pwm to control a dc motor

[mariomoski]

hello,i am trying to control a dc motor (1.5-3) with a transistor 2n2222a, at the 555output i have (supply voltage-1)=7-1=6V and the circuit is attached

my transistor has gain=40 and the maximum load of y motor will be 0.3A,so i need at my base a current at least I=0.3/40=7.5mA to get saturation, and the resistir at the base must be 6/7.5=800 maximum to get this, so i use a resistor=270ohm, all this is correct?

but the problem is that when i make the circuit, the transistor get so hot, why am i having this problem? how could i solve this problem?
pd: the diode on parallel with my motor is in correct way!

could somebody help me with this, i am really loss alot time with this but i don´t get a solution

thanks

View attachment 41668

 

[Hero999]

The transistor is getting hot because you're not driving it into saturation. The data sheet specifies a gain of 40 when VCE is 10V. Try increasing the base current to 30mA.

 

[mariomoski]

i tried it too, with that current but the same problem and when i try to see if the transistor is on saturation with the oscilloscope i see the next signal:

http://imageshack.us/photo/my-images/821/imageniqb.jpg/

 

[mariomoski]

and one question more

when the transistor is on cut-off which voltage is watching the motor,7V ? because the voltage maxium for my motor is 3V.
or it won´t affect because the motor doesn´t recieve energy?

 

[Hero999]

The motor acts as a dynamo when the power is disconnected from it while the shaft is spinning so it will generate a voltage.

 

[mariomoski]

MY MOTOR no load has 0.3A
and max efficient has1.05A
motor works between 1.5V and 3V max

in the datasheet of transistor 2n2222A:

0.8A max colector ,but we never have this value because we don´t have load(no gearbox) on the motor,so the max current will be 0.3A,YES?so it won´t be a problem?

and i saw too that total dissipation for 2n2222a is 0.5W at Tamb<25ºC and 1.8W at Tcase<25ºC,
so when we have the worst case P=V*I=3*0.3=0.9W, are we in the range,yes?

 

[Hero999]

MY MOTOR no load has 0.3A
and max efficient has1.05A
motor works between 1.5V and 3V max

in the datasheet of transistor 2n2222A:

0.8A max colector ,but we never have this value because we don´t have load(no gearbox) on the motor,so the max current will be 0.3A,YES?so it won´t be a problem?

Yes, you do get more than 0.3A, when the motor starts it has to overcome the inertia to get the shaft spinning so the load will be much higher, 5A or more wouldn't be surprising.

and i saw too that total dissipation for 2n2222a is 0.5W at Tamb<25ºC and 1.8W at Tcase<25ºC,
so when we have the worst case P=V*I=3*0.3=0.9W, are we in the range,yes?

If you're running the motor at 7V when it's only rated to 3V, the current will be even higher.

Have you actually measured the motor current?

You've grossly underrated the components so why are you surprised they're overheating?

You should replace the motor with one rated to at least 6V and a suitably rated transistor such as a small MOSFET.

 

[mariomoski]

the motor must be which i said you, i am gonna change the transistor to a BD135,which can have 1.5A at its colector
should it works?
and one question more:
when i check the voltages with my multimeter,i see the effective voltage ?
so to calculate the peak voltage the equation is :
Veff=Vpeak*duty cycle    or Veff=Vpeak*sqrt(duty cycle)

 

[Hero999]

Do you know how your meter measures voltages with an AC component on the DC range?

Does it take the average value, RMS or does it behave unpredictably?

You need to look at the meter's manual or ask the manufacturer.

Failing that, the only way to accurately measure the voltage of such a signal is with an oscilloscope.

 

[mariomoski]

dc range

i don´t know if it takes the average or rms,which is the different?
the formula V=Vpeak*sqrt(duty) is with rms or average?

and i tried with a transistor BD135 but when the supply voltage for the circuit is higher than 3.5V the motor consumes its maximum current o.3V,why?

 

[Hero999]

dc range

i don´t know if it takes the average or rms,which is the different?
the formula V=Vpeak*sqrt(duty) is with rms or average?

Thats RMS, the average is simply the duty multiplied by the voltage.

and i tried with a transistor BD135 but when the supply voltage for the circuit is higher than 3.5V the motor consumes its maximum current o.3V,why?

Sorry, I don't understand what you're saying.

 

[mariomoski]

http://imageshack.us/photo/my-images/705/sinttulo2mn.jpg/

i tried this circuit but with a transistor BD135 not a 2n2222A
and when i tried to supply the 555 with more than 3.5V,the motor consumes more than its maximum current allowed 0.3A
why is this happenin ?because my duty cycle is 60%(to get 3V across the motor)not 5V as i had with 100%duty cycle

 

[Hero999]

So what you're saying is when the motor's supply voltage is increased beyond its maximum rating, the current also exceeds its maximum rating?

Why are you surprised by this? It's what you should expect to happen. Read up on Ohm's law.

 

[mariomoski]

if suply voltage is 5V and my duty cycle is 60%,the average voltage across my motor is 5*0.6=3V(max allowed)

so if i have the duty cyle 60% as i said ,why i can´t give a supply voltage higher than 3.5V,i want until 5V,
my motor won´t read more than 3V with supply=5 because my duty cycle is 60%