Hello all, I need to create a circuit that uses PWM.
I am going to use a 555 timer to create the duty cycle which i would like to range from something like Time high 5% to 95%, so to make an LED go from very dim to nearly full.
However i notice in the equations for time high and time low that you cannot get time high to be less than 50%
Time high = 0.7 (Ra + Rb) C
Time low = 0.7 Rb C
So, time high will always be greater than (or roughly equal to) time low, because with the time high equation you are always adding Ra AND Rb, but time low is just Rb on its own, and since you can't have a negative resistance for Ra, i don't see how it could be lower.
In the diagram on this website
LED Dimmer Circuit - Lighting
I am guessing the way that their duty cycle of 5 - 95% is achieved is something to do with those two parallel diodes. If someone could explain to me how this does this, i would be vastly grateful. Thanks =)
I am going to use a 555 timer to create the duty cycle which i would like to range from something like Time high 5% to 95%, so to make an LED go from very dim to nearly full.
However i notice in the equations for time high and time low that you cannot get time high to be less than 50%
Time high = 0.7 (Ra + Rb) C
Time low = 0.7 Rb C
So, time high will always be greater than (or roughly equal to) time low, because with the time high equation you are always adding Ra AND Rb, but time low is just Rb on its own, and since you can't have a negative resistance for Ra, i don't see how it could be lower.
In the diagram on this website
LED Dimmer Circuit - Lighting
I am guessing the way that their duty cycle of 5 - 95% is achieved is something to do with those two parallel diodes. If someone could explain to me how this does this, i would be vastly grateful. Thanks =)