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Hi to all.
I have a question about how to change my hardware.
I want to be able to change the speed of a 12Volt brush less motor.
This is normally done by changing a programming resistor connected to the motor.
No resister gives 2000 RPM
270 Ohm = 2500 RPM etc
At the moment I simply use a relay that bypasses the resister on the motor and in this way I have two speeds.
Now I would like to change it mo make it work with a solid state method maybe even having three or four different speeds. I want it to be isolated just as it is with a relay.
Is there some way I can do this?
Best Regards:
Uli

hi
have a look at the microchip website as they do a range of chips that use pwm to control speed (very cheap) hth

Hi Uli,
What I did was to use a MOSFET to fully control a 12 motor.
The idea is to use a PWM (Pulse Width Modulation) which was fixed at a frequency of 10KHz.
Take a look here:
http://www.afrotechmods.com/groovy/PWM_ ... torial.htm
& here for meaning of Duty.

The microcontroller just controlled the PWM duty fed in to FET which depended on ADC input that is connected to wiper of RV1.
So as Voltage at wiper of RV1 was increased, the motor speed increased.

As far as the motor was concerned the 10KHz was as good as DC. The 0.1R resistor was use as over current sense. When over current was sensed moto ws stopped and LED Flashed Rapidly.

R3 & R4 formed a potential divider which was use for monitoring battery voltage. When 12V battery fell to 10V then motor was stopped and LED flashed slowly.

If you would like to control a motor using this method, I can sort out a Flowchart for you.

Martin

Thanks this looks like a cool way of Controlling a DC motor.
However the motor I want to control has its own control box that has the option for setting the speed by an external resistor.
If it gets less than 10.5 volts it will think the battery is empty and cut out.
Also it regulates the speed regardless of the Input voltage.
Best Regards:
Uli

Creative25 wrote: However the motor I want to control has its own control box that has the option for setting the speed by an external resistor.

Ah I see.
In that case how about using 2 Fets, Bipolar transistors or optocouplers with a different value resistor in series.

Suppose you normally connect a resistor between A & B to fix the speed.
Have 1 fixed resistor in palce between A and B (Highest value of the range)
Then connect one end of a different resistor to A as normal. connect the collector of transisor (or optocoupler) or Drain of a P type MOSFET to the other end of resistor, and finally the Emitter of transistor (or optocoupler) or Source of a P type MOSFET to B

When base or Anode (With Cathode connected to 0V in the case of optocoupler) or gate is positive then the transistor conducts so the new effective resistance is placed between A and B.
So there will be two resistors in parallel, hence two different speeds.

With the 2nd transistor or optocoupler in place, that will be another speed.

Hi Martin.
I actually was thinking of a mosfet but am not sure if it will work.
For me it is important that I do not introduce extra current or voltage. Does the mosfet leak power from gate to source or from gate to drain?
also if I use an optocoupler it should not draw any current from the driver of the motor.
I have no Ideas which if an optocoupler or a mosfet meets these demands. Since I am new in electronics.

The data sheet says the Following:

Contr.circ. current
2 mA =3500 rpm
3 mA =3500 rpm
4 mA =3500 rpm
5 mA =3500 rpm

R1)
1523 ohm = 3500 rpm
692 ohm = 3000 rpm
277 ohm = 2500 rpm
0 ohm = 2000 rpm

So it means you set the Speed with a control current that is set with a resister.

I guess the speed set resistor must form a voltage divider with an internal resister.

Best Regards:
Uli

Hi I did some research
So I am thinking of two options.
The first option would be a digital potentiometer.
But the ones I found can only handle up to 1 mA
Are there some that can handle 10mA?
The second option would be using Mosfets and resisters.
I read that the gate of the Mosfets are Insulated and virtually no current flows from Gate to source.
Is that true? Are all Fet's like that?
Best Regards:
Ueli