What do the two diodes do of the ne555?

Thread Starter

JamesBond007

Joined Oct 25, 2015
24
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 ?
:cool:
 

BillB3857

Joined Feb 28, 2009
2,497
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.
 

Thread Starter

JamesBond007

Joined Oct 25, 2015
24
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.
 

BillB3857

Joined Feb 28, 2009
2,497
I'll toll along with you, JamesBond0! Explain how the two diodes effect the rise and fall of the output signal.
 

MaxHeadRoom

Joined Jul 18, 2013
19,033
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.
 

Thread Starter

JamesBond007

Joined Oct 25, 2015
24
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.
 

ScottWang

Joined Aug 23, 2012
6,826
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.
 
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