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PWM using 555 timer

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This is how I got confused in the first place. I want to control the phase angle to the triac so I can controll how much power is delivered to my heater. The first circuit I built uses a heat sensor, a comparator, and a standard light dimmer switch. When my temp sensor is below the setpoint, the inverting comparator turns on a mechanical relay to my dimmer switch. This bang-bang controller has poor characteristics, e.g. overshoot, ocsillations, etc.

I'd like the amount of power to the heater to be proportional to the difference b/n my heat sensor and set point. This could be done by rotating the dimmer switch knob, but obviously I'd like an automated way of doing this. This is where I thought the PWM and optoisolator triac driver came into play. No?

If there is some other way to control the phase angle (other than hand-turning a pot), I'm open to suggestions.



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Just to clarify, I know I'm not controlling the phase angle of the triac. I'm simply turning the triac on and off for short periods of time. More heat needed -- longer "ON" time (and vice versa).

I suppose the low frequency is needed to ensure you get some zero crossing with the AC.

As I recall, the reason PWM was recommended in the first place is that the phase control technique has EMI problems because of large current spikes. Right?

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  • 8 years later...

I found a circuit that is very close to what I am trying to do:

This is from http://www.saburchill.com/tech/electronics/elect039.html


This circuit uses a sound sensor to vary the brightness of a lightbulb.

I also figured out some timing formulas based on threshold voltage (Vth) and triggering voltage (Vtr).  As you know, normally, Vth = 2/3Vcc and Vtr = 1/3 Vcc.  These values result in the "standard" timing formulas.  Applying the control voltage, Vc, to pin 5 directly affects these values.  It is applied directly to the 2/3 point on the voltage divider.  3 equal-valued resistors make up this voltage divider.  The reference voltage for the threshold comparator is b/n the first and second resistor (hence 2/3Vcc) and the ref voltage for the trigger comparater is b/n the 2nd and 3rd resistor (hence 1/3Vcc).  If I can figure out how the control voltage affects these reference voltages, the following formulas should work.

In monostable mode:
(pulse width) T = [ln(Vcc) - ln(Vcc-Vth)] * R * C

In astable mode:
T1 = -ln[1 - (Vth - Vtr)/(Vcc - Vtr)] * (R1 + R2) * C
T2 = [ln(Vth) - ln(Vtr)] * R2 * C
(pulse width) T = T1 + T2
(frequency) f = 1 / (T1 + T2)
(duty cycle) % = T1 / (T1 + T2)


I plugged in values Vth = 2/3Vcc and Vtr = 1/3Vcc and got the same values that would result from using the "standard" formulas (discounting some roundoff errors).

Thanks again.  :)


I am interested in knowing how you derive the above equations.

Can you tell me how?

Besides, how does the control voltage affect these reference voltages (threshold and trigger voltages)
ie. what is the relationship between Vc , Vth and Vtr  ?
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