Wiring and Power supplies

04.12.2009 12:59



Wiring and Power Supplies


Extra power supply info added January 2009

There are two ways I could cover track wiring - either give a detailed explanation of why a particular size of wire is needed, or just give a simple guide to what to do.  No doubt some readers will just want the simple approach, and others will want the reasons, facts figures, graphs etc.  I aim to provide answers for both sort of reader - read on for the what to do guide - the reasons, facts figures, graphs etc are in the next article in this series.

The Simple What To Do Guide

For those who don’t want to mess around with the reasons why things work here are a few quick guide-lines on track wiring that should provide a reasonable amount of power.  It assumes a normal club size track, running BSCRA type cars. For lower powered cars the principles are identical, but lower thinner wiring will suffice.  A lot of clubs now use electronic power supplies in place of batteries - these rules apply equally well to both (but I won't keep repeating "battery or power supply"). 

1) Use separate feed wires from the negative battery terminal for each lane. Although a SHORT length (less than a metre) of VERY THICK wire from the battery to the point where the wires separate is tolerable the general rule is DO NOT USE COMMON RETURN WIRING.

2) Obviously the positive wiring has to separate fairly near the battery to go to the separate controller sockets. Ideally use separate feed wires from the positive battery terminal for each socket. Although a SHORT length (less than a metre) of VERY THICK wire from the battery to the point where the wires separate is tolerable.

3) Keep the wiring from the battery to the controller sockets as short as possible.

4) Keep the wiring from the controller sockets to the track as short as possible.

5) Keep any wiring from the battery to the track as short as possible.

NOTE For tracks where the drivers rostrum is next to the track (like most tracks) 3,4 & 5 can be achieved by putting the battery under the track close to the middle of the rostrum.

6) I would recommend at least 5 power feeds for a 30m / 100ft lap length TAPE track. (4 might be adequate for a very compact layout.) More feeds are needed where the lap length is longer ( as a rule of thumb, one feed per extra 6m/20 ft. of lap length).  

7) For braided tracks a single power feed may well be adequate for tracks up to around 30m / 100ft lap length a second feed will be needed where the lap length is longer.

8) Run the first set of extra feed wires run from from the main power feed to a convenient point about half way round the lap length. Its important to keep these wires short, so for example if the feed wires can be 5m shorter if they connect 5m from half way round, then go for the shorter wire. The extra feeds should be distributed evenly round the lap length.

9) Separate feed wires are needed for the positive and negative side of each lane.

10) Use 2.5sq.mm cable (ring main cable or similar) for track wiring (including extra power feeds) (but something much thicker is needed where VERY THICK wire is recommended)

11) Connect up the tape / braid as a continuous loop round the track - a break in the connection increases the resistance considerably.

Which Wire Goes Where?

Making cars run forwards without blowing controllers!


The current BSCRA standard has the cars wired so that  when looking down on the car in the direction of travel the positive braid is on the right.  Most imported American ready to run cars, standard home set cars  (Scalextric, Fly, Ninco etc.) are also wired to go forward when the positive braid is on the right.

(The original standard from the formation of the Association in 1964 was positive on the left.   BSCRA will be changed over to the "plus on the right" standard on 1 Jan 2003. ) 

At first you might think that the wiring options shown in either of the left hand parts of Diagram T would make BSCRA 2003 and "Scalex etc." cars go forward. Well if you use a resistance controller either will work. If you try to use a transistorised controller, the wiring with the big green tick will work fine and your car will go forward. However the wiring on the lower part of Diagram T will blow up your transistorised controller and none of your cars will go anywhere (even on a correctly wired track) until your controller is repaired (Probably with a new transistor).  On the right hand side are the equivalent diagrams to make the cars go backward  ( which is only rarely used).

