problem controlling DC motor with a 500k pot

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
First time attempting something like this (i work with guitars)

I have an A500K potentiometer, 3x1.5V batteries as power supply, and a vibrating motor from a ps3 controller.
The problem is, motor turns on at about 90% rotation. I want it turning at least before 50%, for a finer control. pot is wired as variable resistor, 3-lug voltage divider wiring method produced the same problem.

I have a range of capacitors and resistors, which i know i could use to somehow make the potentiometer more usefull, just don't know how. most posts about Pot modifications talk about guitar wiring, which is AC and has a ground, but i'm working with DC, and a + and - terminal, which makes things more confusing.

so, my questions are:
What can i do to make the pot more useful?
Does current still pass by the pot at 0 (do i need an on/off switch to save power)?

there has to be a simple sollution.
 

Alec_t

Joined Sep 17, 2013
10,350
Welcome to AAC!
there has to be a simple sollution
Despite your assertion, it's not as simple as you'd like :(.
The 500k pot is far from ideal as a speed control. Lower values would normally be used. Even if you used 500k the necessary associated circuit might draw more standby current than the pot itself, so an on/off switch would be desirable.
The most efficient method of control is PWM (pulse width modulation). Modules for this are available cheaply online; more cheaply than the cost of sourcing components and building your own circuit.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
4.5v is not much to work with, most of the Ebay PWM's $2.00 controllers start at 6vdc.
Is the cells just for the motor?
If so increase the battery holder size to 6v or maybe 7.5v.
The motor can be controlled accordingly to prevent over speeding on the higher voltages.
Or a resistance in series with the pot to limit the rpm.
Max.
 

crutschow

Joined Mar 14, 2008
23,291
As Alec stated, a PWM module is the best way to control the motor.
But if you can be happy with a less linear control of motor speed vs. pot rotation and a maximum motor voltage about 0.7V below the supply voltage, you could add an emitter follower to the pot output (below).
Adding an extra battery can make up for that voltage loss.

The circuit will draw the small current through the pot (about 10 microamps) when the pot is at the minimum position.
upload_2019-8-26_11-6-53.png
Depending upon the current draw of the motor, Q1 may need to be a two transistor Sziklai pair (complementary Darlington) as below.
upload_2019-8-26_11-18-21.png
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
As Alec stated, a PWM module is the best way to control the motor.
But if you can be happy with a less linear control of motor speed vs. pot rotation and a maximum motor voltage about 0.7V below the supply voltage, you could add an emitter follower to the pot output (below).
Adding an extra battery can make up for that voltage loss.

The circuit will draw the small current through the pot (about 10 microamps) when the pot is at the minimum position.
View attachment 184822
Depending upon the current draw of the motor, Q1 may need to be a two transistor Sziklai pair (complementary Darlington) as below.
View attachment 184823
I did exactly this, constructed an emitter follower with a pnp resistor, and the pot is still works all the way up, when turned slightly down, the motor actually speeds up, and the Pot starts to smoke... what?:D the emitter follower diagrams seem to illustrate that npn and pnp are interchangable, as long as the same wires are connected to the emitter and the collector. what did i do wrong? B941 transistor was used.

Edit: sorry, missed the 1k resistor on the pot wiper from the diagram, redoing it, will inform on the result
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
Nice! it works! thank you very much!

now the motor starts working at 50% and can be more finely controlled, but the output is decreased, as you said.

So a complimentary darlington connection would lower down the motor activation threshold even further? your scheme uses pnp and npn together, i have both of those, which is good.

And increasing the voltage via more batteries would both increase the output, and activate the mottor at an even lower pot setting?
 

crutschow

Joined Mar 14, 2008
23,291
So a complimentary darlington connection would lower down the motor activation threshold even further?
Not significantly.
It's only needed if the motor takes more current than one transistor can readily supply.
And increasing the voltage via more batteries would both increase the output, and activate the mottor at an even lower pot setting?
Yes.
But you must be sure not to apply more than the rated motor voltage.
 
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