Reduced Track Power

04.12.2009 13:04

Track Building Part 7 - Reduced Track Power 

Page updated January 2009 minor updates

December 2007 (sections on line diodes added)

December 2005 (sections on reduced voltage added)



Why do you want Reduced Track Power?

A fair question - lots of track building effort goes into getting enough power. There are two main reasons for wanting less

1. For low power classes

2. For exhibitions and public open days.

Low power classes are a popular class at some clubs.  It provides a class between Scalextric type cars and  the quicker cars. Typically using a sealed 16d motor, in 1/24 production cars, it keeps costs down and attracts drivers who don't like the higher powered cars. In fact it keeps costs down a lot more than you might at first think - the wear and tare on the car is dramatically reduced - gears, motors, bodies, tyres and even braids last a long long time to the point that its an almost zero maintenance class.  It also dramatically reduces controller heating, so cheap controllers are no disadvantage - and that's a serious cost issue for newcomers where the controller can cost more than the car.

Exhibitions and public open days - The advantages of reduced maintenance, less wear and tare, less controller heating are all to obvious to anybody who has run racing for the public.  When it comes to driving slot cars, the competence of the general public is fairly limited, and there is a real need to slow the cars down to well below the level  that is suitable for (even low performance) club racing.

How does it work?

1 Reducing power with a line resistor

Reduced power is just the same thing as running a "choke" on your controller - except you reduce it rather more. Its simply a resistor in the power line to each lane which absorbs power and slows the car down. The resistors do produce heat (the "absorbed" electrical  power  is turned into heat.

The diagram above shows how to wire it into a track. One resistor per lane is connected at some convenient point between the battery + terminal and the controller full power band. 

 If you wire it into the track, and assuming you don't want reduced power all the time (which is possible on an exhibition track), you need some way of restoring full power. The wiring shown in brown on the diagram provides full power, an on/off switch (one pole per lane) to short out each resistor will provide full power in the on position, and reduced power in the off position. For reasons that are explained elsewhere on this site the brown wiring needs to be as short as possible, and as thick as the main track wiring.

The fuses are for short circuit protection. The full power fuse is described in the track wiring article. The fuse for low power is there to prevent damage to the resistor. If a driver keeps their controller hard down when there is a dead short, all the track power will be warming up the resistor (With a 1 ohm resistor that's 13 to 14 amps on a normal track power supply). Now that is not enough to blow the full power fuse (they should be rated at 15 amps), but it is enough to make the resistor very hot (around 150 watts). To avoid having an unnecessarily big an expensive resistor, and also to reduce the risk of  damage to incorrectly wired controllers, a fuse for low power should  be fitted - 5 amp fuse wire works fine in conjunction with a 1 ohm resistor.

As an alternative to wiring  reduced power into the track, you could do it as a plug in box. The wiring is shown in the diagram above. There is no need for any switches - just unplug it if you want full power. This is also a wiring diagram for a choke box - although resistors in the 0.05 to 0.15 ohm range would be appropriate to a choke.

An example of a club that uses this reduced power formula for some of its club racing is  Bournemouth.  They use 1 ohm 50 watt alloy cased resistors.  (These aren't available from Maplin any more, four 3.9 ohm 10watt resistors in parallel will do the same job, and that'll cost under £2 a lane)  Two pole on/off switches are used - so each switch does 2 lanes (16A DTST rocker switches - about £1.50 from car accessory shops or Maplin).  You need to duck under the track to switch from full to reduced power - but that's not a problem - racing is on the same power all evening.  There's no need to worry about lack of power when its in full power mode - as was demonstrated at our recent BOC nobody will realise you've got a low power option when they are running on full power.  Ipswich and Netley are other clubs with a built in low power option.

Why  1 ohm?  That's what suited the cars we were using  best.  It was easy to try different resistances - an old Parma controller was connected into the power line, this was set on part power with sticky tape, then the car was driven with a normal controller. The old controller was taped in several positions till we found what we wanted - when we measured it - it was about 1 ohm.  The right resistance for other types of car depends on how quick the motor was in the first place, and how much you want to slow it down - just repeat the test described above to find the right value for you.

A higher resistance  would probably be appropriate for public/ exhibition use. Lower  To give some idea, if 1 ohm track resistance is suitable for slowing down the sort of cars you run on a 4 ohm controller, then about  4 ohms should be suitable for slowing down the sort of cars you run on a 15 ohm controller.

