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Silicon Controlled Rectifier Model Railroad Throttles

Silicon Controlled Rectifier Model Railroad Throttles

The Programmable Unijunction Transistor used to trigger the SCR is the key to this design as it ensures that the SCR fires on every cycle of the fullwave DC. This gives a very efficient operation with low voltage loss and very little heat generated.
SCR Throttle Setup Instructions

When the throttle is ready for use the 50K Trimmer must be adjusted so that maximum output voltage can be obtained and keep the SCR from misfiring. To do this perform the following steps.

1. With no load connected to the throttle output and the AC input turned on.

2. Connect an analog voltmeter to the output of the throttle (0-30V).

3. Set the trimmer to its maximum resistance.

4. Press and hold the ACCELERATE push button until the output voltage stops increasing.

5. DECREASE the resistance of the trimmer until the output voltage peaks and then begins to fall again. When the voltmeter needle starts to bounce slightly the SCR is at its maximum trigger angle. STOP.

6. INCREASE the resistance of the trimmer slightly until the meter needle stops bouncing. At this setting the throttle will be able to deliver its maximum voltage. If the trimmer resistance is set too low the SCR will not trigger on every cycle of the full wave DC at full output voltage and may be damaged by high current spikes.

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