Why does it make any difference to your controller which way the track is wired? Looking at the controller socket the standard wiring (Top of Diagram T) has the E terminal (the brake) wired negative and the L terminal (the power connection) wired positive. Transistorised controllers are designed to work this way round. Looking at the lower half of Diagram T, you’ll notice the E terminal (the brake) wired positive and the L terminal (the power connection) wired negative.  This connects the transistors back to front, so they will not work, and unless you are very lucky they are destroyed (this happens far quicker than you can unplug the controller, and faster than a fuse can blow.) Unfortunately there is no simple change that can be made to a controller to get round this problem - the only simple solutions are to wire the track properly or use a resistance controller. (unlike transistors, resistors work exactly the same whichever way the current is passing through them.)

Using the track in the opposite direction

Do you always want to run the track in one direction? Running in the opposite direction gives effectively a different circuit to race on - some layouts work well in either direction. It's sometimes more difficult to drive a track in one direction than the other - bends that open up are often easier to drive than ones that tighten (the Oaklands Park circuit is a good example of this)  There are potential problems with running backward.  Cars will deslot in different places in the reverse direction so the marshalling positions will often be significantly different, and there can be a higher risk of cars landing in awkward places (like under the bridge). Some of the imperfections in track building upset cars much more in one direction than the other. 

If you want to run either type of car without having to rewire each car, or you want the option of running either way round the track without swapping over the wires on the car - the track needs to be wired to allow either. Many clubs now run both types of car, unless you are quite sure the track will only be used for one type, I recommend the track is wired top allow both types of car.   It might appear easiest just to connect the battery / power supply the other way round - unfortunately this produces incorrect controller connections (as the lower half of Diagram T). The right way to do it is to swap over the connections to the lane on the track side of the controller socket as shown in Diagram U. (Cars wired to 2003 standard will run in the reverse direction with the switch in the 2002 position. Cars wired to 2002 standard will run in the reverse direction with the switch in the 2003 position.) 

 I’ve shown a two pole switch, it will also work with relay(s) or plug/sockets. These are carrying the full power to the cars so

(a) The switches, relays, plugs/sockets need to be of a suitably high current rating (20 amp. for strap cars)

(b) The power wiring must not be extended any more than absolutely necessary or else there will be voltage drops in the wiring.

This means the switches, relays, plugs/sockets will almost certainly need to be under the track. If you envisage frequent changes between the wiring polarity, its convenient to use relays and have the switches at race control. (Switches on the drivers rostrum are an option - this makes it easier for the sensible drivers - but gives more opportunities to the less sensible for messing about.)

IF the track polarity is reversible, the lap counters will also need to be suitable for running in both directions - this is covered in the Lap Counter article.

Turning the power on and off

The "power on/off" shown in Diagram V would usually be a relay contact. This should be mounted between the power supply and the socket as shown. This removes power from the controller when track power is turned off which can be very useful if a faulty controller is plugged in. (Putting the power on/off on the "N" lead (the black wire in Diagram V) would still turn off the track but would leave the power permanently connected to the controller)

The power relay should have contacts rated to carry the maximum current a car will take. 20 amps per lane is adequate for BSCRA cars. A separate contact for each lane is ideal. A separate relay for each lane is a good idea - it allows individual lanes to be switched off which can be useful in holding cars on the start line. These relays are available for a few pounds each, and are commonly used in full size cars.

Protection against faults

The fuse shown in Diagram V protects the track wiring and minimises damage to controllers in the event of a faulty (or incorrectly wired) controller or other dead short circuits. Domestic 15amp fusewire (0.5mm) is suitable for this fuse - practical experience is that this does not blow in normal use - even with 25g armatures - even with the sort of short circuit exhibited by a chassis sparking on the tapes as it goes round (yes I hope that's not normal use)- but it does blow instantly when somebody plugs in a controller with the E and L terminal are shorted through the brakes. Just in case you were wondering - a couple of cm. of 15 amp. fuse wire has negligible resistance compared with the rest of the track wiring, so it will not slow the cars down.

Some American tracks use a 10 amp. circuit breaker wired into the brake connection (see Diagram X). This provides similar protection for incorrectly wired controllers, but doesn’t protect against other short circuits. It's also likely to have a small resistance which may slightly reduce the brakes. Incorrectly wired controllers are a more  likely problem - particularly as many American tracks depend on separate croc clips for each wire rather than a 3 pin connector. (With separate stud connections, the careless competitor has the opportunity to wire up his controller wrongly every time he plugs into the track. With a 3 pin connector once the plug is wired up right you cannot go wrong.) 

