Quest for the simplest zero voltage switching

Could you describe this heating element for us? The description above violates the laws of physics; if it really is a capacitive load it isn't a heating element. A true capacitor doesn't dissipate any heat at all. Capacitors also have maximum current at zero-crossing of the voltage... Real-world capacitors are not perfect, which means that they have the equivalent of a resistor in series with them and thus can get hot, but I really don't think you are using capacitors as heaters. So what *are* you using as a heating element?

That chip does not "adjust from zero to one hundred percent duty cycle." It pulses at the zero crossing. That's because it is. when connected to the appropriate thyristor, a zero crossing switch, not a dimmer.

Here is what the output of a zero crossing switch controlling a heater looks like (pretend the signal is a sine wave, not a flat-top trapezoid):

... ... . . . . . . . .

----------------------------------------------------------- 0V . . . . . . . . ... ...

(or it could be hundreds of cycles on followed by hundreds of cycles off).

Here is what the output of a dimmer controlling a light bulb looks like:

... ... ... ... | . | . | . | . | . | . | . | .

----------------------------------------------------------- 0V | . | . | . | . | . | . ... ... ...

...which does not switch on at zero crossing.

If you really are trying to control a heater and you really do want zero-crossing switching, the first (heater) waveform is what you want.

If you really want to adjust the duty cycle, and you really have a heater, not a capacitive load, why not just buy a LUTRON T-2000

The CA3059 requires a DC voltage and a LOT of supply current too. TRIAC trigger current is increased for quadrant III operation.

The 3059 makes its own DC supply.

Best regards, Spehro Pefhany

Spehro Pefhany wrote: > On Sun, 12 Aug 2007 14:55:19 GMT, the renowned Fred Bloggs > wrote: >

It's just a simple half-wave rectifier in series with a user supplied external and relatively high wattage resistor...

The "cycle" in "duty cycle" doesn't have to refer to the AC cycle.

Good point.

Yes, +/- 8V zeners, & a half-wave rectifier. It supplies 50mA or 90mA gate drive (in short pulses).

Best regards, Spehro Pefhany

"JoeyB" schreef in bericht news: snipped-for-privacy@i13g2000prf.googlegroups.com...

The most simple zero cross switching can be obtained by buying a solid state relay with a build in zero cross switch. Building a timer to controll it will be a little bit harder. I ever build one using a classic 555 with a stereo potmeter and some other passives and diodes. The 555 was used in astable mode and set to about 10Hz. The potmeter controlled the duty cycle from about 1 to 100%. As the zero cross switch only switched on zero voltage, I could controll power by the half cycle. Unfortunately I had to add a small power supply. The old 555 required 10-15mA at 15V and the solid state relay added another 5mA. Using a resistor directly from the mains would have made me to get rid of over 3W heat (220-230V/50Hz mains). As you have only 120V mains and can use a CMOS 555, powering via a series resistor will not be difficult.

Nevertheless, this days I'd go for a PIC10F200 and a normal 10k lin potmeter to build the timer. With one or two extra components and a little effort you can build the zero crossing switch into it as well.

petrus bitbyter

So, why not buy a commercial oven controller module? This is a common enough task. Usually there's a thermometer involved, too, so the end result is a thermostat, but you can fake the thermometer easily.

Basic block diagram is: ramp/triangle wave generator with 10 second period, potentiometer from ramp-minimum to ramp-maximum voltage, and a comparator with inputs: potentiometer wiper vs. ramp generator- gives a duty cycle controlled by the pot setting. The comparator output goes to a solid-state relay which powers the heat element.

The CA3059 includes op amps that can perform these functions, but the 'simplicity' is joined with other characteristics (no second- source, high power consumption) that you may not like.

berichtnews: snipped-for-privacy@i13g2000prf.googlegroups.com...

I think if you move up that product line just a bit you will do a little better. You want a comparitor to do the zero crossing and a ADC to read the pot. You don't really need the PWM stuf because 60Hz is so low you can count off the cycles in a loop.

The PIC's output isn't quite strong enough to trigger a triac so an external (gasp) transistor would be needed.

The PIC's ADC uses the supply voltage as a reference. If the pot used the supply as its input, the value of Vcc drops out.

Since you are counting cycles of the mains, the RC clock of the PIC is more than good enough.

The total current draw of the control circuit would only be several mA at the most. Power can be done like this:

R1 C1 Mains---/\\/\\---!!----+---->!----+---- ! ! 5.1V /-/ --- Zener ^ --- D1 ! ! --------------------+----------+-----

The ratings of R1 and D1 are mostly set by the turn on inrush. Once the circuit is up and going most of the mains drop is on C1.

"MooseFET" schreef in bericht news: snipped-for-privacy@i13g2000prf.googlegroups.com...

