The schematic shows R2 as variable though, so it too can be zero Ω. The way it's drawn doesn't make it stick out as a pot right away. It should be a small fixed value, not variable.Actually Bill, if R2 is 1/10 the value of R1, it will limit the PWM range to what the O.P. requested.
I suggest that R2 is not bogus; it limits the peak charge/discharge current through the diodes to/from C if R1 is at one end or the other.
by Duane Benson
by Jake Hertz
by Jake Hertz