Can anyone explain this wiring diagram?

[Mr niall]

Hello there,

Sorry you'll have to excuse my lack of electronics knowledge; I'm looking at barn door trackers at the moment and the most popular version uses a wiring diagram that I see popping up regularly over the web. It's quite simple, and it uses a potentiometer to control the voltage going to a small 3v motor, allowing you to very accurately adjust the turn speed of the connecting rod that opens the barn door.

But - the thing I don't understand, is why the schematics introduce two small ceramic capacitors between the battery and and potentiometer and again between the motor and potentiometer. I'm guessing they're there to regulate the voltage somehow but I thought (probably wrong) the point of a capacitor was to store energy and then release all that all energy in one go, so it would look like a pulse? I'm further confused by the fact that they appeared to be wired in serial with resistors?

As this exact schematic is fairly ubiquitous I have no doubts it is my understanding that is the problem, not the diagram. But if anyone could have a go at explaining it to me I'd be grateful!

 

[Cornelius Varley]

Capacitor C2 is connected across the terminals of the motor to act as an electrical noise suppressor, C1 probably does the same across the switch terminals.

[Mr niall]

18 minutes ago, Cornelius Varley said:

Capacitor C2 is connected across the terminals of the motor to act as an electrical noise suppressor, C1 probably does the same across the switch terminals.

oh - that makes sense - so its acting as a kind of blocker to prevent spikes above the capacitors threshold?

But... if thats true, wouldnt a resistor do the same thing?

[vlaiv]

2 minutes ago, Mr niall said:

oh - that makes sense - so its acting as a kind of blocker to prevent spikes above the capacitors threshold?

But... if thats true, wouldnt a resistor do the same thing?

Resistors provide voltage drop (depending on current). Capacitors in this case provide filtration - removal of ripples in current.

Think of current as having two components - one constant (DC) and one variable (ripples, AC) - capacitor acts as open circuit component for DC, while acting as closed circuit for AC component.

If there are both constant 9V DC and some ripples on voltage supply - C1 will act as no connection for DC current - thus forcing current to go to U1 component, while AC/ripples will short circuit thru C1 and go directly to negative terminal thus not entering the rest of the circuit (path of least resistance).

C2 acts in the same way, but preventing any sort of ripples created by turning motor (created by magnetic induction of spinning magnets and wires in motor) to go back into circuit.

Resistance components don't act this way - they let current flow whether it is AC or DC, and depending on how much current is flowing that way - they create voltage drop across terminals.

In schematic above R2 has fixed resistance, while R1 is variable resistor, so by changing R1 resistance you are in fact varying voltage drop between Out (exit from U1) and point between R1 and R2 (which feeds into Adj port of U1).

U1 is acting as a sort of valve - depending on voltage difference between In and Adj - it lets certain current flow thru it. Thus adjusting resistance on R1 regulates how much current is flowing to the motor (and resistors).

[Mr niall]

Super - that makes sense now. Thanks both!

[ringz]

The capacitors are to stop the voltage regulator from self oscillating.

[jacksdad]

While it's a simple circuit, it might be a bit too simple really...

 

I'm assuming the component U1 is a linear voltage regulator, something like an LM317, and these aren't the most stable of devices...

 

Also, by just cutting the voltage you reduce torque along with speed on a small DC motor, which makes it much easier to stall, which also affects the stability of the drive speed.

 

A much better bet would be a cheap PWM - about a quid off Aliexpress.

 

Because of the way a PWM regulates the voltage it exhibits better stability and less loss of torque.

[Dr_Ju_ju]

Put this on a motor of your choice   https://www.ebay.co.uk/itm/Replacement-Ultra-small-DC-3V-35V-PWM-Mini-Motor-Speed-Controller-Switch-5A/253546073334?hash=item3b08860cf6:g:-~UAAOSwdzVXoC2V

There are plenty of small, high torque, dc motors on the bay to choose from.