You will need a relative good 5V power supply. I'd use two 1W series resistors (to dissipate 1W), a 10V zener and a 7805 type stabilizer. Plus, of course, some capacitors. The PIC I mentioned requires less then 0.5mA @4MHz. Add another half for the potmeter and 5mA to drive the SSR. It's good to add some margin but an 8mA power supply will be enough. The PIC can also sink the 5mA required to drive the SSR directly. (If you want to use a triac, things change of course). The PIC also has an 8 bits ADC so you can read the potmeter. Using the 5V power supply wil not give a problem. (Not in the appliance I'm currently building anyway.) Only if your SSR has no zero cross switch build in, you'll need some extra components to produce a zero crossing pulse for the PIC to do the zero cross switching.

petrus bitbyter

berichtnews: snipped-for-privacy@i13g2000prf.googlegroups.com...

I don't see why. The PIC doesn't need one, the pot doesn't need one and the SCR trigger doesn't need one.

Why would you want to waste all that power and use all those parts. If you really do need a better supply, you can raise the 5.1V zener to a 6.8V zener and use a regulator but I really don't see where the need comes from.

If you use a triac, you need far less average current. The PIC only needs to make the output high long enough to trigger it.

Remember your LM7805 is eating about 5mA. You need to provide for that too.

If commented on the fact that there is an ADC. You will also see that I pointed out that the ADC and the POT both use the supply as a reference so the exact value of the supply voltage drops out of the equations.

You don't need a "zero crossing pulse". You only need a divided down copy of the AC waveform. The PICs have comparitors in them. You just need to use one comparitor to detect the zero crossing.

Thanks all. I never realized I could get away with a simple RC network and a zener for DC good enough to drive these ICs. I thought they would be more particular. I've been looking at electric ranges and they use what they call an 'infinite' switch. Based on a bimetal on/ off duty cycle caused by a heating element in a wholly mechanical device. I would feel like I was cheating to use one though. I think I'll give the solid state method a whirl ! I'm curious why these inifinte switches have persisted so long. I wouldn't imagine they are any cheaper than what is being described in this post. Maybe because of the manufacturing base developed over the years. Are modern ranges going solid state?

"MooseFET" schreef in bericht news: snipped-for-privacy@x40g2000prg.googlegroups.com...

Maybe you're right. I tend to stay on the safe side.

Maybe you're right too. But I've had some bad experiences using series capacitors for current limiting on the 220-230V mains.

Depends heavily on the triac. Some types require 50mA, and - as you say - the trigger pulse needs to last long enough. It has been some time now but I had problems with trigger pulse duration. Don't remember the required pulse length anymore.

The same applies for the SSR. If you use one with build in zero cross switch, the PIC does not need to look for the zero crossing at the cost of some energy.

You're right. I used some low quiscent current types (three leg, not 78xx) lately but the standard 78xx series needs that current so it has to be taken into account.

You're right again. I misunderstood.

That small PICs have either a comparator or ADCs, not both. But speaking about the PIC10F222, it has two ADCs so you can use one to look after the mains phase.

BTW. Re-reading the posts I see I mentioned the PIC10F200 in my first reply. Should be PIC10F222 as the 10F200 has no ADCs. Sorry for that.

petrus bitbyter

berichtnews: snipped-for-privacy@x40g2000prg.googlegroups.com...

The safe side does have its attractions but remember one goal is keeping the costs down. The OP said "simplest" not "reasonably simple and yet reliable".

You have to apply gate current until the device is passing much more than the holding current. In a zero crossing switch this can force you to pulse longer than normal. If the load is inductive matters are extra messy. The OP is running a resistive load so we don't have that to worry about.

It happens to us all.

You are right. Either we need to add a comparitor (perhaps just a transistor) or we have to get very clever. First lets see how few parts the comparitor could be:

The PIC has weak pull-ups. We can use that as the pull-up on a comparitor. 0.7V is very close to zero when considering mains voltage so we could:

-----------PIC pin 100K !/ AC----/\\/\\/----+-----! Small NPN ! !\\e With large HFE at low I 1N914--- ! ^ GND ! GND

A small JFET may actually be a better way to go if you have a low Vgs(off).

If you really want to go cheap and nasty, you could just run the resistor to the pin of the PIC. The substrate diodes would clip the signal.

Now for getting very clever:

I had assumed you meant a member of that family not that specific one.

Lets see if we can do without the ADC.

If we run a simple RC from an output to one pin of the comparitor and the pot to the other, we can use a sort of delta-mod ADC. We have to be careful about how we code the whole thing to be sure to get around the loop at the same rate but I will leave that detail out and write a bit of pseudo-code to show what I mean:

if RC < POT then OUTPUT = HIGH inc MeasuredPot else OUTPUT = LOW

LowerPart = LowerPart - MeasuredPot if BORROW then dec MeasuredPot

The MeasuredPot will settle at the point where the rate it causes borrows from the subtract matches the portion of the time that the OUTPUT has to be high to cause the filter to match the setting of the pot.

Because they're simple (one moving part), reliable (like 25 years lifetime), and cheap.

Labor.

I'm afraid I don't know this one - the high-end ones (i.e. $$$$) probably are.

Cheers! Rich