2 Reducing power by reducing the voltage

Some clubs have track power supplies with adjustable voltages.  If you've got one of these, then reducing power is just a case of turning a knob on the power supply - it'll be easier than inserting resistors! Indeed reduced voltage has proved very successful when the BSCRA National championship track has been used for charity events.  If you have a fixed voltage supply, then line resistors are going to be the easiest way of  reducing power..  

For the sake of technical accuracy, I suppose I should mention that reducing the voltage and inserting a resistor don't do exactly the same thing.  Voltage reduction has more effect on top speed than on acceleration.  Line resistance has more effect on acceleration than top speed.  From a practical point of view, both reduce both acceleration and top speed so choose whichever is easier .  

Some tracks use the track power for the lap counters and for powering the on/off relays - turning the voltage down too far could cause a problem with these.

3 Reducing power with a line diode

This works by fitting a diode (or more than one diode) in place of the resistor in the above diagrams.  The effect is a combination of reducing power with a line resistor and reducing the voltage.  This is because a rectifier diode produces a voltage drop and some resistance. To reduce track power you need  ordinary silicon rectifier diodes, NOT Zener or Schottky diodes.  
Choosing a suitable rectifier diode.
Firstly decide on the most powerful sort of motors you want to use with the diode power reduction - and choose a diode with sufficient current rating to cope with it
For example 3 amp diodes such as 1N5400 or 1N5401 or ....1N5408 are suitable for Falcons and home set type motors
                    6 amp diodes  are suitable for 16d etc. motors 
Each diodes will produce a voltage drop of about 0.6 volts plus some resistance. The resistance is typically up to another 0.6 volts at full rated current. You can do a calculation on this if you want to*  but probably the simplest  way to get the speed reduction to suit you  is-
  (1) Try one diode and see how much that slows the car
  (2) Add a second diode in series and see how much more that slows the car
  (3) Continue adding diodes in series till you've slowed the car enough. 

Diodes pass current in one direction only.  When connected in series, they must all be connected the same way round.  They are marked to show which way round they fit, (typically with a white band round one end of the body).  Electronics experts will know the correct way round - if you don't know about this just try one both ways and see which works. (If connected backwards the car won't go, but no damage will be caused).

(* For those interested in the detail, manufacturer's spec sheets normally give a maximum voltage drop at full current. Typically this is 1.1 - 1.3 volts. As the manufacturers specify this as a maximum, you cannot expect it to be a precisely the same with all diodes.  If you want an example of how to calculate approximately what will happen, a 3 amp diode will have a voltage drop of about 0.6 volts plus another 0.6 volts at 3 amps - so 0.6 volts divided by 3 amps gives 0.2 ohms.  So this diode will behave approximately like a 0.6 volts drop plus 0.2 ohms resistance.)

Why not fit a slower motor rather than reduce track power?  

Yes this is certainly an option, and  it can produce much the same effect.  Which is better depends on what's available and what you want to achieve - here are some things to think about.

 If slower motors are available and cheaper then the slower motors have obvious advantages. If you cannot get a reliable motor of the appropriate power then reducing the track power has the obvious advantage.  If you want to make use of a large stock of existing motors that are a bit too powerful, then reducing the track power is attractive. The table shows the main differences between the two approaches. 

Differences Lower Track Power Lower Powered Motors
Motor performance Power can be reduced to what ever level is needed The power available depends on what motors can be obtained.
Gear reliability

Both produce improvements

Controller reliability Improved particularly under fault conditions Similar improvement under normal running, not as good under fault conditions.
Motor reliability Improved reliability May be problem where low powered motors are not being used as the designer intended - see below
How do you change the amount of power? 1 Changing the resistor values fitted to the track 
2 Adjusting the voltage on the track power supply
3 Changing how many diodes are fitted
Buy new motors.

 Generally motors designed to produce low power are no more reliable than high powered ones, where as motors running on reduced power will be much more reliable. This is particularly a problem with some motors intended for home set type applications where they will not be working hard much of the time .... put them in a better chassis on a club track where they are being called on to produce their maximum power most of the time and reliability can be poor.  

At a recent charity event where a track was open to the public for 9 days, JK Falcon powered 1/24 cars running on reduced power proved entirely reliable.  A number of raceway owners report 16d motors last a long time in  hire car use 

C can style motors that are designed to handle more power can be expected to be very reliable when using an extra mild wind. C can motors are not generally available with anything thinner than 30g winds which many people would say is far too powerful for novice drivers with 1/32 production - and is certainly a lot quicker than the "low power" we are considering here.


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