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Help. I can't get my potentiometer to work with my motor

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atsay714

New Member
I've read up so much, and received so many different opinions. I have a feeling it is a lot simpler than I'm making it.

I currently have a crappy motor I pulled from a toy car. I wanted to control the rate at which it ran, so I slapped a pot' in there. It works, however, not how I would like it to. I would turn the knob of the pot slowly and nothing would happen, I will turn it until it hits around 270 degrees. (300 degrees is the total). Once it neared the end, the engine would shoot up and go on full blast.

Some say I need a pot' with a lower rating. I tried one with 10 ohm resistance, and now I'm using 5 ohms. It still does the same thing.

THEN I was told I needed to introduce a transistor into the circuit. So I went and bought a bread board and a pc board. Now, it goes from battery -> collector -> out the emitter -> pot' -> back to battery. (A side question: Where does the "base" of the transistor go??)

Once the transistor was installed, nothing changed except the motor speed lessened. It didn't give the full range like I wanted. It still shot up to max speed at around 270. I want a gradual increase from 0 through 300.

Here is my potentiometer:
Resistance: 5k ohms
Rated power: 0.5 watts
Sliding Noise: <47mV
Total rotation: 300 degrees
On the front it says 500VDC...

I believe I used a 600mW NPN transistor.

Thank you for the help guys!

Just realized. This might be in the wrong section. Please move it if you can... Sorry
 
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You can't use a potentiometer by itself to control the speed of a motor. It has to do with ohm's law and how potentiometers are constructed. First off, the motor needs x voltage to run and will draw y current. In most cases the current needed will exceed the current handling capability of the pot therefore burning it out. Secondly, the most commonly found potentiometers are "audio" taper potentiometers and not "linear" taper, therefore the resistance change across them is "bunched up" to one end.

While you might be able to setup a transistor and a potentiometer to change the speed of the motor you are going to burn the transistor out in most cases because once again of current requirements of the motor.

What you need is PWM (pulse width modulation), more specifically in your case since it appears you want to be able to change the speed at any time you need variable PWM. While the easiest way to do this would be with a PIC I will instead suggest the good old 555 timer method. See this website Pulse Width Modulation - Electronics in Meccano for an example. If interested in the PIC method then speak up and someone will enlighten you.
 

Hero999

Banned
What voltage are you running the motor from?

Check out the following link for a circuit that just uses on 555 timer and will control the motor's speed simply by wiring it in series with the motor.
Two wire dimmer
 

atsay714

New Member
So I went and bought myself a 555 timer. I set everything up and low and behold I can control the motor speed now! Step 1 complete. To my dismay, more issues have arisen. It seems the range of the motor's overall speed has decreased. Meaning, when I have the potentiometer completely turned off, the motor is still running, but at a relatively slow speed. As I turn it up, the speed increases, but the max speed when turned all the way up, is quite slow. Relative to the motor's max, that is. Ideally, I would like the motor to be completely still when the pot is turned down, and max speed when it's turned all the way up. It most likely has to do with the ratings of my components I bought.

Here is everything:
555CN Timer IC
1 10uf capacitor
1 1/2-watt 470-ohm resistor
Potentiometer: I've tried 3 types. 5, 10 and 100 ohm. All linear-taper and 0.25 watts. And All = Fail

and of course a 9v...

read on if you want to know how I set it up, I'm pretty sure I set it up right though.. Seeing as how it sort of kind of works

These are all the terminals of the 555.
1. Ground
2. Goes to 6
3. Goes to motor
4. battery +
5. nothing
6. (capacitor to ground) & (goes to pot, then back to 7)
7. resistor to battery +
8. battery +
and of course motor is hooked to ground also.

Thank you! You guys are wonderful
 
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BrownOut

Banned
Sounds like you're controlling the timer's frequency, and what you really want is to control the duty-cycle. We have a current thread about this some where on this site. I'll try to find it.
 

georgetwo

Member
the speed of the motor will not be at the highest speed like the direct connection.
this is because, the output current of the 555 ic is not enough to totally increase the speed. I dont know of any ic that pumpes current like a relay or direct contact switch. If you dont get any post on how to increase the current, you shuld create a switch for full speed.

as for the off case, you shuld try using higher resistance for potentiometer.
 
As you will notice, in all 3 links posted that show ways of doing this there is a transistor connected at pin 3 of the 555 driving a transistor. The transistor will handle more current than the 555 is capable of handling. I have not built the circuit(s) referenced by Hero999 and BrownOut but trust that if they say the circuit will give full range control then it will do so. Since with that circuit you only need 1 555 then I would use it. Please note the transistor used in that circuit is a mosfet and not a bipolar transistor.
 

Hero999

Banned
I'll post all the circuits and breifly highlight the advantages and disadvantages.

