Guru
Joined: 05/10/2019
Location: United KingdomPosts: 6027
| Posted: 07:26am 20 Nov 2022 |
A resistor drops the voltage. The torque falls. Assuming the load remains reasonably constant the speed will fall, although the speed:resistance_value ratio isn't usually very linear. A simple PWM controller with a single mosfet output is pretty much ideal. Avoid H bridges unless you have voltage to spare as you'll usually lose at least 2V, possibly more, in one. A 555 can be used as a PWM generator, you don't need a micro.
For the simplest system I'd keep a resistor in circuit as many switches will break the circuit between positions and this will keep a supply to the motor. It does no harm to wire it this way anyway. You just have to choose parallel combinations for the speed steps. Just copy the whole circuit for another pole of the switch on a 2-motor system. If the current is low then the cheap rotary switches can be 4-pole 3-way as standard. You can wire two poles in parallel for each motor. Beware though, the maximum rating of these switches is only about 300mA per pole and you'll probably get about 500mA with two poles in parallel.
+ --------+
|
+------------+
| |
R1 O
| /
| O O O
| R2 R3
| | |
+------------+---+
|
motor
|
- --------+