Various Transistor Throttles

With Add-On and Experimental Circuits

The following is a list of the circuits and related sections that appear on this page. Select one of the internal page links or scroll down the page.

There are quite a few images contained in this page and it therefore may take a long time to download.

  1. Throttle Power Supply
  2. The Basic Throttle
  3. Walkaround Control for The Basic Throttle
  4. Auxiliary Throttle Controls
  5. Basic Throttle with Unplugable Walk Around Control
  6. Full and Half-Wave DC Test Throttle
  7. Variable Width Pulse Type Throttle
  8. Regulated Current Pulse Throttle
  9. A Modernized "Toy" Throttle
  10. 2012 - FWDC Throttles
  11. 2012 - DC Throttles
  12. Fine Print Section for Transistor Throttles

  This page has a number of variations on transistor type throttles and related circuits. The throttles range from 'tried and true' to experimental designs. The basic transistor throttle serves as a base to which other circuits are added.

  The Waveform diagrams shown on this page are not technically correct but are meant to illustrate of the throttle output wave forms for a given control setting.

  The diagrams and texts start with the transformer and rectifier circuits and some throttle designs and on to the addition of other features such as unplugable walk around control. The circuits although functional are meant to be used as guides for other designs.



Throttle Power Supply

  The first schematic is of the Direct Current power supply for the throttles. Shown is the primary and secondary circuits and the components needed to power the throttles. As this section of the throttles uses 120 volt AC power great caution should be taken in this area. All transformers should be mounted in a fully enclosed metal cabinet and properly grounded. Transformers for multiple units could be centrally located and the low voltage AC then routed to each throttle.

Power Supply For Throttles Schematic

  If you are unfamiliar with high voltage circuits please get help from qualified personnel. This not an area to in which to learn from your mistakes.


Laptop Computer Power Supplies

Perhaps A Better Way

  An excellent source of high quality, regulated DC power supplies is AC power adapters for laptop PCs. The best choices are 12 and 15 volt models with a current capacity of 3 amps or greater.

  Power supplies with 3 prong plugs should be avoided unless a grounded output is needed. The Toshiba brand supplies are more likely to have the 2 prong power cord that is desirable for use on model railroads and for testing power supplies.

  15 volt power supplies would be ideal for home built DC throttles and accessory decoder boosters for DCC systems.

  Even though these power supplies have built in overcurrent protection, a fuse should be used at their outputs as the internal protection may pass more current than the downstream circuits can handle.

  Used laptop power supplies can be can be purchased inexpensively at many computer and electronics recycling facilities. (Have them tested before purchasing.)

Additional Power Supply Information



The Basic Throttle

  The first circuit is a 2 Amp, 0-12 Volt, voltage regulator type circuit. It uses a 10K ohm potentiometer to control the output voltage and has a light emitting diode that will indicate an overload condition. A DPDT toggle switch controls the train direction. The TIP142 output transistor will need a heat sink to keep it cool and the 0.33 ohm resistor should have a 5 Watt power rating. The OPAMP is one half of an LM 358. One quarter of a LM 324 could also be used.

Basic Transistor Throttle Schematic


Walkaround Control for The Basic Throttle

  In the next circuit walk around control is added to the throttle. A 4 conductor, 22 Gauge stranded cable will allow the operator to move around with the 'Hand Held Controller' while the power section of the throttle remains at a fixed location.

  A DPDT 24 Volt DC relay and a toggle switch has been added for direction control. The direction switch, speed control potentiometer and a 10K ohm resistor are mounted in the Hand Held Controller.

Walkaround Transistor Throttle Schematic



And Now for Something Different

"Auxiliary Throttle Controls"

  A variation on the walkaround throttle is in the next circuit. If extra wires are added to the control cable they can be put to use for operating accessories on the layout.

  For every extra wire 2 such loads can be controlled. Electromagnetic uncoupling ramps could be turned on and off, train whistles could be blown or a reverse loop switch can be thrown after the train has passed.

Walkaround Throttle With Auxiliary Controls Schematic

  By using 4N33 Darlington output Optoisolators as low power relays and adding two push buttons or a toggle switch to the hand held controller a wide variety of external devices could be operated with only a small current draw on the throttle circuit itself.

  Unplugable operation would still be a feasible option even with six wires in the control cable. This would allow four auxiliary control outputs.

