SWITCHING CONTROLLERS (including PWM controllers)

Sometimes you want a motor to stop, sometimes you want it to go as fast as possible, and sometimes you want to control it between the two.  Two ways of making your slot car motor run at part power are either to put something in the supply line to waste some of the power or switch it on and off quickly.  

Typical ways of wasting some of the power are resistor controllers and linear transistorized controllers.   You might wonder where  the waste power in the controller goes to,  it MUST go somewhere.  Waste power heats up the controller. Heat is not a problem if you are able to get rid of it without too much temperature rise.  The chunky pieces of aluminum, cooling fans and the like you see on transistorized controllers are dedicated to keeping  the temperature rise within acceptable bounds.

The alternative option is switching.  When its fully switched on or fully switched off the controller isn't wasting any power - so it won't get hot.  The power from the motor can be controlled by having it switched on some of the time and off some of the time.  How much power it produces depends on the ratio between on time and off time.  (Terms such as variable mark to space ratio, or pulse width modulation (PWM) are used to describe this.)   It is fairly obvious that if this on -off  cycle takes place too slowly the car will speed up and slow down enough to upset its cornering - but if it is done fast enough (some people reckon a  hundred times a second is enough, others prefer a couple of thousand times a second or even more) then the car behaves just as it would with an "ordinary" controller wasting part of the power.  How frequently it does the on-off cycle is often called the switching frequency and expressed in Hz (Hertz). For example  500 on - off cycles in a second is 500 Hz.     Some switching controller have adjustable  switching frequency.

Switching controllers do not waste power like a linear one - this is a huge advantage to battery powered model aircraft or radio controlled cars which have to carry batteries on board.  It's rather less important on slot car tracks where there is enough power available even if a bit is wasted.  However, switching controllers do produce some waste heat, very little in some designs, in others there is enough heat to get rid of that the designers have included a heat sink and cooling fan.

Are switching controllers new?  Not really,  Transistorized switching controllers for slot cars were built using germanium transistors as far back as 1963.   1960s slot racers will remember the Revell Professional which achieved this switching with a vibrating relay.  The vibrating relay has its limitations, both in switching speed and reliability.  Properly designed  transistor  circuits don't suffer from the limitations of relay technology. There are an increasing number of these switching controllers coming on the market.  Nearly all modern switching controllers use a type of transistor known as MOSFETs (metal–oxide–semiconductor field-effect transistor), some liner controllers also use MOSFETS.

There are two ways of switching, both have been used in slot racing controllers.  Both types switch the power full on during the ON part of the cycle, the difference is what happens during the OFF part of the cycle.  
One type  disconnects the supply. With this type the voltage between the car's braids  during  the OFF part of the cycle is whatever voltage the rotating motor generates. 
The other type switches to brakes for the OFF part of the cycle.  With this type the controller connects the car's braids to one another during  the OFF part of the cycle so the voltage between the car's braids is low.  (It's not quite zero because of the resistance of the track wiring). 
I haven't attempted a detailed explanation of how the switching circuits work, enough to say they are a good deal more complicated than the simple linear circuits I have described elsewhere on this web site.  

I've suggested a few maintenance tips that are applicable to switching controllers, although they tend to need less maintenance than linear circuits.  If there are any obvious physical faults (for example a wire fallen off the plug) then go ahead and fix it. The electronics are complicated, and have some delicate devices so  I would recommend consulting the supplier rather than attempting to repair the electronics yourself.  Fortunately they seem very reliable so there shouldn't need repairing.