problems to control a dc motor with a pwm signal

[colin55]

Use a Darlington so it turns off.

 

[mariomoskis]

ok,i am gonna try all that you said and i will say you tyhe results if it works or not
i have a question:

when i try to check voltages with a multimeter, as i have a pulse signal, all voltages which i check in each part of the circuit must be calculate like this:

V(multimter)=V(that i should see with a oscilloscope)* duty cycle?
or V(multimter)=V(that i should see with a oscilloscope)* sqrt(duty cycle)?

and this is in each part of the circuit where i have a pulse signal or in all circuit?

 

[mariomoskis]

before try with another transistor i saw that maybe i can do it with a power opeartional amplifier like follow voltage(3V) with 1A output,like this:
View attachment 62416

but i have this question:
if the supply voltage for the amplifier must be 5V,
why when i have supply voltage=6V with duty cycle 50%,the motor is already consumes the 0.3A maximum that it can? and i see too that the voltage at the output of the amplifier is higher than at the input,but it shouldn´t happen?

and i see too that if i increase the supply voltage for the amplifer to more than 6V,the motor is gonna comsume more current,why is this happen?

 

[alec_t]

when i try to check voltages with a multimeter, as i have a pulse signal, all voltages which i check in each part of the circuit must be calculate like this

Unless you have an expensive digital multimeter it will probably give wildly inaccurate measurements of pulse signals. An analogue meter, on the othe hand, will tend to 'average' any pulse voltage applied.

i see too that the voltage at the output of the amplifier is higher than at the input,but it shouldn´t happen?

It can and does happen, because the motor is not a simple resistive load. Its winding has inductance (read up about that if you're unfamiliar with the term).

when i have supply voltage=6V with duty cycle 50%,the motor is already consumes the 0.3A maximum that it can?

Whatever the duty cycle is the motor maximum current can be > 1A, even with a 3V supply. Read the motor spec again.

if i increase the supply voltage for the amplifer to more than 6V,the motor is gonna comsume more current,why is this happen?

Ohm's Law.

 

[mariomoskis]

Whatever the duty cycle is the motor maximum current can be > 1A, even with a 3V supply. Read the motor spec again.
Ohm's Law.

but if i am fixing the output of the AO to 3V because the input is 3V too, and it is working as a follow voltage,
if i increase the supply voltage, the voltage at the output should be fixing to 3V ,not?

and one think taht i forgot to say before:
this problem is when i have 100hz or 1Kz PWM,
but when the frequency is 10kHz the voltage at the input and output is exactly the same maybe because it isn´t working well at this frequency?

i tried to fix the voltages at the input and output to 2V instead of 3V and it looked better about the motor current consumed

do you know some solution to solve this problem about the current consumed by the motor?

 

[alec_t]

but if i am fixing the output of the AO to 3V because the input is 3V too, and it is working as a follow voltage,
if i increase the supply voltage, the voltage at the output should be fixing to 3V ,not?

Yes. But A0 is not a rail-to-rail output type of opamp. At 1A current the datasheet for A0 says the output voltage is typically never below 1.3V (when the opamp supply is 24V....its not stated what it would be if the supply was only 5V).

but when the frequency is 10kHz the voltage at the input and output is exactly the same maybe because it isn´t working well at this frequency?

The opamp bandwidth is specified as 1MHz, so the problem may lie in stray external (to the IC) circuit capacitance reducing the A0 gain at high frequencies.

i tried to fix the voltages at the input and output to 2V instead of 3V and it looked better about the motor current consumed. Do you know some solution to solve this problem about the current consumed by the motor?

That sounds as though you've found your own solution

 

[mariomoskis]

but if i fixed to 2V and the motor works between 1.5 and 3V ,i am reducing the speed range of my motor so much ,yes?maybe i will try with 2.5V too

is there a way to reduce the current which the motor consumes or i will be always limited to do it with less voltage at the ouput and input?

and about this kind of circuit, why this works for control pwm motors too ? which is the different bewteen this and when iused the transistor?

 

[alec_t]

is there a way to reduce the current which the motor consumes or i will be always limited to do it with less voltage at the ouput and input?

You can put a resistor in series with the motor, but I wouldn't recommend it because the motor needs a high current to kick-start it.

and about this kind of circuit, why this works for control pwm motors too ? which is the different bewteen this and when iused the transistor?

