Capacitor Discharge Power Supplies

For Twin Coil Switch Machines

  This page presents various capacitor discharge power supply circuits for use with twin coil switch machine motors. Also shown are some optional wiring methods that can be used to enhance the availability and control of these supplies.

  The schematics on this page have sections that have heavier lines. These lines represent the portions of the circuit that are subjected to large current surges when the switch machines are operated. Careful attention to good wiring and soldering techniques should be used for these sections to ensure good results.


Basic Twin Coil Switch Machine Power Supplies

  The following diagram shows two basic twin coil switch machine power supplies.

Basic Capacitor Discharge Units

  Circuit A is the simplest type but suffers from a relatively slow recovery rate.

  Circuit B is a very popular power supply as it has a very good recovery time. This circuit has a high current surge when the capacitor is charging.



Simple But Effective Switch Machine Power Supply

  The first circuit of this page is for a classic Resistor / Capacitor unit but with a few modifications made.

Simple Resistor / Capacitor Unit schematic

  The standard bugaboo with this type of circuit is the relatively long charging time of the capacitor. But if you are willing to wait the 1 second that a 2200uF capacitor takes to charge to 90 percent of its maximum voltage when a 220 ohm resistor is used this can be a simple and cost effective power supply. Using a lower resistance charging will shorten the charging time proportionally.

  Shown in the top power supply circuit is an indicator circuit that shows when the capacitor is near its full charge. This is an optional feature and can be left out as shown on the second unit.

  To increase the availability of switch machine power more than one discharge unit can be connected to a central transformer and rectifier / filter capacitor. This would allow smaller discharge units to be placed around the layout and used to operate machines that are nearby.

  The diode in front of the 220 ohm resistor will prevent units from draining the voltage from each other when turnouts are thrown. Although this is unlikely to happen with an adequately sized supply transformer.

  If the recharging time is not too important such as for machines that are not thrown often, the value or the 220 ohm resistor could be increased and its wattage reduced. The maximum potential load on the circuit will be reduced accordingly. If not as much pulse current is needed to throw the turnouts then the value of the capacitor could also be reduced.

  With a 16 volt AC supply transformer the DC voltage across the capacitor will be about 21 volts and the maximum charging current will be 0.2 amps.

  For a circuit of this type the charging time is dependent on the values of the resistor and capacitor used. For example if a 220 ohm resistor and a 2,200 microfarad capacitor are used then the charging time constant would be as follows.

220 Ohms X 2,200uf = 0.484 Seconds (1 Time Constant)

  This is the time that the capacitor would take to reach 63 percent of the supply voltage. The time needed to reach approximately 88 percent of the supply voltage is 2 time constants.

2 X 0.484 = 0.986 Seconds (2 Time Constants)

  For practical purposes the time to reach the supply voltage is 5 time constants.

  If you have a little patience, about one second's worth, the basic resistor / capacitor power supply can be a very effective and economical system. However if you have the need for speed then one of the more sophisticated supplies is required.


  The 5 watt resistors in the following circuits are for safety.

  If there was a short circuit in the discharge portion of the CDU circuits, the 5 watt resistors would limit the current value to a safe value.


Current Blocking Switch Machine Power Supply

  The Current Blocking type of switch machine supply is a widely used device and is well documented. It is reliable and practical design.

  Simply stated; This type of supply blocks the charging current to the storage capacitor anytime current is flowing from the output of the circuit, such as when a switch machine is activated.

  The only modification that this circuit might use is: (1) A resistor to limit the maximum charging current to a reasonable level. (2) a "Capacitor Charged" indicator. These additions are shown on the next diagram.

  The first schematic on the following diagram shows a current blocking switch machine power supply with a 10 ohm resistor in the collector circuit of the blocking transistor. This resistor should be a wire wound type power resistor as it will have to handle peak currents of approximately 2.4 amps.

  The second schematic shows a "Capacitor Charged" indicator added to the circuit. The LED will begin to glow when the voltage across the capacitor is approximately 16 volts and be fully lit at about 20 volts.

Upgraded Current Blocking Switch Machine Power Supplies



Split Supply - Current Blocking Switch Machine Power Supply

  The next circuit is essentially the same as the one above except that a split of dual output is provided. The circuit therefore has a PLUS, MINUS and NEUTRAL output terminal but functions in the same manner as the single sided power supply circuit.

  A 1N4001 diode has been placed in the emitter circuit of the power transistors to ensure that the proper voltage differential is achieved between the base and emitter junctions of these transistors.

  This circuit has not been tested on a layout but should work as designed. Please take some time to experiment before actually installing the circuit.

Split Supply - Current Blocking Switch Machine Power Supplies


A Power Supply For Use With Silicon Controlled Rectifiers

  The next power supply is designed specifically for use with silicon controlled rectifiers. The SCR's would take the place of push buttons in the high current paths of the switch machine control circuits.

  These devices are ideally suited for use where large current surges are expected and in many cases they are less expensive than darlington transistors.

SCR Compatible - Current Blocking Switch Machine Power Supply schematic


SCR Compatible Circuit Operation

  For the purposes of this description the storage capacitor will be fully charged and the circuit ready for operation.

  1.   When pushbutton S1 is operated the voltage across capacitor C1 will be bled off through the 330 ohm resistor and into the GATE of the silicon controlled rectifier.

      The SCR will turn ON and the storage capacitor will discharge through the "A" coil of the switch machine and the turnout will throw.

  2.   When the voltage across C1 falls below approximately 2/3 of the supply voltage the output of comparator IC 1 will go HIGH. This will cause the transistor Q1 to turn OFF and block the charging current to the storage capacitor C2.
  3.   When S1 is released, capacitor C1 begins to recharge. When the voltage across C1 reaches 2/3 of the supply voltage the output of IC 1 will go LOW and Q1 will turn ON allowing C2 to recharge.

      There will be a 0.2 second delay between the opening of S1 and the output of IC 1 going LOW. This delay allows the storage capacitor to fully discharge and the SCR to turn itself OFF before recharging of C2 begins. With out this delay the SCR might remain in an ON state and the storage capacitor could not recharge.


SCR Compatible Circuit Notes



A Power Supply For Use With Transistors

  The next circuit is a variation of the SCR compatible power supply. This version can be used if transistors were used to control switch machines.

  The only significant differences between this and the SCR compatible circuit is the removal of the time delay before recharging begins and the higher control current that would be required for transistors as opposed to silicon controlled rectifiers.

  As with the SCR compatible circuit this version cannot be used with pushbuttons in the coil circuit as the charging current would not be cutoff when the button is pushed.

Transistor Compatible - Current Blocking Switch Machine Power Supply schematic

  Incidentally, this circuit would be a good choice if optoisolators were used in place of the pushbuttons, as might be the case with computer controlled switching. This technique is shown in the "B" COIL section of the above schematic.



Transistor Control Of Switch Machines

  The next schematic shows how a Darlington type transistor could be used to replace a push button in the coil circuit.

Transistor Control for Switch Machines

  Using transistors to control the machine would allow operation of the turnout over long distances or via a computer interface. By using a small current to control a large current just as with a relay.

  In this type of operation the control current should be from a filtered or regulated DC source.

  Also using transistors to replace push buttons would allow a diode matrix to be built using signal diodes as these would be subjected to only the smaller control currents required by the transistors.



Twin Coil Switch Machine With Latching Relay

  The next schematics show how 'latching' type relays could be used with twin coil switch machines.

Single Coil Latching Relay

  The SCR circuit may be the better choice for CD type power supplies as the SCR's don't need any gate current once they are switched ON.

Dual Coil Latching Relay


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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.

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