Silicon Controlled Rectifier Model Railroad Throttles

London Model Railroad Group SCR Throttle

  The following is a schematic drawing of a Silicon Controlled Rectifier type throttle for use on larger scale model railroads. Three versions of this throttle are shown on this page. They are not sophisticated designs but work well and are tough and reliable.

Push Button SCR Throttle Schematic

  It was designed for use on a "The London Model Railroad Group's" large O scale layout located at London, Ontario, Canada. The prime requirements for the club throttles were that they be rugged, reliable and produce as little heat as possible. Throttles of this type have been in service at the club since 1987 with excellent results. The throttle will deliver 5 amps continuously and up to 18 volts DC. See Notes.

  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. Please refer to your electronics books if you need more information on how the SCR and PUJT in the circuit function.

  The SCR throttle has momentum effect built in due to the memory provided by the 50uF capacitor at the PLUS input of the OPAMP. There is no decay but this can be added by placing a 1 to 2 Megohm resistor in parallel with the 50uF capacitor. Increasing the value of the memory capacitor will slow the action of the throttle controls.

  Due to its unregulated voltage output, the SCR throttle will have a prototypical feel as trains will slow when climbing grades and speed up when going down grades. The operator must work the throttle to keep their train from speeding or slowing too much in hilly terrain.

SCR Throttle Setup Instructions

  The 50K Trimmer is adjusted so that maximum output voltage can be obtained but keep the SCR triggering on each cycle. 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 could be damaged by high current spikes.


* NOTE 1 At full throttle and with no load this circuit will deliver more than 25 volts. Therefore care must be taken with auxiliary equipment and train lighting that may be connected to the track.

(Remember that most voltmeters only indicate average or RMS voltage and not the peak voltage which can be 1.4 times greater that the average.)

* NOTE 2 If your trains will run as fast as you need at lower track voltages than 18 volts then lower the input to 12 or 14 volts AC. The under load DC output voltage of this type of throttle is roughly equal to the RMS AC input voltage.

* NOTE 3 Add your own reversing switches to the output and a suitable circuit breaker for added protection if desired.

* NOTE 4 The use of good quality push buttons is a must as these are the most likely components of the throttle to fail due to the heavy thumbs of some people.

Please feel free to use this design if you wish. It worked for our club it might be useful to you.

  The following is a photograph of a typical SCR throttle installed at the London Model Railroad Group's layout. The throttle has two reversing relays so that when the direction switch is in its centre position the throttle is disconnected from the track. This is a club standard for all main line throttles.

Typical L.M.R.G. Throttle Construction

  The throttle boards are hung under the layout by hooks so that they could quickly be changed if a failure occurred. This was a hold over from a previous throttle design that was trouble prone.

  The power transformers were located in electrically safe enclosures to keep the weight of the throttle boards to a minimum. Volt and Amp meters are connected at the terminal strip so that they are ahead of the relays.

Potentiometer Controlled Silicon Controlled Rectifier Throttle

  The second schematic is a basic version of the SCR throttle circuit. It uses a potentiometer instead of push buttons for control. All of the above set up instructions apply to this design as well except that the potentiometer is set to maximum when adjusting the 50K trimmer.

Potentiometer Controlled SCR Throttle Schematic

SCR Throttle Waveforms At Half Output

The "MoAT" Project Throttle.

  The third schematic is a throttle that was built for the London Model Railroad Group because of a quest for more power (amps) to triple head an all brass 21 hopper coal drag with scratch built Steam locomotives.

  A quest ranking somewhere between that for the Holy Grail and the Golden Fleece. The fact that the transformer (22 Volts and 15 Amps) that powers this throttle just happened to appear at the right time helped get it off the ground.

"MoAT" SCR Throttle Schematic

  As the schematic shows this design uses two silicon controlled rectifiers that are triggered through a special SCR trigger transformer. This transformer allows the cathodes and gates of the SCR's to be isolated from the control circuit and thus permitting the incorporation of the over current protection into the throttle.

  The SCR's form half of a rectifier bridge that provides the track power while the two diodes in the bridge module that the SCR's replace supply power only to the PUJT and the control circuits. In effect the power flow through each SCR's is halved because each is triggered on every other cycle of the full wave DC.

  The current limiting simply bleeds the charge off of the memory capacitor if the voltage across the 0.05 ohm resistor rises higher than the voltage at the plus input of Ic2. The voltage across this resistor is directly proportional to the current through it (E = I X R). The level of limiting is adjusted by the 10K ohm potentiometer.

P.S. MoAT stands for the "Mother of All Throttles".

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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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17 January, 2013