Because this IC has been designed for driving motors and can handle more current than the transistor you were using.

 

[mariomoskis]

ok,i tried with 2.6V output and the motor doesn´t comsume more than 0.3A maybe sometimes 0.31A,and the voltages input and output are almost similar. i have used a supply voltage 5V for the AO(i think that is the min limit)

and i have a question: should the motor consumes almost the same current when i change the duty cycle from 80% to 50%, because i have seen that there is variation of the speed but not so much when i change the duty cycle, do you think that it is working well?

how could i know the speed of the motor in each moment to know if this is working well?

 

[alec_t]

should the motor consumes almost the same current when i change the duty cycle from 80% to 50%

The average motor current should increase as you increase the duty cycle, more or less proportionally provided the motor winding doesn't saturate (at which point the current is limited more by the motor resistance than its inductance.

how could i know the speed of the motor in each moment to know if this is working well?

You would need a tachometer.

 

[mariomoskis]

i don´t have a tachometer
the average motor current is which i can see in the supply power ,yes? this current sometimes change a little for a same duty cycle,but later it becomes the same value

so it is difficult to know if all is working as it should

 

[colin55]

If you haven't changed the circuit, I told you it would not work.

 

[mariomoskis]

yes,i know
but i am trying with a new transistor BD135 with more current at its colector 1.5A,with this circuit:
View attachment 62466
but i have the problem that when i try to supply the circuit with more than 3.5V,the motor consumes more than 0.3A(max) for a duty cycle 50%,i don´t know why?
should it work now?

i have this too with a power AO:
**broken link removed**
but i still must check some results

 

[Reloadron]

The specifications for your motor clearly spell out with a 3.0 volt constant the motor speed will be about 14,800 RPM and the motor will draw about .300 amp. That is for the motor free running and doing no work.

Let's forget the motor for a moment. Draw a series circuit with a battery and resistor. The battery voltage is 3.0 volts and the resistor is 10 ohms. Therefore the circuit current is .300 amp. Now what happens if we increase the battery voltage to 5 volts? What happens to the circuit current?

Why do you insist on trying to run a 3 volt motor at voltages above 3 volts?

Ron

 

[colin55]

Take the line going to the capacitor LOW Then take it HIGH. Use a jumper lead. Does the motor completely stop for one of the readings?

 

[audioguru]

I think the 1N4001 rectifier diode is too slow for PWM. It should be a much faster diode like maybe a 1N5817 Schottky 1A diode.
I get them from cheap old solar garden lights.

 

[mariomoskis]

I think the 1N4001 rectifier diode is too slow for PWM. It should be a much faster diode like maybe a 1N5817 Schottky 1A diode.
I get them from cheap old solar garden lights.

my frequency is 100Hz or 1Khz,so it won´t be a problem,the capacitor isn´t the same that in the picture

 

[mariomoskis]

Take the line going to the capacitor LOW Then take it HIGH. Use a jumper lead. Does the motor completely stop for one of the readings?

sorry,i don´t understand what do you mean with this

 

[mariomoskis]

The specifications for your motor clearly spell out with a 3.0 volt constant the motor speed will be about 14,800 RPM and the motor will draw about .300 amp. That is for the motor free running and doing no work.

Let's forget the motor for a moment. Draw a series circuit with a battery and resistor. The battery voltage is 3.0 volts and the resistor is 10 ohms. Therefore the circuit current is .300 amp. Now what happens if we increase the battery voltage to 5 volts? What happens to the circuit current?

Why do you insist on trying to run a 3 volt motor at voltages above 3 volts?

Ron

but the duty cycle which i am using is 60%,it will give me the 3V no more
for example if i try a diode zener o 3 or 4 normal diodes at the emittor i can 100%

about the other you said me ,i will try too and i will say you the results

 

[Reloadron]

but the duty cycle which i am using is 60%,it will give me the 3V no more
for example if i try a diode zener o 3 or 4 normal diodes at the emittor i can 100%

No it won't. If the voltage is 5 volts when the transistor (or any switch) is on the applied voltage to the motor is about 5 volts regardless of duty cycle. What you seem to be doing is wanting to say if the voltage is 5 volts and the duty cycle is 50% that the voltage becomes 2.5 volts and it doesn't. You are getting at average current I believe?

Ron