  The optoisolator by itself can only handle a load current of 20 milliamps, larger load currents require adding another transistor. The next drawing is shows a more detailed view of the auxiliary outputs.

Auxiliary Output Schematic

  Push button switches are shown in the schematic but these could be replaced with a SPDT switch if desired. The only limit to the auxiliary outputs is that both loads can not be operated at the same time or a short circuit will occur. The 100 ohm resistor protects against this in the push button system. A little creative wiring or a second wire could provide a solution when needed. Any number of extra wires could be added for more control outputs.



Basic Throttle with Unplugable Walk Around Control

  next up is the basic transistor throttle with unplugable walk around control added. The direction memory uses an LM339 quad comparator and has a delayed action so that the direction can not change if the throttle output is above a predetermined voltage.

  The delayed reverse section, comparator section D, of the circuit grounds the bases of the transistors whenever the voltage at its PLUS input is higher than the reference voltage at the MINUS input. The PLUS input is connected to the throttle control voltage line.

  When the transistor bases are grounded they cannot conduct and any changes at their emitters will have no effect until the control voltage again falls below the preset level.

  Comparator sections B and C function as a voltage window detector. That is to say the outputs of both section Band C are high as long as the direction signal voltage is between 4 and 8 volts. If the signal goes below 4 volts or above 8 volts the appropriate output will go low and the direction will change accordingly.

  If the controller is unplugged the direction signal will go to 6 volts and the train will maintain its last direction.

  Please refer to the voltage comparator information page on this web site for help on how comparators work.

Voltage Comparator information page

Unplugable Walk Around Control and Delayed Reversing Schematic




A Full and Half-Wave DC Test Throttle

  The next throttle is one that was put together for testing purposes only. As can be seen it is a modification of the basic transistor throttle shown above. The SPDT switch allows the throttle to have a full or half wave DC output.

  The value of the 8 uF capacitors is calculated to allow the voltage across the 1000 ohm potentiometer to decay to 1/3 of the supply voltage before the next cycle starts. At 120 Hz; 8 uF is needed and at 60 Hz; 16 uF is required to achieve this result.

  This means that the peak output voltage of the throttle has value that is three times the lowest voltage of the output for a given throttle setting. Please refer to the wave forms diagram below the schematic for clarification.

All-wave DC Throttle Schematic

All-wave DC Throttle Waveforms

  If the 8 uF capacitors are omitted, the circuit becomes an overly-complicated, fullwave DC throttle.

  The throttle as shown has an automatic current limiting of about 2 amps and a quasi-regulated output voltage.

  This throttle gave pretty good results with an Atlas locomotive and one from a starter set of the type that make their appearance at Christmas time.



A Variable Width Pulse Type Throttle

  The next schematic is for an advanced type of pulse throttle. Simply put, as the output voltage is increased by the 10K pot, a pulse is injected onto the the control voltage at the PLUS input terminal of the second LM358 OpAmp. As the control voltage increases the width of the pulse is also increased.

Pulse Type Throttle Schematic

Pulse Type Throttle Output Waveform

  The height of the pulse is adjusted so that at some point it is overtaken by the output of the first OpAmp section. From this point on the output from the throttle is pure DC 

  The duration of the pulse is adjusted so that it is at approximately 50% when the pulse voltage is overtaken by the output control voltage.

  The pulse rate can also be varied but whenever the rate is adjusted the other variables of the oscillator will be thrown off and all would have to be reset.

  To properly set up this particular throttle an oscilloscope is required.

  Overall this particular throttle did not perform as I had hoped it might, but seemed to work well for locomotives with good quality motors.



A Regulated Current Pulse Throttle

  The next throttle is more than a little different, it's very different but in its own unique way it works just fine. It is however posted here for interest sake only and should not be viewed as a major discovery or some sort of magic trick.

  The design uses the variable pulse width oscillator that is the at the heart of the "Variable Pulse Type Throttle" as shown above. The oscillator in this circuit however switches a current regulator constructed from an LM317 voltage regulator.

  Rather than regulating the output voltage this throttle instead produces variable width pulses of a fixed current value, for example 200 or 400mA. The voltage can rise to a level that is determined largely by the B+ input voltage and the drop across the LM317 but the motor can only receive the maximum current that the LM317 will pass. This current value is determined by the value of R1.

