PWM using 555 timer

[audioguru2]

Ante,
I grow hair on my face most of the time, and especially when I see a circuit with its common-mode input voltage limit exceeded.
Have you seen what happens to a TL07X? Its output suddenly inverts! It makes really bad overdrive distortion.

I relate the moon being full with Murphy's Law, and Murphy wasn't even a vampire.

 

[darrins]

Ante and audioguru,

The voltage difference between my heat sensor and reference will be, at most, around 300 mA. I've read that, as a rule-of-thumb, that the control voltage at pin 5 should be between 45% and 90% of Vcc. Obviously, I'll have to amplify my signal if I'm going to use a Vcc of say 5V.

Given the limitations that audioguru described (with respect to the 555's PWM capability), will this still be adequate to drive my optoisolated tric driver?

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I have another dumb question. The triac that I bought came with a mica washer. It is my understanding that the metal part that connects to the heat sink is LIVE . I've read that the mica washer should be installed between the metal part and the heat sink, along with some "thermal grease". What is this thermal grease and can I use silicon compound instead (or is silicone compound the same thing)? Looking at the inside of a light dimmer, I noticed that its triac had only a non-metalic washer (no grease). I've heard that there are silicone rubber washers that can replace the mica/grease. This seems a lot less messy. Can these washers be found at most electronics stores?

There is supposed to be a full moon tonight, so maybe I should hold off on building this circuit.

As always, thanks for the help.

Darrin

 

[darrins]

Ante and audioguru,

Just found another interesting circuit. This one is from http://home.cogeco.ca/~rpaisley4/LM555.html#7

This circuit looks like it might be a little easier to predict and the pulse width can be between 5% and 95% of nominal.

 

[ante1]

Darrin,

This last circuit and diagram is really interesting, I can

 

[darrins]

Thanks Ante and audioguru. I will try experimenting with a few different designs, including the one Ante simulated. I think I will also try stuffing a few of my temperature sensors (LM34) in a copper tube and use a summing amplifier to calculate an average temperature. A lot of the temp control circuits I see use only one sensor. A body of water can have a large temperature gradient (as can large pieces of metal, etc.).

Well, I better hurry and post this. We are having a big storm here in Texas and the power keeps going out. Too bad I already dismantled my UPS. :-\

Have a nice weekend.

Darrin

 

[audioguru2]

Wait a minute, Darrin,
You mentioned, Triac. All along we thought that you were PWM'ing from a DC supply.
A TRIAC works like a latch, since once it is triggered, then it conducts and stays-on until the current through it drops very low (when the mains voltage crosses 0V for resistive loads), when it turns itself off. The TRIAC must be triggered for each 1/2 cycle of the mains. You don't need a PWM circuit, the TRIAC does it. All that you need is a circuit that is syncronized to the mains frequency, and gives a short pulse to the TRIAC's opto with a time delay from when the mains voltage crosses 0V.
With a short time delay, the TRIAC is turned-on early for each 1/2 cycle, and therefore gives nearly full power to the load. With a long time delay, the TRIAC is turned-on late for each 1/2 cycle, almost when the TRIAC will turn itself off anyway, and therefore gives a very low power to the load. The time delay must not exceed the time of 1/2 cycle of the mains frequency.
Recently, 2 other posts discussed "lamp dimmers", with circuits to do this. I would find them for you, but lately this site's navigation is very slow.

 

[darrins]

audioguru,


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.

Thanks.

Darrin

 

[ante1]

Darrin,

I believe you are right, the PWM is the way to go. However it must be with a very low frequency to suit this application, something like 0.1

 

[darrins]

Ante,

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?

 

[ante1]

Darrin,

Yes the low frequency enables the circuit to

 

[22]

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)

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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.  

Darrin

Darrin:
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  ?