Arduino-controlled slot cars - alternative to MOSFETs for speed control?

Question

I'm attempting to control the speed of two slot cars in a set using Arduino.

Each car is controlled by a controller with a variable resistor that looks like this:

They're wired in parallel to the same 17V DC power supply:

Simplified:

For Arduino control, I experimented with manipulating a single car's speed using a MOSFET. It worked really well. Using a PWM output on the Arduino, I was able to pulse the MOSFET on and off to throttle the current and change the car's speed. The behavior of the MOSFET seemed to be exactly the same as the variable resistor controller; the resistance, voltage, and current fluctuated the same way on various parts of the circuit.

Enter car B. I added a MOSFET to the setup and mimicked the MOSFET wiring for car A, grounding them both to the Arduino ground. The result looks like this:

Here's a sketch:

Close up, MOSFET A:

MOSFET B:

At first, all seemed well. I sent signals to MOSFET A and successfully changed Car A's speed. I stopped Car A and pulsed MOSFET B, and I was able to control Car B's speed just fine.

Then, I turned Car B to a constant speed, and sent a signal to MOSFET A while Car B was still running. Car A started, but Car B's speed immediately dropped as a result of running Car A.

I hooked a multimeter up across the rails of Car B and watched its voltage while it ran on its own. Then, sure enough, when Car A started running alongside it, Car B's voltage dropped dramatically.

I figured this was because of my wiring configuration, so I left the multimeter on Car B, and made one change. I replaced MOSFET A with the original slot car controller:

I started running Car B again using its MOSFET. The multimeter showed a steady voltage, so I began to squeeze Car A's controller (the variable resistor), and Car A's speed increased. This time, however, unlike using a MOSFET to control Car A, the variable resistor did not affect Car B's voltage at all. Car B remained at a steady speed the whole time.

Any idea why this happens? The MOSFET seemed to function the same as the variable resistor (empirically) with a single car, but had totally different behavior with two cars in parallel. Is it because the MOSFET is not really changing resistance to current, but just switching it on and off, so it opens up another path for current to flow fully every time it closes? Is there an Arduino-controllable alternative to a MOSFET that would fluctuate actual resistance like the variable resistor? Digital potentiometer? Servo hooked up to a potentiometer? ;) I'm really curious what the difference is and what causes the major differences in behavior.

Answer 1

YOu are entering a world where Electro Magnetic Compatibility is at risk. Pay attention to the stray current spikes that radiate noise (Egress) and their influence on other circuits of high impedance (Ingress). YOu probably also have ripple or the conducted egress and ingress as well.

For radiated noise a handy tool is a small AM radio (not FM) nearby, to locate such noise issues if you dont have a scope.

Noise suppression management may include some common mode chokes to the track or pot lines. Perhaps some supply rail filtering may need to be added around the track.

I would use ferrite beads for each driver with caps after the bead and then use twisted pairs to send the current after going thru a ferrite torroid or similar common mode choke, like those used in Video cables. Its good to have a kit of parts for such issues.

Add decoupling caps to the breadboard for both sides.

Answer 2

I suspect that your two PWMs are in phase: on at the same time, off at the same time. This is likely to cause the voltage to drop at the output of the power supply. Try a large electrolytic capacitor of suitable voltage rating across the power supply, and making sure that your PWMs are out of phase.

You should also have a diode connected in parallel with your driver MOSFETs, in the normally non-conducting direction. This shouldn't make much functional difference other than avoiding damage to the MOSFETs from back EMF.

Answer 3

I ended up finding that the L298 driver provided all the circuitry internally to control two motors separately and keep them isolated. It worked really well using the part directly, but it needs some extra stuff to control heat. This got complicated pretty quickly, just like trying to isolate the MOSFETs did.

Finally, I settled on buying this motor driver from SparkFun, which uses the L298 at its core:

https://www.sparkfun.com/products/9670

It has worked incredibly well for months, and it was a good investment to not have to design the circuit around the L298 myself. Plus, it adds a few extra really nice-to-have features and packages it all together in a nice board with screw terminals for your power supply and motors.

Answer 4

You have got diodes across the motors so that is ok...

I think you problem is one of current, a breadboard and the jumper wires will limit the current flowing to the motors from the supply, you need a thicker wire for the common ground from the FETs to the supply negative.

One motor is ok, but when you start to use two, the extra current will cause problems.

Try taking 2 identical jumper wires from the motor supply ground, and have 1 to each fet source pin (leave the other wires to your ground rail on the bread board (Yes you will get a ground loop, but for testing this will prove if the breadboard / links are limiting the current))