capacitors for disengage a power source

[mariomoskis]

hello,i am new in this forum
i am working about control of dc motor with a pwm signal
i have the next circuit to do it,but i would like to disengage the power source with two capacitors,but i don´t know how much value they must have

http://imageshack.us/photo/my-images/405/desacople.jpg/

could somebody help me ?
thanks

 

[(*steve*)]

Do you mean "decouple"?

You may need to decouple the 555's power from the motor. To do this, place a (say 1000uF 10V capacitor across the power supply to the 555 and add a small resistor in series with the power supply to the 555 (but before the capacitor). Something like 47 ohms may be sufficient.

Your diagram also doesn't show a power connection to the motor -- is it running from the same 6V source (if not then decoupling is almost certainly not required)

 

[mariomoskis]

i forgot to draw the supply power for the operational amplifier, and it is 6V too

is it that you refered?

 

[(*steve*)]

Yes and no.

I was actually referring to the power supply for the motor.

Decoupling (if that's what you want) is normally done if you have electronics that need a clean supply rail sharing a common power source with one that generates electrical noise.

The idea is to stop the electrical noise from getting into the more sensitive electronics via the power supply by making them less tightly coupled, i.e. by decoupling them.

This can be done by creating a totally separate power supply, or by employing a (normally low pass) filter or additional regulation to the power supply requiring lower noise.

In your case, the motor (which is also not shown connected to a power supply) will probably generate noise that you don't want affecting the 555 or the op-amp.

 

[mariomoskis]

this is the new image, so the motor has its supply power of the AO,
because the pwm signal has amplitude of 5V at the output of 555,but i do there a voltage divider and i get 3V at the input and output of the amplifier because it is a voltage follower, i need 3V because my motor must work from 1.5 to 3V,then for example i chose 3V

http://imageshack.us/photo/my-images/853/desacople.jpg/

is it correct?

 

[BobK]

Using an opamp to run a motor seems like overkill to me. What you really need is a 3V supply and a transistor to switch the motor current on and off. They way you have it, the power opamp is going to dissapate as much power as the motor since it is reducing 6V to 3V.

Bob

 

[mariomoskis]

so maybe it is the problem that i have

because when i try to supply the amplifier with more than 4V,my motor consumes the maximum current that it can 0.3A,so is it because this voltage which i am supplying the amplifier is the same for the motor???

but the amplifier is a power operational amplifer with output 1A!! i think that i should work