Slot Car Throttle booster

To achieve optimum control a slot car should just start to move as the trigger is advanced from the rest position, then accelerate smoothly as the trigger is advanced until full speed is achieved with the trigger fully depressed. A common problem occurs when the car does not even start to move until the trigger has been advanced some way. This means that the useable range of trigger movement between starting and full power is reduced, rendering the car difficult to control.

This situation often arises when modern cars, with traction magnets and powerful motors, are driven with a standard throttle out of a set. A standard throttle is a simple electrical device, just a variable resistor. The type of resistor provided is fine for many cars, but for others it just does not deliver enough current until the trigger is advanced some way

There are various ways to solve this. The easiest is to replace the throttle with a lower resistance variety. These are available from various manufacturers. Some have interchangeable resistors. Another alternative is to use some sort of solid-state controller.

This device addresses the problem by electronically changing the resistance of your existing controller. Because it is electronic, it is easy to change the effective resistance quickly to suit the car being driven. For example, using a standard Scalextric 70 ohm controller, this device can behave like anything from 14 ohm to 56 ohm.

It connects between the power base and the throttle. A rotary switch is used to select the required resistance. You then drive the car as normal. You can continue to use your favourite throttle to control any of your cars.

Circuit description

Component	Type
Q1	TIP42 Power transistor
Q1	TIP42 Power transistor
R1	1 ohm Power Resistor
R2	1 ohm Power Resistor
R3	1 ohm Power Resistor
R4	0.5 ohm Power Resistor
D1	1N4002 power diode

The circuit works by allowing more current to flow than would normally be the case for a given value of throttle resistance. The throttle is connected in series with Q1 and R1. In the absence of any other components the throttle would work pretty much as normal, the effects of Q1 and R1 being negligible. The current boost is achieved by using a second transistor, Q2, with Q1 to form a current mirror. The current flowing in Q2 is proportional to that in Q1. The resistors connected to the emitters of the transistors determine the current ratio. With the switches SW1 and SW2 open we have 1 ohm (R1) on Q1 and 1ohm (R2) on Q2. The ratio is 1:1 so the same current flows in Q2 as in Q1. The current from both transistors passes through the power base, giving the same effect as using a throttle with half the resistance.

By changing the value of the resistors you get different current ratios in the transistors, which in turn makes the unit behave like different resistance controllers. R3 and R4 are provided for this purpose. By operating switches SW1 and SW2 different combinations of resistors are connected to the transistors. A combination can be found to suit most cars.

Purists will realise that some of the power available from the power pack is lost in this circuit, even with the throttle fully closed. This slightly reduces the maximum speed available. Personally this does not bother me, because it is rare that cars have the opportunity to drive flat out at home, and if both drivers use this booster, then neither is disadvantaged.

Construction

As you can see, the prototype was cobbled together on a piece of wood, using Meccano as heat sink for the transistors. A circuit like this must dissipate power during normal operation. It is also important to consider what happens under fault conditions, such as a short on the track, or connecting the unit the wrong way round. For the prototype I couldn't be bothered working out the various heat dissipations, so I just used big beefy power resistors and bolted the transistors to large steel plates. If I were ever to productise this unit I would do the maths and reduce the component ratings accordingly to reduce cost. Rather than buy a selection of resistor values I just bought 1ohm and connected as many as necessary together in parallel to get the desired effect.

The prototype proved that the concept was sound. A significant improvement in handling was achieved. I housed the electronics in an aluminium box. This same box was used as the heat sink for the transistors. Insulating kits were used to electrically isolate the transistors from the case. The original toggle switches were replaced with a 2-pole 6 way rotary switch, wired such that the control became more sensitive as the switch was rotated clockwise.