pwm transistor

[mariomoskis]

Hello,i am working control a dc motor(from 3 to 1.5V, Imax=0.3A) with a 555 and a transitor 2n2222 and i have the next questions:

my supply voltage is 8V and at the output of 555 i have 7V,so to have a voltage range which neeed my motor i need duty cycle 3/7=42% to 1.5/7=21%

are there a way to get this:

100%duty cycle 3V

0% duty cycle 1.5V

for example if i would put a diode at the emisor of the transistor or something like that??

and the second question is: later i will use crio ni9401 instead of 555 to generate de pwm signal, i thought about this:

but ni9401 need current=[100microA,2mA], so 2n2222 have gain 40 and the motor load 0.3A,so i will need at least 0.3/40=7.5mA(so i will be out of the range), won´t I?? how could i solve this problem?

thanks

[johnsold]

1. The voltage at the output of the 555 is not relevant.  If it is high enough to turn on the transistor fully, the actual voltage does not mean anything.

2. What you want to calculate is the average voltage across the motor.  The peak voltage is probably a little closer to 8 V than 7.  However your duty cycle calculation is close: For average motor voltage of 3 V the duty cycle will be about 40% and about half that at 1.5 V.

3. Your question about 100% duty cycle 3 V is meaningless.  I think what you want to say is Maximum speed setting produces 3 V average and Minimum speed setting produces 1.5 V. Those correspond to duty cycles of about 40% and 20%.

4. You do not need any changes in the output circuit.  If the values in the 555 timing circuit will produce the duty cycle range you need it should be OK.

5. For the cRIO circuit use a Darlington transistor connection.  For this project two 2N2222s should be fine.  Combined current gains should be on the order of 40^2 = 1600.  The base current will be less than 1 mA.

Lynn

[mariomoskis]

about darling i already resolved it

and about the 555,i change the sypply to 5V,and i put 3diodes 1n4004 at the colector between the motor//diode and the transistor,so with this i will bet the 3V average at 100%duty cycle,is it correct?

but when i check the average voltage across the motor i see 1.8V for duty cycle 50%,but i should see 1.5V

why is this happening?maybe the voltage acroos the 3 diodes isn´t 2V as i want

[johnsold]

You do not need any diodes (except the reverse polarity diode parallel to the motor).  The idea of PWM is that you can use a higher voltage source. By switching much faster than the load can respond, the load only "sees" the average voltage as set by the supply voltage multiplied by the duty cycle.

With a 50% duty cycle and an 8 volt power supply oyu should have about 4 V across the motor. For 1.5 V the duty cycle would be about 18-19%

Lynn

[mariomoskis]

my teahcer said me that the motor reads averages voltages from 3 to 1.5 V

so i think that with this diodes i can get,not?

[mariomoskis]

and when i see the voltage across the motor whith a multimeter,i see 2.7V with duty cycle 100% and 1.7V with duty cycle 50%,it looks work well not?