The standard wiring for the studs on American tracks is 

L pin - To Battery Positive - White Stud

E pin - To Battery Negative - Red Stud

N pin - Power to car - Black Stud

Is there a Constant Voltage all round the Track?

It would suit slot cars very well if the voltage arriving at the motor was always the same whenever you put your thumb hard down. So why isn’t that just what you get on any slot track?

There is a popular misconception that copper wire has no resistance - this is not true - the first thing to understand is that copper wiring has resistance and that resistance is enough to reduce the voltage to your car by a very noticeable amount. There is also a misconception that car batteries produce a constant voltage under varying loads - this is not true either - the voltage drops with increasing load. Generally, electronic power supplies provide a more constant voltage than a battery.   The combination of these voltage drops is the reason the lights on your full size car go dim when you turn over the starter motor.

In fact it doesn’t matter much if the power available is exactly equal all the way round the track (Good job too because there's no practical way of making it exactly equal all the way round as I’ll explain later). Certainly adequate power is needed all the way round, but less power is "adequate" in a bend where you cannot put your controller full down than on a straight where cars are accelerating on full power. As long as the power available on any particular part of the track is the same every lap, it just becomes part of learning the track .... Drivers learn to deal with the different levels of power just as they learn to deal with different radii bends on different parts of the track. The voltage from some club batteries go down slowly by half a volt during a 3 min race, and the drivers naturally compensate (by braking a little later and applying a bit more throttle in corners) without realising they were doing it. What drivers cannot compensate for is power going up and down by the split second depending on how much power the other cars are taking

Separate wiring to each lane is important. If the wiring is common (see "wrong!" half of Diagram W), when one lane is drawing power the voltage to all the lanes will drop by say 1 volt. So the power to all the lanes will go up by 1 volt when one car brakes, and the power on all the lanes goes down again when the driver on one lane puts his thumb down. With separate wiring each lane has the same voltage available regardless of what the other lanes are doing! (see left hand half of Diagram W)

The maximum power available to the car is limited by -

(1) how much power is lost in the resistance between the car and the battery / power supply.

(2) the power available at the battery / power supply.

The next article in this series explains what sort of wire to use, why, and includes some graphs to show  what happens all the way round the track.  If you just want a simple what to do guide go to the top of this page. 

Power Supplies

The power for the cars comes from the track power supply - traditionally this was a car battery with some sort of charger.  These days the use of batteries is less common.   High current electronic "regulated" power supplies are available at reasonable cost and are often used  without a battery. For home set type cars low cost unregulated power supplies can be used.

A 12 volt car battery is a good source of high current dc at a fairly constant voltage, and was the standard choice for many years (although they are now less common). The battery needs to be recharged otherwise it'll go flat fairly quickly.    The voltage from a battery is at best only fairly constant. The combination of clapped out batteries and  poorly regulated chargers, that used to be all to common, produces disappointingly large variations in voltage.   In fact poorly regulated chargers can quickly convert  a good new battery into a clapped out one!

So what do you need in a battery charger?     
(1) A trickle charger will do the battery no harm, and will recharge it eventually.  This means only a few amps of charging current, and unfortunately means that high powered cars will drain the battery rather much more quickly than the trickle charger can replace it.  
(2) A higher current charger that turns itself off very quickly when full charge voltage is reached.  This is how traditional car charging systems work, and in the early days of slot racing car parts were the most common way of doing it.
(3) A constant voltage charger set to the correct float charge voltage for the battery (13.8v 
is usually recommended for batteries with lead/antimony plates, 14.2v is usually recommended for batteries with lead/silicon plates).  An electronically regulated supply is usually used - ideally 10 amps per lane (e.g. 40 amps for a 4 lane track) so you can deal with any motor, but many clubs manage with considerably less.

