555 based PWM motor controller

[boylesg]

broken link removed
I have seen version of this where the top of the 1K resistor is connected to pin 3 rather than Vcc.

What is the purpose of doing this?

 

[d123]

Hi,

Maybe to provide feedback to control the PWM rate.

I think the "555 transistor tester" operates a similar way - the output provides power/feedback to the timing section.

That motor is the generator to power the 555, or is "reset" an alternate V+ pin? Where does the initial power come from to start that loop up?

I'm interested, does that circuit you've posted the schematic for work, or do you need to spin the motor first?


Now I see, there are no dots to show junctions on that schematic, my mistake.

 

[goldsmith]

Maybe to provide feedback to control the PWM rate.

Hi
Why would anyone take feedback from base of a transistor in such an application ?!!

I have seen version of this where the top of the 1K resistor is connected to pin 3 rather than Vcc.

May you attach the circuit which you say you've seen please ? perhaps then we can help you much better ?


Best Wishes
Goldsmith

 

[Audioguru]

Here are a few ways a 555 can make PWM:

 

[boylesg]

Here are a few ways a 555 can make PWM:

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Wow, the first one is novel looking circuit - haven't seen that one before. Output from pin 7??? Is there any significant advantage over it compared to more conventional astable 555 circuits in your view?

Apart from that I am just curious though how the feedback from pin 3 to what is usually labelled as R1 (normally connected to Vcc) is supposed to work?

 

[Warpspeed]

The output from pin 3 switches high and low as the 555 changes state.
Pin 7 follows pin 3, but it can only pull down to ground, its an open collector output.

There are two ways this circuit can be made work.
As shown above, pin 7 is either pulled down to ground, or pulled high through the 1K resistor.

The other way it can work is to connect where the two diodes join together to pin 3. If you do that, the 1K resistor is not needed and pin 7 remains not connected.

 

[d123]

Why would anyone take feedback from base of a transistor in such an application ?!!

I have seen version of this where the top of the 1K resistor is connected to pin 3 rather than Vcc.

The poster asked about a different circuit, I replied in reference to that, not to the schematic posted in the first post. You even asked to see the circuit in question afterwards, so I don't see the "helping" purpose of your post...

 

[FvM]

Curiously no astable circuit with feedback from output pin is ever shown in 555 datasheet application circuits. But it's used quite often by designers, e.g. to make a minimum part count square wave oscillator. Depending on your design objectives, one or the other circuit may be preferred.

The poster asked about a different circuit, I replied in reference to that, not to the schematic posted in the first post. You even asked to see the circuit in question afterwards, so I don't see the "helping" purpose of your post...

Unfortunately there's no link or attachment in your post.

 

[BradtheRad]

This site is a comprehensive collection of 555 IC information, with numerous schematics.

**broken link removed**

 

[Audioguru]

The output voltage of an ordinary 555 does not go high enough to turn on many Mosfets if the supply voltage is low because the pin 3 output high voltage is typically 1.3V less than the supply voltage when the load current is low. The pin 7 voltage goes as high as the supply voltage when driving a Mosfet.

 

[BiNa2605]

That is used to calculate the duty cycle of the PWM. Let think about when the variable resistor when it comes to zero ohm, you still have the 1K resistor for the duty cycle T = RC.