I hate to be that guy who revives an old thread, but I had some questions about how these things work, specifically for AC current.
Everywhere I've read says you can't PWM an AC circuit, let alone an inductive heating element. Obviously that isn't true as you're all doing it - so I want to understand exactly how they work, before settling on a wasteful Solid State Voltage Regulator (SSVR) to 'trim' the voltage.
So from my understanding, once the board is modified to PWM at 1Hz (swapping the capacitor), the potentiometer is used to adjust the Hz of the signal which can be calculated by using that formula shared by
interplexr.
—————————————————————————
Charging Time - T1 = 0.693(Ra+Rb1)C
Discharge Time - T2 = 0.693(Rb2)C
Frequency - f = 1/(T1+T2)
If you're using the circuit above, Ra and Rb are both 1k ohm resistors.
Rb1 = 1k + one side of 1M pot depending on where it's position is
Rb2 = 1k + other side of 1M pot depending on where it's position is
Pot all the way to one side: Rb1 - 1k ohm and Rb2 - 1M ohm
Pot in the middle: Rb1 - 501k ohm and Rb2 - 501k ohm
—————————————————————————
A few questions then:
How does the above equation work?
- The pot when turned fully 'on' should give no resistance so it's just the 'Ra' 1k resistor + 'Rb1' 1k resistor that creates the Hz output.
Charging time: T1 = 0.693(1000+1000)C = 1386C. What is C?
- Does this mean that the PWM cannot ever be 'fully on', and that you will always be pulsing the SSR/heating element?
- How does the 'Pot in the middle' bit calculate?
Rb1 - 500k would = 1000 - 500,000 which is negative resistance?
How does switching the AC work?
- Is the key to this working for AC, in using a zero-crossing Solid State Relay so that it doesn't really matter how the PWM Hz doesn't match/align to the input voltage Hz? E.g. if you end up with a .31Hz PWM signal, that's a duty cycle every 3.2258 seconds. in NZ our AC is 50Hz so does that mean the AC will alternate [50 * 3.2258 = 161.29] 162 times as the zero-crossing will allow the signal to both start at 0 and finish at 0?
Can someone tell me the functional switching potential from the device? E.g. the fastest and slowest switching available?
Thanks for any and all help.