So what do you need in a power supply (without battery)?
You need a power supply that can give each motor the maximum current (amps) it needs.  That means the maximum motor current multiplied by the number of lanes.   Here are some examples
(1)  For high power cars 20 amps per lane is needed - so a 40 amp supply shared between two lanes etc. will do nicely.
                    A 75 amp supply shared between 4 lanes seems to work fine.
(2)  For group 12 powered cars 10 amps per lane is needed - so a 40 amp supply shared between four lanes etc. will do nicely. 
(3) For Falcon powered cars 5 amps per lane is more than adequate - so a 20 amp supply shared between four lanes etc. will do nicely. 
(4) For home set type cars 2 amps per lane is more than adequate - so a 4 amp supply shared between two lanes etc. will do nicely.

Is a higher current power supply a acceptable?  
YES   Motors only take as much current as they need.  For example if a low power motor running at speed  needs half an amp then it'll only take half an amp even if the power supply is capable of supplying 100 amps.  
Even for home set use it makes sense to buy a big enough supply to cope with the highest current motors you are likely to want to run.  Cost is a reason for not going too far above the current you need.  
Higher current power supplies will put more current into a fault, so protection against faults is important.

Does lap length makes a difference to what power supply is needed? No (except possibly with digital tracks)  - BUT extra wiring is usually needed for extra lap length. 
Some electronic power supplies can be connected in parallel satisfactorily, some cannot.   The best way to avoid this problem is to connect supplies to lanes individually - so for example if you have two 40 amp supplies for your 4 lane track connect two lanes to one power supply and the other two lanes to the other power supply as shown in the diagram below.

NOTE - The  blue wire "x" in the diagram is often necessary to get the lap recorders working - the power supply to the cars will work properly if it omitted.

There seem to be plenty of suitable power supplies about.  For example, the BSCRA Nationals track currently uses four Rapid Electronics  40 amp switch mode power supplies (part number 85-1828) - two lanes from each supply and no batteries. The output voltage is adjustable, they are used on the fixed 13.8 volt setting for championship racing, but for charity events lower voltages are used. 

Adjustable Voltage Power supplies with an adjustable voltage are often used on slot tracks.  Many clubs simply want a fixed voltage, and never make use of the voltage adjustment. Adjustable voltage provides a useful way of reducing power, for example when opening a track to the public (see section 7). 

Capacitors - some tracks (particularly in North America) use large capacitors connected to the power supplies.  I haven't measured the supply on a track with these fitted so I'll only offer a theoretical observation.  The capacitors will maintain the the track voltage over very short periods (fractions of a second) of high current load, which can help with the peak current when starting from rest. They should also be useful for reducing ac ripple ( ac ripple was a problem with simple mains frequency transformer power supplies, but shouldn't be a problem with switch mode power supplies). There is no guarantee all power supplies will start up with capacitors connected. 

Home set power supplies
Many home set tracks come with a low cost unregulated power supply.  These are suitable for their intended purpose, but can present problems for the enthusiast who wants consistent power to his car.
The problem with unregulated power supplies is that the voltage goes up and down as the current changes.  Think about what happens when two cars share the same unregulated power supply.  One car taking current reduces the voltage for the other car.  When one car suddenly stops taking current (as it will when the brakes are applied, or it falls off) the other car suddenly gets more volts.  At worst this means when one car falls off the other one gets enough extra power that it also falls off !  This can be described as a power surge problem.
regulated power supply (as described above) is a great solution to these problems.  
A low cost solution to this power surge problem that makes use   of these unregulated power supplies is to have a separate one for each lane.  The voltage still goes up and down depending on how much power the car is taking but the driver is unlikely to notice.  Drivers have no trouble in learning to drive a car on 13 volts in corners and 10 volts under low speed acceleration.  They are looking at the car not a voltmeter! Consistent voltage differences are just part of learning the circuit.  The diagram below shows the right way to connect them (separately), also for clarity I've shown the wrong way to connect them (in parallel).  

NOTE - The  blue wire "x" in the diagram is often necessary to get the lap recorders working - the power supply to the cars will work properly if it omitted.

Chris Frost


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