Turnout Controller

Eight of the PIC outputs may be used to control eight turnouts. The controller circuit is quit simple and no extra logic is required. However, some of the components may be bulky. Besides, the card edge may not be able to accommodate so many connections. Changes in the driver circuit may involve using two outputs per turnout. For these reasons, each controller will dive only four turnouts.

The driver consists of a transistor driving a relay discharging one of the two capacitors for each line. The capacitors are charged using a constant current source running off a +16V supply.

Constant Current Source

Turnout motors which use a solenoid type mechanism like the Peco ones, need a large surge of current for a short time. A capacitor discharge circuit is ideal for providing this. The capacitor will have a resistor in series to limit the charge current. This current may also flow through the motor coil in case of a malfunction. For this reason and also to limit the initial surge, the series resistor has to be kept relatively high. This, however, will result in a longer charge time and reduce the cycle time. The surge current demands place a heavy load on the power supply with a large number of turnouts used.

The constant current supply gets around all these problems. The current level can be kept fairly low while still allowing a low cycle time. If the current rate is kept at 20mA for a +16V supply, a 1000uF capacitor should take about 0.8 seconds. For the same value of max. current you would need a 820 ohm resistor in series. This would take about 4 seconds. For the capacitor to charge in the same time, it would take a 220 ohm resistor with a current of 100mA. With the lower current requirement for a constant current charge, a smaller accessory power supply can be used. With a 1A supply you can run 50 turnout motors at a rate of under a second even if the were all switched at the same instant.

  Command    Function
  ---------  ---------------------------------
  0100 0000  Poll
  0010 pppd  Set point ppp to direction d

The circuit uses a DIP relay which has a fairly low profile. However, a couple of power MOSFETs may be a better solution here. They will replace the relay contacts and will need two inputs per turnout to avoid adding a 74LS04.

Updated on 2 Nov 95. Feedback to