Guru
Joined: 06/08/2020
Location: United KingdomPosts: 367
| Posted: 01:32pm 19 Nov 2022 |
Hi -- slightly off topic - delete if not right for here...
I want to incorporate a rotary switch into a project. I want to use this to introduce one of 3 resistors into a circuit to make a rudimentary speed controller.
I have two motors, each powered from the same source, but controlled independently. The supply to each motor can be controlled independently by varying the voltage and polarity. This allows the speed and direction to be controlled by varying the speed of each motor independently. All simple stuff.
All the above works fine.
BUT the default speed as constructed is far too fast, so I want to add inline resistance. Again, fine if I do this for each motor independently.
If I constructed this from two rotary switches, I would be fine with what I think is referred to as a 1 pole, 3 position switch to bring different resistances into the circuis for each of the motors.
How do I do this for both motors using one rotary switch, so that the same resistance value is introduced to each motors circuit at the same time?
Having not used anything different than the above switch - I am unsure if what I am looking for is referred to as a 2 pole, 3 position switch? I guess I could just buy some and bust out the multimeter...
Long question - hopefull simple answer....
Cheers
N
Guru
Joined: 30/03/2016
Location: United KingdomPosts: 1646
| Posted: 04:01pm 19 Nov 2022 |
Resistors...Yuk
Grab a cheapie H-bridge and drive it from at least a 555 or better yet, a PWM from a Picomite
Guru
Joined: 06/08/2020
Location: United KingdomPosts: 367
| Posted: 04:29pm 19 Nov 2022 |
LOL -- this is something I am retro fitting to an existing device in schools - rough and ready is best.
At presented the set up drives to motors far far too fast to use on a desktop - more like a school hall -- so this is almost like a "calibration" I am trying to hack back in.
I'll give it a go and see what happens.
N
Guru
Joined: 24/12/2017
Location: AustraliaPosts: 1601
| Posted: 05:38pm 19 Nov 2022 |
Yes, a 2 pole, 3 position switch is what you are describing. It could have more poles and more positions, just don't use them.
If there are more positions, there is often a washer on the mounting spindle with a tag which is used to restrict the number of switch positions.
You don't say what current you need to control, just make sure that the switch can carry the current (and more) and that the resistors have a high enough power rating.
Bill
Guru
Joined: 06/08/2020
Location: United KingdomPosts: 367
| Posted: 05:46pm 19 Nov 2022 |
Perfect.
Will measure the current to check when its running in its unmodified current state. And yes, will make sure over the resistors.
N
Guru
Joined: 07/08/2011
Location: AustraliaPosts: 5962
| Posted: 07:14pm 19 Nov 2022 |
Don't worry if you have to get a switch with extra poles. You can always parallel them up to help with current capacity.
Jim
Edited 2022-11-20 05:16 by TassyJim
Regular Member
Joined: 25/07/2019
Location: United KingdomPosts: 64
| Posted: 10:50pm 19 Nov 2022 |
Oversimplification warning!
Resistors control the torque of the motor, not the speed.
Voltage controls the speed, and PWM is a good way to control voltage in this context.
It's so easy to do PWM with almost anything with MMBasic.
Suggest use any transistor, Maybe Darlington, or 20p ULN type chip to power motor.
Don't forget a freewheeling diode though.
Alternatively.. post project requirement in details here,
TBS is full of clever helpful people !
Guru
Joined: 24/12/2017
Location: AustraliaPosts: 1601
| Posted: 05:53am 20 Nov 2022 |
If the motors can be connected in parallel, you could get away with a single pole 3 way switch and 3 resistors, each resistor in series with both of the two motors.
Bill
Guru
Joined: 12/10/2011
Location: AustraliaPosts: 1828
| Posted: 06:10am 20 Nov 2022 |
I'm guessing they are DC motors, why not use a motor speed controller like this
Motor Speed Controller
AC Version
Edited 2022-11-20 16:15 by palcal
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
|
- --------+
Guru
Joined: 05/03/2018
Location: NetherlandsPosts: 3811
| Posted: 12:31pm 20 Nov 2022 |
Post 1: you have independent control over voltage and polarity. Why resistors?
Guru
Joined: 06/08/2020
Location: United KingdomPosts: 367
| Posted: 04:42pm 20 Nov 2022 |
This is a commercial product that I am "hacking", so I am limited to adding something in - the MCU is a "blob".
Control as in - each motor is controlled independently - and is either "on", "off" or "reverse". I can't control the voltage -- its almost like I just need to bring the level down. The "on" is far far too fast so I need to scale it back.
All the feedback here has been great --- I'll be building something over the week and will report back.
Cheers all
N
Guru
Joined: 05/02/2015
Location: AustraliaPosts: 647
| Posted: 10:47am 21 Nov 2022 |
G'Day All
Perhaps you may consider using diodes instead of resistors. I have no idea of your voltage and current requirements but 1N400x diodes are cheap and can be strung in series to give a range of voltages.
I am reminded how years ago the son of my late wife's friend wanted to drop the speed of two windscreen motors. He had tried resistors, but the regulation was poor, so I designed a switching circuit that did the job but when he saw that it incorporated a pot. which varied the speed a whole new world of possibilities opened up. It was a classic example of me, the Engineer, not asking questions of the end user at the start.
I was very good at that
Peter
Edited 2022-11-21 20:49 by PeterB
Guru
Joined: 24/12/2017
Location: AustraliaPosts: 1601
| Posted: 11:21am 21 Nov 2022 |
That's a good idea Peter, it would give a more or less constant voltage that would not vary much with load but still very simple. Don't know how much voltage Nimue wants to drop though.Bill