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· Premium Member
318 Posts
Discussion Starter · #1 ·
Simple rotary switch voltage drop unit

I started this project to give me the ability to independently drop the voltage of each lane of the track, this was initially to have my daughter run with me and not come off every corner. I soon found that this was also great for running unmatched cars together for example a Slot it McLaren F1 against a SCX rally car.
This guide shows the parts and method I used in its rough prototype form. I have gone one step further and actually made up a circuit wiper board to replace the resistor in my Parma controller allowing me to use Diodes instead of resistors.
Rotary switch

Parts Used

Switch - 12 way is the one I have
Diodes - I don't remember which I selected

My track is powered by a digital adjustable DC supply 0-30V 0-3amp. The negative goes direct from the DC supply to the track and the positive goes through the controller. This switch is placed in the positive feed between the controller and the track. This is the configuration of my track. I'm sure the switch could be placed in either wire either side of the controller providing the current flow direction was correct.

The Build

The main thing to remember is that power will only flow one way through a diode, this means all diodes in the switch need to face the same way. It doesn't matter which way they are soldered to the switch.

The first two pictures show the switch on the cable. The third picture shows it mounted to the circuit board of the controller. Both are acting in exactly the same way and are on the same wire.

The Diode controller

I had the issue of controller resistance not matching car motor resistance, I'm sure we all have this at some point but don't realise what it is.
With resistor controllers like the Parma's you get a different feel with different cars depending on the motors resistance. Hotter/faster motors like lower resistance in the controller 10 -15 ohm and Scaley type motors like higher resistance in the controller like 45-55 or higher and all the others somewhere between. if the resistance in the controller does not match the resistance in the motor you get a strange effect such as the trigger can be pulled 50% before the car moves or the other way round where the car is off the line as soon as the trigger is moved and at full throttle when the trigger is only 50% pulled. That is because these controllers restrict CURRENT to the cars.
The Diode controller I made does not restrict the current, it controls the voltage. Each diode in the line drops around 0.5 - 0.6 volts so as the trigger moves along the wiper it passes through more or less diodes hence dropping more or less voltage. The effect on the cars is far smoother power delivery. Its exactly the same as the rotary switch.

You can see from the pictures that this still in its rough experimental stages, I will be making much neater versions when I get chance. It really has been great having the smoother control of the Diode wiper, I found I have been using the cars that were previously "uncontrollable" a lot more now, I's like having a load of new cars.
I realise I could have saved myself the bother and bought an all singing all dancing controller but to be honest I really enjoyed learning a little about the electronic of the hobby and It's cost almost nothing to make up these switches and wiper board.
I hope this has been of some help to everyone.

· Registered
263 Posts
Hi, I use these switch's/diode's alot making test bench's you can also use them with schottky rec diode's 1amp 20v for adjustable brake's, solder in the same way then put in the brake line direction of diode pointing away from controller toward's track you could fit it to your controller as per the other switch to keep it simple but I prefer to mount my control's in a seperate box. There is also a way by keeping the parma resistor in place to have a basic form of adjustability over the sensetifity by useing the switch again but this time useing resistor's for this im useing the 3watt wire wound one's also from maplin's there the smallist in there group at 22mm long you can use the next up at 7watt but size also increase's to I think 33mm anyway the 3s are working ok.I would suggest encloseing all the switch ware in a box as not only does it protect from damage/short's but weight in the hand if you want more info on this just ask, from the photo of the wiper board there didn't seem to be a dead track between brake and first power step there's normally a un-used strip to keep a gap incase the wiper didn't fully return to brake and was touching both contact's. phil.

· Matt Tucker
3,739 Posts
I have used this info and completed my own black box for my parma plus that offers me adjustable brakes as well as the ability to reduce the voltage by up to 2.8v in 0.7v steps - just what I was wanting and with the self build a nice smug satisfaction! Will look in to the adjustable sensitivity build now

many thanks
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