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Electrical Circuits For Model Railways

Description

This page was written before I started to use DCC to control my model railway. I've kept the page here as it may be useful to modellers who don't use DCC.

The points / turnouts on my first "Intres" layout were remotely controlled, electrically actuated. Point motors were of the form of Peco PL10 - twin solenoid - two solenoids aligned co axially. Energiseation of a solenoid causes the magnetic field within to increase, so attracting a metal bar which is free to move axially. Mechanically connected to the moving bar, is a linkage to the tiebar on the point. Remote control was achieved by wiring back the power circuit for the solenoids to a capacitive discharge unit via push to make switches mounted on the control panel.

Route setting

Instead of remote controlling each point, solenoids are electrically grouped such that energisation of the group results in a predetermined combination of point settings. These groups are track routes and this process is known as "route setting". A press of a particular push to make switch results in various solenoids being energised. The configuration of the track groups is determined by a home made diode matrix circuit. The diodes allow switches to control particular solenoids, whist preventing reverse current or back feeds.

Capacitive Discharge Unit - CDU

When a point is to be changed, you can switch it so that a 12V DC or 16V AC power supply is connected across it. Whilst this can work, it can result in the point motor overheating, and it is likely to result in contact wear on the switch (due to arcing). A solution to this is to power the solenoid through a Capacitive Discharge Unit, CDU.

Route Indication

When you've set a route, you'd like to see feedback that the route has been set correctly. That is where a mimic diagram of the trackplan can help - with some indication of which way the point is set.

 

Point Power

The capacitive discharge unit is a very straightforward circuit. The capacitors store charge and the stored charge is large enough to cause sufficient current to flow in the solenoid to create the necessary magnetic field. The capacitor is connected by a 1M bleed resistor in parallel to ensure the capacitors are discharged when the CDU is de-energised. The other resistors exist to bias, or set up, the transistor. The transistor is a switch. Without the capacitors connected across the solenoids, the transistor is switched on, and allows current to charge the capacitors. When the capacitors are connected across a solenoid, this circuit causes the transistor to switch off, so the only current from the mains is tiny transistor bias current. i.e. it is very small in comparison with the peak capacitor discharge current.

I cobbled this circuit together after having a good look round the web - especially at Rob Pearce's design (see this link: The Sump, Laisse and Huneausware Railway)

Above: Schematic Diagram for a capacitive discharge unit.

Above: CDU as assembled on a strip board.

 

Item

PN

Unit cost

Qty

Line total

Stripboard

as req

 

1

 

1N4004

3525338

0.031

5

0.155

4700uF Capacitor 35V

652088

1.79

2

3.58

0.25W 10k Resistor

 

0.012

1

0.012

0.25W 150r Resistor

 

0.010

1

0.010

Transistor 2n2222

 

0.61

1

0.61

Wire

as req

 

1

 

Total

 

 

 

4.367

Note: All part numbers are Farnell unless noted otherwise. Prices are ex VAT

 

Route Setting

The CDU is the power source, but the selection of multiple points for simultaneous switching is the job of the diode matrix circuit. This simple circuit uses diodes to enable multiple coils to be connected to multiple switches. It should be noted that simple routes can be set by simply putting two coils in parallel (eg for a cross over).

Above: Track schematic showing point / turnout motor numbers and wiring references for route indication. This is also the overlay I used to create the control schematic on the control panel.

 

Above: Circuit diagram for diode matrix showing wiring references for point operation.

Above: The diode matrix as assembled on a strip board

Item

PN

Unit cost

Qty

Line total

Stripboard

451058

4.15

1

4.15

1N4004

3525338

0.031

23

0.713

Wire

as req

 

1

 

Total

 

 

 

4.863

Note: All part numbers are Farnell unless noted otherwise. Prices are ex VAT

 

Route Indication

With DCC, track rails are always live. However, this is not entirely accurate. Beyond a point / turnout , the voltage appearing on the rails on the various ways off the point depend on what direction the point is set to, and indeed, what type of point is being used. As electrofrog points are being used, an isolation gap is installed to prevent short circuits...

By sensing the power on the ways off the point, It can be inferred what direction the point is set to. So you can take voltage tappings are taken from the rails beyond a point, and are connected to LEDs on a control panel schematic.

It should be noted that this technique is for Peco points and DCC track power only. There has also been much discussion about the reverse voltage withstand of the LEDs. All I can say is that I had this circuit in use for 6 months, for almost 30 LEDs (part numbers below) and did not experience any problems.

If you were not using DCC, you could install a changeover switch in line with the point motor eg Peco PL13. This would be able to switch indicating LEDs as appropriate.

No microswitches are required for the DCC configuration, and no electrical/electronic/mechanical log is needed of which way the points were last set.

 

Above: Schematic of the route indication circuit.

Item

PN

Unit cost

Qty

Line total

Control panel

as req

 

1

 

LEDs

637117

0.1520

16

2.432

0.25W 10k Resistor

 

0.012

16

0.192

Wire

as req

 

1

 

Total

 

 

 

2.624

Note: All part numbers are Farnell unless noted otherwise. Prices are ex VAT

Above: You can make the control panel track diagram fairly easily. Here I used MDF board. I drilled holes for each LED and switch. (The switch is a momentary normally open switch) The switch is connected to the diode matrix, and then to the points via a CDU. The LEDs are connected to the track immediately beyond a point / turnout. On the board, I've cut strips of red insulation tape to depict the track arrangement.

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