#1

I wouldn't recommend doing it this way, it uses too many parts and the transistor is connected as an emitter follower when it would be better connected as a common emitter amplifier.

If you do choose this option, then I'd recommend using the dual 556 timer and connecting the motor between the transistors collector and +V, using a MOSFET would also be adventurous.

#2

Not meaning to be cocky but it's a good circuit. :D It uses minimum parts, the MOSFET could be N-channel if switching the negative is acceptable. This circuit works better with a 7555 but it will work with the NE555.

#3

Should be good, works well in simulation. 7555 only really but will work with the 555, especially with larger motors. Apart from requiring an extra diode and capacitor this circuit has the disadvantages of having a minimum load requirement and will never fully turn the load on, although it's easy to get >99% on. The advantages are that it only required two wires (it's just connected in series with the load) and can drive the positive wire using a N-channel MOSFET thanks to the bootstrapping action. See the origional thread for more information: Two wire dimmer
 

atsay714

New Member
This makes it much easier. Thank you so much hero. Where do you buy your 7555 timer? Radioshack doesn't carry it, and surprisingly, I can't really find it online either. Would the TLC555 work?

Also, for diagram number 2. I have a few questions.
1. What does 10R and 4k7 on top of the resistors mean? And what is the yellow circle with the X?
2. Where does my variable resistor, or pot' go?
3. That symbol on the very right, middle of the diagram is a mosfet transistor? Does that mean the gate goes to power, and source and drain go to ground?
 
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rmn_tech

Member
10R = 10Ω

4K7 = 4.7KΩ

The circle with cross = Load (your motor)

The 100K Variable Resistor in the centre of the diagram is your pot

Gate 555 Pin 3
Source +ve
Drain motor
 
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Hero999

Banned
This makes it much easier. Thank you so much hero. Where do you buy your 7555 timer? Radioshack doesn't carry it, and surprisingly, I can't really find it online either. Would the TLC555 work?
The TLC555 is the same part and will work.

Also, for diagram number 2. I have a few questions.
1. What does 10R and 4k7 on top of the resistors mean?
There are no 10R and 4k7 resistors on diagram 2.

You must mean diagram 3.

10R means 10Ω and 4k7 means 4.7kΩ

It's fairly standard practise to replace the decimal with the prefix and use R for Ohms.

And what is the yellow circle with the X?
It's the load you want to power, in the schematic it's a bulb but it could be a motor or heating element.

2. Where does my variable resistor, or pot' go?
See R2 on # 3 or the 100k resistor on #2.

3. That symbol on the very right, middle of the diagram is a mosfet transistor? Does that mean the gate goes to power, and source and drain go to ground?
Which schematic? I'll answer the question for both.

#2
It's a P-channel MOSFET, the gate goes to pin 3 (it's optiontional but you might want to consider adding 10R in series with the gate). The source goes to the +V and the drain goes to the load.

You can use an N-channel MOSFET if you like but the load and source/drain connections will need reversing: connect the load to +V, the drain to the load's 0V and the source to the dimmer's 0V.

#3
Look at Tr1.

The gate goes to the 7555 via R3, the drain goes to the +V and the source goes to the load.

Look up MOSFET in Wikipedia.

Note that the only ground on the schematic is connected to the load, the entire dimmer circuit floats on top of the load's positive power rail.

How much current does the motor use with no load?

How much current does the motor use when fully loaded?

What's the power supply voltage?

Some of the component values on #3 might need tweaking depending on the answers you give to the above questions.

Did you read the thread, I suggest you do before asking more questions?
Two wire dimmer
 

rmn_tech

Member
Sorry in my post I reversed the source and drain I have now edited it.

I'm just getting old.
 

Hero999

Banned
D1, D2 and D3, any silicon diode such as the 1N4148 or 1N914 will do, you could use the 1N4001 but it's a bit overkill, same for both schematics.
 

Hero999

Banned
You still haven't answered the questions in my post linked above.

De honest, you haven't even read it thoroughly have you?
 
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atsay714

New Member
Yes. In fact, I've been scrutinizing your answers. I really do appreciate your help. So, these questions?

How much current does the motor use with no load?

How much current does the motor use when fully loaded?

What's the power supply voltage?
Like I said, the motor was salvaged from a toy car.. So I don't know. Is there some way to find out.. I'm sorry... I'm fairly new to circuits, but I'm learning a heck of a lot just from this one thread.

In fact, if it makes it easier, can I just purchase a new motor?
One of these? http://www.radioshack.com/search/index.jsp?kwCatId=&kw=motor&origkw=motor&sr=1





Also... If it's not too much to ask. Is this correct. According to #3?
http://img9.imageshack.us/img9/8755/dsc00815tjc.jpg
http://img18.imageshack.us/img18/5968/dsc00816h.jpg

Don't laugh...
 
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