Fried chips and smokin' motors

Keeping the vital parts of a Scalextric car cool by understanding "duty cycle"

 

In order for your Scalextric car to give the best and ongoing performance it is necessary for the key components not to get too hot. The key components discussed in this article is the motor and the digital chip (if fitted). This article uses the notion of "duty cycle" to keep your Scalextric cars cool and running well.

Duty cycle
In this World not all machinery or equipment is capable of running or operating continuously. Some equipment needs to be serviced, some may wear out and some may get hotter and hotter in use. This would indicate that the equipment needs a period of rest between usage periods. This "on" verses "off" time is known as the duty cycle.

Scalextric duty cycle

For example one of our instant soldering guns can be used for 12 seconds but must then rest for 40 seconds to allow the transformer to cool down. After the 40 seconds cool down period it can be used again for 12 seconds, etc. This on / off period is known as the duty cycle for this soldering gun.

Duty cycle may be expressed in time as in the soldering gun example or as a percentage. For example a 75% duty cycle could mean the equipment can be operated for 3 minutes and then rested for 1 minute.

Smokin' Scalextric motors
None of the motors used in any of the Scalextric cars is capable of continuous racing conditions. They are capable of long term use at slow speeds. As the race continues the motor is unable to dissipate the heat it generates and the motor just gets hotter and hotter. That is until the internal solder joints melt and the motor stops working with permanent damage.

Sometimes a tiny amount of oil contamination on the commutator will start to burn and the motor will emit smoke and smell of burning.

It therefore follows that a Scalextric motor has a duty cycle to let the motor cool down between races. All motors, cars and track layouts will be different but to be safe it is best to assume a duty cycle of 50%. That is race for 5 minutes and let the motor cool for 5 minutes, or race for 10 minutes and let the motor cool for 10 minutes. I'm sure you've got the idea by now.

Fried Scalextric digital chips
The Scalextric digital chip is nothing more than some basic electronics and a single micro chip computer to manage the communication side of things. The chip has 3 main sections; the power supply, the micro processor and the motor power drive. Even when operating in analogue the chip is still regulating the power to the motor. Note, there is no overcurrent protection on the Scalextric digital chips.

The power supply and micro processor are well enough designed to not usually cause a problem. The cause of failed digital chips is the motor power drive transistors. These drive transistor turn on and off many times a second to control the power to the motor. The transistor on time and off time is controlled by the micro processor based on the hand throttle position.

When the motor power transistors are switched on they effectively become low value resistors allowing the electrical current to flow through the motor. When the transistors are switched off they effectively become high value resistors stopping the electrical current from flowing through the motor.

When the motor power transistors are switched off there's no current flow and therefore no heat generated within the transistors. Therefore off is no problem for heat generation.

When the transistors are switched on, the full motor current passes through the transistors and heat is generated within the transistors. So, the more on time the more heat generated per unit time. The more on time, the more power to the motor, the harder the motor is working, In this case full speed racing will generate more heat in the motor power transistors and as this heat builds up there comes a point where the internal silicon of the transistor effectively melts. This will destroy the transistor function - fried chip.

Again the Scalextric digital chip has a duty cycle depending on the chip installation, motor, car and track layout. In this case Silicon cools very quickly so a duty cycle of 75% would be OK. However, we should limit the race time to prevent the heat building in the first place. We recommend not racing for more than 5 minutes to prevent permanent damage to the Scalextric digital chips.

 

This great article is brought to you by Scalextric Car Restorations at:

www.scalextric-car.co.uk