Hi Malcolm,
I don't think Denis is missing anything. What you are doing is some form of PWM, probably at a reasonably low frequency. 100% means the output is on all the time 0% means it's off all the time. At 50% it's on half of the time and off for the other half.
The question Denis is asking is how long in terms of milliseconds, seconds, hours or days is one period of this PWM signal.
On slow responding systems or "loads" like your heater for example one period can be long, like several seconds or even longer while on fast responding systems like regulating the current thru a motor or whatever the period needs to be MUCH shorter.
A 50% duty cycle or output means the output is on 50% of the time but in the first case, with the heater, it means it's on for 5 seconds and off for 5 seconds while in the later case with the motor it means it's on 5us and off for 5us. Still 50% in both cases but in the first the period is 10 seconds, in the later the period is 10us.
Really it's all a matter of how much "jitter" you can live with on the resulting response from whatever it is you're driving. There's usually no need to use 20kHz PWM if regulating the temperature in an oven where applying "100% power" for one minute will result in a temperature rise of 0.5° - as an example.
Sorry for the long winded response.
/Henrik.
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