Re: Quest for the simplest zero voltage switching


"MooseFET" <kensmith@xxxxxxxxx> schreef in bericht
news:1187014014.937032.70700@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Aug 12, 3:58 pm, "petrus bitbyter"
<pieterkraltlaatdit...@xxxxxxxxxxxxxxxxx> wrote:
"JoeyB" <joseph.bur...@xxxxxx> schreef in
berichtnews:1186924269.912470.58160@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



All,

I'd like to do proportional control on a 120VAC, 1.5kw heating element
(capacitive load) as simple as is possible. Ideally, I'd like to have
a single pot that I could adjust from zero to one hundred percent duty
cycle input to a zero voltage switching controller chip. This ZVS
controller chip would then control the Triac swiching the load. I've
found several chips out there that provide zero voltage switching but
only one that DIDN'T require a dc voltage for operation AND simply
allowed a pot to be connected to the control pins, the Motorola
CA3059. Or at least, it was the only one that had a circuit in the
data *** showing such a configuration. Unfortunately, its
discontinued and I cannot find any suitable substitute for its
operation. The MOC3081 seems like a possibility however it requires a
digitial input control. This would mean some kind of timer circuit
(again DC would probably be required) unless I'm missing some obvious
passive solution to providing this input. Any help from the power
circuit design pro's out there would be greatly appreciated.

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.

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.


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


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