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[QUOTE=erick;583319If the pot at minimum gives me about 40% on-time, then reducing R2 value should reduce that below 40%. OK, I can play with that easily and check the results on the scope.[/QUOTE]
Eric I have played extensively with 555s PWM. The mark space ratio (range of control) is set-up by the Pot and (capacitor) Cx value. F = 1.44/(Rx * Cx).
I found if you have these wrong the PWM starts at around a duty cycle (mark space ratio) of around 40%...this is pretty much what you are seeing. Thus you have less range of control.
By changing the Rx & Cx (Pot & Capacitor) you can get this down but you also effect the PWM freq.
Since your Pot is a switch/pot you may have troubles finding a higher value pot that suits. In any case I found a pot of around 500K (& up) and a Cx value of around 1 to .3 microfarads gives a freq of oscillation of 2 to 7hz and the mark space ratio is 4.25% to 99.9% (range of control).
I can send you a spreadsheet I did on pot/Cx PWM freq & mark space ratios if you want.
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