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Controller Power Relays

Quite a few of the top drivers in the UK are now experiencing controllers with full power relays for the first time. Although the first time many UK racers saw full power relays was on Difalco or Cidex controllers, this is not a unique feature - The first one I remember seeing was built by Dave Stevens in the early 1970s but I expect somebody can tell me about earlier examples.. Keith Oborn won the main grade Nationals in 95 with a relay added to his otherwise fairly standard Parma resistance controller. It's time to take a closer look, and explain to the more adventurous how to do it yourself.

OK so why is a full power relay a good idea? It makes the car faster in a straight line - in particular it gives more acceleration and a small increase in top speed. Yes that’s exactly the opposite of what a "choke" does. That should give you a clue to how it works. A "choke" reduces acceleration by putting a small (usually between 0.05 and 0.15 ohms) extra resistance between the power supply and the motor. A full power relay will increase the power by removing the small resistance in the controller wiring between the plug and handle.

Hang on - why not just use a thicker controller lead rather than messing about with a relay. Yes, thicker wire would improve full power. The table below shows the resistance of the types of wiring you might use. A controller using ordinary "13 amp." mains flex might have 0.03 ohms in this wiring - this could easily be halved with thicker wire. Its practical to go up to something like 12 AWG wire giving about 0.01 ohms (although the plug might need a bit of extra work to get it all in!) but there are practical limits to thicker wire (Do you really want a controller that stands up on its own flex?). A shorter length of wire would have lower resistance but practical considerations limit this to about 3 ft. - just under a meter.  The practical way to less full power resistance is a relay. Incidentally you still need thick wiring for brakes (I guess you could have a relay for brakes as well, - it should provide better brakes -  but I don't know of anybody who has tried it .  In fact most quicker cars these days are driven with the brakes backed off - that is to say with extra resistance in the braking circuit - so there isn't that much interest in a controller providing even more braking.  )

Yes we are talking a small difference in resistance. How much difference does this make? Lets look at some figures. Removing 0.015 ohm from the supply to a sports car motor can increase the voltage available by 0.3 volts on acceleration and give an extra 0.15 volts at full speed. The gain would be about half that for a Group 12.

Conductor

Area sq.mm

Resistance ohms per meter

Typical mains lighting cable

1.0

0.017

"13 Amp." Mains Flex

1.25

0.014

High power cable (2.5mm²)

2.5

0.007

18 AWG wire

0.823

0.021

16 AWG wire

1.31

0.014

14 AWG wire

2.08

0.008

12 AWG wire

3.31

0.005

How do you make it work ?

The connection diagram below show how a typical full power relay is wired. ( There are other ways of connecting it that work equally well.) The switch in the controller is arranged to switch on when the controller is pressed down to full power. This switch delivers power to the relay coil, which pulls the relay contact to the n.o. (normally open) position. This provides a very low resistance path between the N and L pin on the controller plug for the full power to the motor. The relay needs to be mounted close to the plug.

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Obviously all this needs to work very reliably otherwise its more trouble than its worth - in particular it needs to be arranged so there is minimal risk of the relay switching on when the controller is not pressed onto full power (that's likely to blow a controller transistor and put your car into the wall).  A fault that causes the relay not to switch on is a nuisance, but at least it will get you to the end of the race!

Overloaded relays can fail with the contacts welding together. To avoid this chose a relay with a current rating comfortably in excess of maximum it should see in use. Powerful slot motors can take 20 amps on acceleration, so relays with a contact rating of at least 40 amps should be safe even if you end up with two cars on your lane. Fortunately there are plenty of relays "for automotive use" around at under £10.  (In fact we've used 30 amp relays to switch the power on our club track and they have proved reliable ever since I fitted them in 1994 - and they are often switching the full motor power at the start and end of each race)

Most controllers have a sliding "full power" contact, these can be used to enhance relay reliability. If you arrange it to make after the full power slider has made contact and to break before the wiper has left the full power contact. Typically the relays are rated for 100,000 operations switching full power, and 1,000,000 operations where they are not switching much power (as they won't be if the wiper is arranged as above). I know that sounds like an awful lot of use so why worry, but just think about hitting full power 5 times a lap, at a meeting like a 1/24 BOC you do 800+ racing laps plus practice – a little over 20 meeting like that and your relay has done 100,000 operations.

The switch in the controller should present few reliability concerns - its only driving a small current for the the relay coil so a sub-miniature microswitch will be than adequate. It's as well to protect the switch from arcing by putting a suppression diode across the relay coil - a 1N4001 costing a few pennies will do fine. At the full power position there is no problem with pushing the switch hard enough to it on... most drivers push their controllers far harder than necessary on full power (doubtless in the subconscious hope of getting a bit more speed). Changing the subject slightly, there isn’t all that much pressure available at the top of the controller stroke, which is one good reason why brake microswitches went out of fashion. A bit of extra friction in the mechanism could mean that the controller spring couldn’t operate the microswitch, and worse still the microswitch spring prevented the slider from returning to the brake position.

The relay is wired so the coil is taking power when the car is on full power. It’s done this way round so that its "fail safe" if a wire to the really gets broken. (This failure causes your car to go fractionally slower; if the relay was wired the other way round a broken wire would smash your car into the next wall.) Doesn't this waste a bit of power that should be driving the car? Yes it does - the sort of relay needed for this job typical have coil resistances of 80-90 ohms which mean they will take around 0.15 amps - on a well wired track this will reduce the voltage to the motor by about 8 millivolts. So a typical theoretical gain of 0.15 volts for a Group 12 on full acceleration will be reduced to 0.142 volts.

This is all well and good for your high power cars on a track wired to the best BSCRA practice - what about a Scalextric / Fly / Ninco car running on a transformer power supply? Common sense may tell you that this controller technology is way over the top for Scalex cars - but in fact a full power relay could even make these cars go slower! These motors take much less power, so the gain in reducing the controller resistance by 0.015 ohms would only be around 0.01 volts. If the power supply is a transformer, the 0.15 amp relay current could easily drop the supply voltage by a good deal more than  0.01 volts. In short, if you are running off a "home set" style transformer, the car will probably go faster if you don’t use the full power relay.

Is it a good idea?

Its a good way of getting the absolute maximum out of your motors in a straight line. It gives most extra power with the highest powered motors. When you are using a choke you don’t need a relay, indeed it might be a good idea to incorporate a switch to isolate the microswitch to give you a little bit of choke effect when the circumstances demand it. The relay life might also benefit from being switch off when it is not really needed. 

Is there a transistorized option instead of a relay?  "Ordinary" (Bi polar) transistors always produce a voltage drop, unless your controller has very thin wiring the transistor will have a larger voltage drop than the wiring - so it will do no good.  There is another type of transistor called a Power MOSFET which doesn't exhibit this kind of voltage drop - but they have some resistance.  Some controllers using very high power MOSFETs have such low full power resistance that relays are not needed. 

Chris Frost