Watt No Power!!!

Repairing Slot Track Power Faults

Link to Slot Racing home page

Link to Track Building start page

Link to "Maintaining braided tracks" 

Link to an explanation of track wiring resistance 

updated January 2011


Ever felt there was a lack of power part way round one of the lanes? Worse still, ever had somebody complaining there is a lack of power! You don’t think there’s a problem but how can you be sure - sure enough to convince the racer who is complaining?. We all know that cars go wrong far more often than tracks, so if a car seems to be short of power try another car, try cleaning the tapes/braids and try another controller. If there really does seem to be a shortage of power on part of one lane then there is likely to be a problem with either the power feeds to the track,  a broken tape or the joints between sections. (Contact problems between sections is the most likely problem with  home set (Scalex etc.) tracks)

Assuming that power is usually OK on this track, what do you do if the power has gone down on all the lanes ? 

Check the battery/ charger ! .... Is the battery charger plugged in? Is the charger connected to the battery? Has a fuse blown on the charger? Is there any water in the battery? Is the battery flat? (Its also worth measuring the battery voltage with the charger disconnected , it should register about 13 volts if its in really good condition, 12 and a bit volts is alright (ish) and 11 volts indicates the battery is as flat as an exhalation of wind. Flat batteries need charging, that takes time, so its best to find out if its flat a day or more before the big meeting. Occasionally you’ll find a battery that scarcely reaches 12 volts off load and once a charger is connected rapidly rises to 15 volts plus - beware these batteries have about as much go as a 2CV with an empty petrol tank and should be scrapped as soon as possible.) 

If the power at a track was always awful, then its worth looking at the design of the track wiring - see the track building page. Rewiring a track is not likely to be a quick job, so allow plenty of time - if you realise there's a problem on the day of a big meeting, there's no choise but to race with you’ve got!

One possible problem is a short dead section. If cars are being driven a full speed they will coast over a 8 foot dead section in the middle of a main straight and not stop, but they will be noticeably slower than they should be. In portable tracks a dead section is quite possible (this is equally true of Scalextric and similar home set tacks), although it’s less likely in a permanent track where there is a complete loop of tape so you need two breaks to give you a problem. Anyway the best thing to do when setting up a portable track is to drive a car very slowly round each lane - it will stop on any dead sections. Then you need to know which side is not making contact - use a coin (or anything conductive) to bridge the tapes across the joint - if the car is still dead  then try the other side - once you've worked out which bit to bridge to get the car going again then make the necessary repair. 

This is fine for sorting out dead sections, but the problem is often reduced power rather than none. One break in a tape means that power has to come the long way round the track, so more power is wasted in track resistance - see track building pages for more detail of why.  To sort out these sorts of faults you need something more than a slot car and a 10p coin (no I don’t mean a 50p coin!). I would recommend a digital multimeter and a suitable load resistance.

The track wiring and the tapes / braids have an electrical resistance - typically 0.1 to 0.2 ohms. If there is a break in the tapes this can easily go up to 0.4 ohms in some parts of the track (for more explanation see the article on "Wiring Resistance"). Now a quick comparison with controller resistance will give an idea of how much difference that makes. . If a motor is drawing 10 amps the best track wiring will drop 1 volt, where as the faulty 0.4 ohm section will be dropping 4 volts so (on a 13v battery) your motor is running on 9 volts instead of 12 - not surprisingly it will feel slow. Now just putting a volt meter across the tapes applying full power won't tell you very much - it will register the same voltage all the way round unless you draw some power from the tapes. A slot cat isn't a very useful load for measurement purposes because the current changes with motor revs., what is needed is a suitable size resistor

So what equipment is need to check the power? Something to measure voltage - a digital multimeter is recommend - somebody in your club will probably already have one, but if you can’t blag one (Are we allowed to mention that on this site?) its well worth buying one (they have loads of other uses). A 3 1/2 digit type (one that reads up to 19.99 and reads 20.0 up) is suitable and can cost from as little as about £6.

You also need are a load resistor - this consists of a slot guide with a resistor connected between the braids (see diagram 1). The resistor needs to take enough load to represent a car - about 1.2 - 2 ohms for BSCRA tracks ( Scalextric etc. cars take much  less power .. and power supply probably isn't up to supplying the sort of current needed for a BSCRA track - so use and say 27 or 39 ohms). These resistors will get quite hot, so you need something with an adequate power rating and preferably don't leave them connected too long. (This heating effect will grab your attention when you pick up the resistor if its been drawing power for too long!) The cheapest load resistors are made by using up old controller resistors or some other electronic junk you happen to have, but if you are buying new resistors ten 15ohm resistors in parallel would be suitable for BSCRA tracks (for Scalextric track a 27 ohm or 39 ohm 7 watt resistor is about right.)

Load resistor

The last piece of equipment needed is something to apply full power. One option is a controller with a club member to put his thumb/ finger full down, (once he gets bored you can tape the plunger full down)! The alternative is a "full power" plug - a plug with no controller and the appropriate pair of pins shorted together (see diagram 2 below).  ("Digital" sets have full power on the rails all the time so you don't need to do anything with a controler.)

Full power plug

To check the power simply plug in full power, go to the appropriate part of the track, measure the voltage with no load then put the resistor in place in the slot and measure the voltage again. Its wise to check the voltage with no load from time to time just to make sure the change you think you are measuring in the track wiring isn’t just the battery voltage changing. Repeat this on the other lanes at the same point on the track, and if the track is working properly you should get roughly the same on load voltage on all the lanes. (Differences between the lanes of 0.1 volts are not significant).  Diagram 3 (below) show what to do. Its important that the load resistor is always further away from the power feed than the fault you are trying to find - the whole point is that a high current is passed through the fault to produce an easily measurable voltage drop. It may not always be obvious which side of a joint is nearer to a power feed, particularly when you are about half way round the track - try measuring with the load resistor in a few different places - remember the lowest voltage across the resistor is always furthest away from the feed - pause to think and it should become obvious which way the power is coming from. 

Checking track

Once you have worked which joint is faulty, measure each side as shown in diagram 4 (below) to work out which side needs repair. The other thing you can do is to go round the track checking there are no "step" changes in the on load voltage. (on most tracks you can expect to see the on load voltage being highest where the power is fed in, and dropping by perhaps 1 1/2 volts at the far side of the track. If you find the voltage drops by half a volt or more over a few feet then look for a broken tape.

Which Tape is faulty

How you repair the fault depends on the type of track - soldered joints are ideal for low resistance, but may not be necessary - or desirable for portable tracks. (A group of people standing around urging you to get on with a repair so they can continue racing is not a pretty sight - particularly when nobody can find an extension lead, and you’ve only got a soldering iron that takes 10 minutes to heat up!) A small length of braid stuffed down the gap has often proved completely adequate on portable braided tracks. Scalextric track can often be sorted out by taking it to bits and putting it back together again! This is a good trick with any connector, plugging them back in again often wipes off the insulating tarnish that has formed on the pins. Its always as well to repeat the measurement after you’ve completed a repair, just to check the fault really has been fixed.

If your club track has poor power at the far side from the power input, some extra wiring - "jump leads" as they are sometimes called - can help. These methods provide a way of checking how much you’ve improved the power - and may enable the wiring to be moved around to best advantage. Best of luck with the racing, and I hope you don’t have to put this article into practice too often!

Chris Frost

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