should run your led.

For the bridge to work, you need a load, which you would not normally have with a caboose. I normally see bridge rectifiers to run lights in engines, the bridge ~ terminals go on one rail pickup, splice it in. Then the -/+ terminals on the bridge go to the led.

For a no load situation, just use resistors if the power supply is constant. Otherwise you will need a load (properly sized resistor) to make the bridge rectifier work.


----------



## SRV1

The bridge rectifier came with the sidemarkers I ordered for the caboose. Its going to have lighted, controllable, lanterns on the sides in back along with interior lighting.
With these ribbed steel wheels I was unable to see insulators on them. There a rounded bulge in the back of the wheels and the axles go in. I dont notice any insulator. The packaging says the axles are insulated.

Sent from my SCH-R950 using Tapatalk 2


----------



## timlange3

With metal wheels there must be an insulator somewhere, otherwise they will not roll! 

Use your multimeter to see where the insulator is located.


----------



## Howard1975

briangcc said:


> I like Howard's suggestion and the only thing I would add is this...it only works if the truck is plastic. Mount that same setup on a metal truck and you'll cause a short circuit.


Yes that is very true, I forgot to mention that in my post. It slipped my mind.

Howard


----------