Regulated Current Pulse Throttle Schematic

Regulated Current Pulse Throttle Output Waveform

  It'll never work you say. It does work, however there are some drawbacks.

For example:

  To determine the current pulse level for a particular engine, trial and error is required to find the best pulse current limit and frequency of pulses. A scope is necessary for this work.

  The throttle should perform best with a single locomotive used for switching and yard work. It might be ideal for switching games on portable layouts.

  This throttle although functional is also rather odd and as such should be viewed as perhaps being in the realm of weird science. The testing done on this design indicated that it will not harm motors in any way. In fact when the motor is operating at high RPM's the voltage across the motor will behave little different than for a normal power pack and current draw may be reduced. As the motor always receives a fixed current pulse, low speed cogging should be reduced to a very large extent. Under short circuit conditions the output current is restricted to the pulse current value.

  While the LM317 performed as expected in this circuit it certainly not designed for this application and therefore may suffer a high failure rate although it held up during testing.



A Modernized "Toy" Throttle

  This throttle is an upgrade of the type of throttle that is supplied with many of the HO train sets that are popular at Christmas time. These throttles do not have a reversing switch but instead have a center OFF position on the speed control knob.

  While this throttle may seam like a step backward in the evolution of train controls it does have features that make it useful in certain situations.

  Due to its one control operation it would be ideal for small children and persons with reduced hand dexterity.

  There is no chance of the direction suddenly being reversed while the train is under way as the output control would have to pass through ZERO in order to change direction.

  It would also be possible to have a spring loaded, single axis joystick type control that would automatically stop the train when the lever is released and as there is only one control it could be as large as desired. Again this might be helpful for someone who could not easily manipulate small controls or small children who would have an easier time with just one control.

  The following schematic shows the basic Toy Throttle. It is powered by a 24 Volt, Center Tapped transformer to provide PLUS and MINUS voltages.

  The throttle uses a Push-Pull power amplifier type circuit that eliminates the need for a reversing relay or switch.

Basic Toy Throttle

  When resistor R2 is in the CENTER position the throttles output voltage would be "ZERO". Adjusting R2 up or down would produce positive or negative voltage to the track thereby controlling the trains direction.

  Resistors R1 and R3 limit the maximum output voltage to between PLUS or MINUS 10 Volts.

  The next diagram shows the basic circuit but with a full wave DC output that might be better for older motors.

Toy Throttle With FWDC Output


  This next circuit is a variation on the Basic Toy throttle. In this version three push buttons are used to control the output.

Basic Toy Throttle - Push Button Control

  Button S1 causes the train to move in one direction while S2 moves it in the other. Button S3 is the "Quick" or emergency brake that will stop the train very quickly.

  An interesting feature of this system is that the train could be brought to a stop and then move off in the opposite direction by pressing only one button. Useful for shunting perhaps.

  Resistors R5, R6, R7 and diodes D1, D2 form a voltage limiter that reduces the maximum output voltage swing as mentioned in the potentiometer controlled circuit description.


  With this type of throttle would be difficult to know when the output is at "ZERO". To help with this a ZERO indicator has been added to the next drawing. The indicator can also show the trains direction.

Toy Throttle with Zero Indication

  The above schematic shows the addition of a "Zero Indicating" circuit to the throttle. When the output voltage is between +0.7 and -0.7 Volts both of the LED's, D3/4 will be OFF. As the output voltage increases, either PLUS or MINUS, one of the LED's will turn ON.

  This will allow the operator to know when the throttle's output voltage is near ZERO and the train is safely stopped.

  As there are two LED's, they can also show the direction of travel based on the polarity of the output voltage.

  A two colour LED could be used for D3/4. This will give a GREEN indication for one direction and a RED indication for the other.

  The next schematic shows an optional "ZERO" indication method. In this case the LED will be ON when the throttles output is near ZERO.

Zero Indicator Option

  In Option "B" the single LED would be ON when the output voltage is between +0.7 and -0.7 Volts.


  The next schematic shows a full featured Toy Throttle Upgrade. This version has Zero Indication, and Momentum.

  The transformer and bridge rectifier have been omitted to make the drawing smaller.

Full Featured Toy Throttle (Mk II)

  All of the features in this circuit can be found in conventional throttle designs. The only difference here is the PLUS or MINUS output voltage capability.


Toy Throttle Upgrade Notes