What do the two diodes do ?
Do you understand the circuit, because I'm not sure I want to spend the time ?



Mod edit:
Please don't hijack other member's post, now you have your own.

This thread was split from :
Help me improve this PWM controller
http://forum.allaboutcircuits.com/threads/help-me-improve-this-pwm-controller.116850/

 

Do you understand the circuit, because I'm not sure I want to spend the time ?

Then don't post!

 

!OK!

 

MikeML
Perhaps u misunderstood me.
I am saying that I don't understand what role those two diodes play ?
And therefore, I can't give a constructive answer.
In other words, I won't give an answer that is useless.
"Ditch the Darlington. Replace it with a modern NFET that has a Vth ~ 4V, Vds >50V, Id > 20A, Rds < 30mΩ, and mount the NFET on a suitable heatsink..."
Do u no wot the diodes do, or do u not ?

 

NO ?

 

If you understand the operation of the 555 (charge and discharge of the .1Uf tied to Pins 2 and 6 and wiper of the pot) the diodes make perfectly good sense to give wide PWM control.

 

The 555 uses two comparitors, one that detects a voltage across the capacitor that is one third of the supply voltage, and one that detects a voltage across the capacitor that is two thirds of the supply voltage.
I have shown the CCT diagram of the timer so that we can see how those two diodes affect the rise and fall profile of the output signal, which ultimately effects the inductive kickback of the motor.

 

I'll toll along with you, JamesBond0! Explain how the two diodes effect the rise and fall of the output signal.

 

u should never answer a question with a question, but here goes. Back in II.

 

I wonder what the reason was for marking the + sign on the diodes, it would not matter in this case as long as they were each connected back to back.??
Max.

 

I think MaxHeadRoom is on the right track. Those diodes give a 0.6 V reference to the threshold pin 6 of the 555. Because the threshold goes below zero volts (capacitors do this) both diodes offer a stable reference during both half cycles. I need to build this CCt and CRO it, but I wont.

 

Ok, enough is enough. The diodes provide two different paths for the charge and discharge current to the capacitor. By doing so, the frequency stays fairly constant and the duty cycle varies from near zero to near 100%, just as MikeML said.
This link has a good write up explaining the circuit.
http://pcbheaven.com/circuitpages/PWM_Fan_controller_using_a_555/

 

BillB3857 already given the answer for two diodes.
MikeML is a good member here and he help a lots of members to solve their problem, so be nice to him, he could give you the knowledge you didn't know, if you want to ask for help here then you should be polite to other members who trying to help you and be careful about your attitude, thanks.

 

...
I am saying that I don't understand what role those two diodes play ?
...
Do u no wot the diodes do, or do u not ?

Yes, I think I know. And you will too, if you can follow this: