I was not thinking of them as linear but as equal. What I realise now is that although the charge/discharge time constants are equal, the charge/discharge voltage are not equal. In the charge case it is the positive power supply. In the discharge it is the present voltage across the capacitor, which is clearly different.
My next step was to model with spice and this I will do.
Ian
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At last, the light finaly dawns on me. Of course the capacitor does not discharge fully because the voltage on it is very small. So every cycle the cap will chage up a bit until equilibrium is reached when the small voltage across R is just enough to restore the charge loss during the off period.
Many thanks for your patience. I find it is always of benefit to revisit fundamentals and renew your understanding.
Ian
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