Can anyone help with these parallel diodes in 555 circuit?

[seb1982]

Hi all!

I was looking at this simple 555 adjustable PWM circuit below today.  I understand the reason for placing a diode between pins 6 and 7 when you want a duty cycle of <50%, but I can't get my head around why you'd have two diodes with opposite biasing like in the diagram below.  No matter how hard I think about it, I keep coming back to thinking that they'll just cancel each other out, as it were.

If any of you guys can shed some light on it for me, I'd be very grateful - I've been pondering it for hours now today!

Thanks again.

[c4757p]

They're not in parallel! The charge current will go down one side of the pot and the discharge current will come back up the other.

[digsys]

If you Redraw it like this, it will make a lot more sense. R1+R2 make up the pot at one extreme position.
C1 charges 100x faster than it discharges.

[seb1982]

Ah, of course! 

God, I feel mighty stupid right now!  What an absolute brainfart! 

Thanks ever so much for the replies poking my brain in the right direction!

[SeanB]

The quick and dirty way to change duty cycle while keeping something closish to a constant frequency at the output.

[dentaku]

You might also be interested in seeing Dave build this circuit in one of his videos.
http://www.eevblog.com/2012/11/29/eevblog-392-555-led-pwm-hack/

[vlf3]

A quick and dirty get you going... remove pin 7 discharge from the diodes with no connection, move R2 from +12 supply end, re-connect to pin 3 output; the duty to frequency timing cap might require adjustment ?... otherwise one can go that bit further, and remove the diodes altogether and the pre-set/pot too, just take R2 from pins 2 and 6 to pin 3, alter R2 value for required period, duty-cycle is near 50/50.