Since by tests on the DATA ACQUISITION UNIT, several major brands of switch machines drew over 40 amps for a few milliseconds, large power packs or transformers are required for reliable operation. There is also the danger of burning solenoid windings if pushbuttons are depressed too long, using unlimited current. Pushbutton and switch machine accessory contacts often burn due to heavy arcing caused by inductive kick at break time. Although the maximum UL approved 48 volts peak can be used with no damage to coils, this agrevates the other problems. AC can be used but it produces excessive noise and more heat. Capacitive power supplies were developed to eliminate most the problems, however they introduced some of their own.
They are not useful in normal diode matrix applications, where more than one turnout is thrown at the same time. A very large capacitor would be required and worse, a machine(s) with low resistance could shunt current from the others with higher. For domino or sequential applications, insufficient recharge time or lack of recharge prevents throwing successive turnouts. This also applies to rapid pushbutton operation.
NOTE: In selecting parts, economy is not the key to success, but reliability is. Within reason safely overrated paramters avoid failures. Using a 35 V capacitor in a 35 V unregulated circuit is asking for trouble. Line voltage surges could cause punch through, A 50 V rating is safer and the small amount saved may be negated by failure, costing more in the long run.
The basic supply consists of a small DC input supply of the right voltage, a current limiting resister and a large capacitor. After complete charge time, the capacitor will always reach full input peak voltage. Inexpensive surplus filtered switching supplies of the right voltage can provide fastest charge time. Unfiltered FULL OR HALF WAVE supplies may be used to increase voltage, but charge time increases respectively with half-wave slowest. An old powerpack may be used, by using DC accessory output or track output, using throttle as part of the limiting resister. These should yield voltages up to about 30 V. Most throttle rheostats are in the 50 - 100 ohm and 10 - 50 watt ranges. The capacitor and the solenoid inductance form a low frequency, series resonant circuit, which effects the output pulse.
In circuits with inductive kick, it is wise to include a diode to eliminate possible negative spikes which may punch through capacitor. Any diode with a rating of 1 A and 100 V, such as 1N4002, will do. Also these should be placed across all switch machine cutoff contacts to reduce contact and push button burnout potential.
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Basic switch machine power supply
First the input voltage must be determined and selected. Using an unfiltered input, the capacitor will charge to the peak voltage, which is 1.414 times the RMS value normally measured on a meter. Using the National Electrical Code maximum of 48 V for low voltge curcuits, the highest usable input would be:
48 / 1.141 = 33.94 V RMS.
This would permit use of a common capacitor working voltage of 50 V. Using a more common 25 V RMS filament transformer and unfilteréd output the peak voltage will be:
1.414 * 25 = 36.36 V.
Since the total charge stored in a capacitor is measured in a very large number of electrons called Coulombs Q and amperes = coulombs / second, the greater the charge, the larger the current that can be supplied by it. Charge on a capacitor equals the voltage * capacitance C in Farads.The higher the voltage and the larger the capacitor the greater the charge.
Charge Q = C * V
To avoid overheating, the resistor limits the input supply current that is fed to the solenoid coil, when its impedance drops off. Since I = V / R, a 500 ohm resistor would limit current to .05 amp, which should not overheat anything. The minimum wattage for 25 V is:
W = V^2 / R = 625 / 500 = 1.25 watts.
A 2 watt should work well. The internal resistance of the input supply should be considered part of the total. This can be very small with filtered supplies and very large with power packs using older rectifiers.
TIP: In empirical circuit design, the use of variable components can reduce extensive calculations. In this case, using a potentiometer or rheostat can help. Once optimum operation is established, resistance can be measured with an ohmmeter and the variable can be relaced with a less expensive fixed resistor.
The use of cutoff contacts on switch machines can reduce the size of the resistor and coil heat, since current is removed immediately when solenoid is toggled. Since many machines are spring loaded and although adjustment is tedious, contact leaves may be bent to break just past the toggle point. The spring will continue throw and possibly the capacitor will not be completely discharged. The residual voltage can be measured with a high input impedance voltmeter to verify.
Charge time is derived from RC time, which is time in seconds = resistance in ohms * capacitance in farads.
T = R * C = 500 * 4700 µf = 500 * .0047 = 2.35 sec.
But since the charge rate is an exponential curve, this only gets to about 63.7% and for a nominal full charge greater than 99%, five RC times are required yielding 11.75 sec. This is an intolerable wait in emergencies or setting up yards.
Selection of components is dependent on switch machines and input supply used. Construction can be very simple by just securing the resistor and diode to the side of the capacitor, slipping spaghetti over leads to insulate; then soldering connections and external leads. The assembly can be slipped into any insulating container. Since heat build-up is cumulative, the walls should be perforated to allow air flow for cooling.
With additional circuitry, some of the disadvantages can be reduced or eliminated. Examine the problems with the domino system desirable in yards, where machines are toggled sequentially; each fed though contacts on the preceding . At closure time the capacitor will probably be discharged to below a usable level and a large portion of current for the next machine would have to come from the input supply, limited by the resistor. This probably will not toggle the machine. Worse the capacitor will not recharge until the pushbutton is opened. Throwing ten turnouts would require nine additional delayed pushes.
Since common silicon controlled rectifiers (SCR), or possibly triacs, can be triggered on at a specific voltage level and off by interrupting current through them, these can be used at the supply output to pulse each succeeding turnout when sufficient charge is attained. The contacts must adjusted so that the cutoff breaks before the feed to the succeeding machine makes.
Note: Adjust brightness and contrast for optimum viewing.
SCR added to output.
The potentiometer VR1 sets the desired charge voltage that triggers the SCR back on. Using a volt meter, adjustment should start at zero and be increased gradually until the throwing works well. Care should be taken not to exceed maximum gate voltage rating for the SCR used. Although a 50 V break down voltage may work, 100 V is safer. Single cycle surge current rating should be greater than 50 A. Depending on your final setup, the limiting resistor R1 may be reduced to speed charging.
NOTE: For maximum heat dissipation, all power semi-conductors should be mounted directly on a heat-sink with heat conducting grease and no mica insulation between. If isolation is necessary, insulate the heat sink from chassis with nylon screws, nuts and washers.
Note: Adjust brightness and contrast for optimum viewing.
Fast charge added to output.
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