how to get 0 rpm out of a dc motor controller

Hello again,

I have built PWM circuit out of two 555's and a third that generates a 10KHz tone which is modulated by the PWM. THis is going to be transmitted to an RF receiver at 27MHz.

On the receiver there is a tone detector which extracts the PWM to be applied to an electronic speed controller for a DC motor.

I will share some drawings.

When I hook it all up. after days of deliberation, I manages to get some control of the speed of the motor But when the pwm is adjusted all way back the motor still turns.

I don't have the RF portion worked out yet, so the tone generator is connected to the tone decoder, just to tset out those two parts of the system.

How can I get it to go to 0 rpm?

View attachment img-Y06064200.pdfView attachment img-Y06064015.pdf

Most of your attachments do not work now but at first they worked (maybe on another website?).

A single 555 makes PWM as shown in its datasheet but you can use two opamps to make a square wave and a triangle wave then compare them if you want.
Why do you have two tones? For two channels of control?

Your "radio receiver" is still not a radio receiver.

Why do you have an H-bridge driving the motor? Its upper Mosfet will never turn on because it is a follower and needs its gate voltage to be 10V higher than your supply voltage.

Zero RPM is when the "on" pulses of the PWM are absent.

Audioguru said:

Most of your attachments do not work now but at first they worked (maybe on another website?).

A single 555 makes PWM as shown in its datasheet but you can use two opamps to make a square wave and a triangle wave then compare them if you want.
Why do you have two tones? For two channels of control?

Your "radio receiver" is still not a radio receiver.

Why do you have an H-bridge driving the motor? Its upper Mosfet will never turn on because it is a follower and needs its gate voltage to be 10V higher than your supply voltage.

Zero RPM is when the "on" pulses of the PWM are absent.

Click to expand...

Thanks for responding so quickly audio guru. The drawings I put up at first so I removed some. I'm not using those.I haven't implemented an H bridge yet I am still working on forward. It seems that the on pulse never goes away completely, because whether I try to control it by varying the resister or appling a variable voltage to pin 5, it does the same thing. I even put a short in place of R and brought the voltage from 0V to+Vcc. Same thing. I don't get it!

George

Shorting the winding will give you zero at zero applied torque
but you will still have a load-line. An active, continuous-reversing
drive would be needed to hold true zero against a varying load.
H-bridge centered about 50% duty, would do that. A 555 has
a very limited duty cycle range; difficult to approach 50%, even.
There are better PWM options out there.

His original schematic that he deleted showed a square wave from one opamp oscillator, a triangle wave from a second opamp integrator and a comparator. It should be able to produce "PWM with no output" for zero RPM.

dick_freebird said:

Shorting the winding will give you zero at zero applied torque
but you will still have a load-line. An active, continuous-reversing
drive would be needed to hold true zero against a varying load.
H-bridge centered about 50% duty, would do that. A 555 has
a very limited duty cycle range; difficult to approach 50%, even.
There are better PWM options out there.

Click to expand...

Would something like the attached circuit work in order to get 0 RPM?

View attachment PWM CONTROLLER.pdf

obrien136 said:

Would something like the attached circuit work in order to get 0 RPM?

Click to expand...

You show a strange H-bridge circuit. An H-bridge should have a low voltage drop but since your output transistors are emitter-followers then they have have a voltage drop.
your zener diode also has a huge voltage drop.

An H-bridge is able to reverse the rotating direction of a DSC motor. Is that what you want?

Here is an h-bridge that has the output transistors as common-emitter switches with a low voltage drop. The motor gets almost the entire supply voltage when the PWM pulses are wide and gets nothing for zero RPM when the PWM pulses are extremely narrow.

For 0 rpm I would use a simple ON/OFF switch in motor circuit. This stops the motor.

Audioguru said:

You show a strange H-bridge circuit. An H-bridge should have a low voltage drop but since your output transistors are emitter-followers then they have have a voltage drop.
your zener diode also has a huge voltage drop.

An H-bridge is able to reverse the rotating direction of a DSC motor. Is that what you want?

Here is an h-bridge that has the output transistors as common-emitter switches with a low voltage drop. The motor gets almost the entire supply voltage when the PWM pulses are wide and gets nothing for zero RPM when the PWM pulses are extremely narrow.

Click to expand...

Is there an attachment?

obrien136 said:

Is there an attachment?

Click to expand...

My attachment is an ordinary PNG file type. The thumbnail small image is there and if you click on it then it will be full size.

What is your motor Rs winding resistance and supply voltage? Amp rating or stall current?

This will determine the start/stop surge current which your switches must control and is usually 5~8x average rated current. Then duration depends on load or acceleration/brake time you need. Keep in mind it will coast to a stop with with an open switch and shunt motor current and stop faster with low Rce or RdsOn switches.

ALso keep in mind gate or base current has to be almost 10% of output current peak so as many cascaded stages are required depending on rate of change of current, which translates to force.

For example Audioguru's bridge is a 3A switch with 30mA drive or a current gain of 100 with two stages of buffering or 10x each or in other words the drive is 10% for each stage. Now MOSFETS can be much more efficient until you start to see high commutation current from PWM and for optimum control but lower efficiency the current gain ends up being the same as bipolar devices but better RdsOn per dollar.

Your drivers should be lower resistance when ON and low Vsat or RdsOn MOSFETs.