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PICList Thread
'Suggestions for AC chopper?'
1997\03\27@162613 by Aaron J. Miller

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Hello all,

This question is only marginally related to pic's since I am going to use
a 16C84 to control the chopper, but there seem to be quite a few bright
people out there who may be able to help...

I am trying to make an incandescent lamp dimmer from a 16C84 and planned
on doing a simple waveform chop using a triac.  I realized, however, that
since the pic is going to be doing a bunch of other time sensitive stuff
that I don't have the processor time to manually sense the zero and chop
the wave using the pic.

So, is there any easy method to passively (from the pic's point of view)
chop the AC wave?  i.e. use 8 I/O pins to a D-A to give a reference to the
chopper circuit and have it do the rest?  That way I can just output the
light level to the pins and it would stay put leaving the rest of the
processor available?

Thanks for any help,
aaron

-------------------------------------------------------------------------------
-     Aaron J. Miller           Email: spam_OUTmilleraTakeThisOuTspamstanford.edu
-     Physics Graduate Student  Web  : http://www.stanford.edu/~millera
-     Stanford University       Phone: (415) 372-5575 (until 4/12/97)
-------------------------------------------------------------------------------

1997\03\27@174827 by Chris Atkins

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part 0 2131 bytes
Like you, I wanted to use the timer to keep track of other things.
One thing I found was an I2C real time clock - Phillips PCF8593.
You could set the RTC to give you an alarm interrupt, freeing up
the timer. This was optimal for me because some of the delay times
were very long. The application might want to wait an hour and then
dim the light for 5 minutes.

I drove the gate with a MOC3033 triac optoisolator. You can
built a zero crossing detector from a comparitor to synchronize
the the gate transitions with the zero crossings.

If you want the chopping to be pasive to your PIC, why don't you
use two PICs?

Regards,

Chris Atkins

----------
From:   Aaron J. Miller
Sent:   Thursday, March 27, 1997 2:15 PM
To:     .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject:        Suggestions for AC chopper?

Things are changing at NetForward!
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hello all,

This question is only marginally related to pic's since I am going to use
a 16C84 to control the chopper, but there seem to be quite a few bright
people out there who may be able to help...

I am trying to make an incandescent lamp dimmer from a 16C84 and planned
on doing a simple waveform chop using a triac.  I realized, however, that
since the pic is going to be doing a bunch of other time sensitive stuff
that I don't have the processor time to manually sense the zero and chop
the wave using the pic.

So, is there any easy method to passively (from the pic's point of view)
chop the AC wave?  i.e. use 8 I/O pins to a D-A to give a reference to the
chopper circuit and have it do the rest?  That way I can just output the
light level to the pins and it would stay put leaving the rest of the
processor available?

Thanks for any help,
aaron

-------------------------------------------------------------------------------
-     Aaron J. Miller           Email: milleraspamKILLspamstanford.edu
-     Physics Graduate Student  Web  : http://www.stanford.edu/~millera
-     Stanford University       Phone: (415) 372-5575 (until 4/12/97)
-------------------------------------------------------------------------------


1997\03\27@175712 by Aaron J. Miller

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face
Thanks for the response!

> If you want the chopping to be pasive to your PIC, why don't you
> use two PICs?

I am thinking about it...  I just took apart one those dimming halogen
lamps and noticed that they do an elegant chopping thing with a RLC
circuit somehow (I don't understand the circuit).  I figured if there were
an easy way to simulate the variable resistor in the circuit it would be
nice.  Any clues there?

-aaron

-------------------------------------------------------------------------------
-     Aaron J. Miller           Email: .....milleraKILLspamspam.....stanford.edu
-     Physics Graduate Student  Web  : http://www.stanford.edu/~millera
-     Stanford University       Phone: (415) 372-5575 (until 4/12/97)
-------------------------------------------------------------------------------

1997\03\27@181533 by Harold Hallikainen

picon face
On Thu, 27 Mar 1997 13:15:44 -0800 "Aaron J. Miller"
<EraseMEmilleraspam_OUTspamTakeThisOuTLELAND.STANFORD.EDU> writes:
{Quote hidden}

       We are using the 16c74 to phase control four triacs.  We detect
zero cross by doing a negative edge interrupt on the external interrupt
pin.  Positive zero cross is determined based on a timer.  We determine
when to turn on a triac by using the compare mode of a capture/compare
timer register.  At zerocross we determine when the first turn-on will
be, then put it in the compare register.  On receiving the interrupt from
that compare, we turn on that triac and load the next.  The external
interrupt port pin is driven directly from the power supply transformer
secondary with a series current limit resistor.  The clamp diodes in the
16c74 square it nicely and the hysteresis on the input prevents noise
problems.  Note that we actually get the interrupt a little BEFORE the
zero-cross, but that's a lot better than a lot after (which would be the
case on a positive edge trigger).
       A 16c74 running at 16 MHz handles this quite nicely.  While doing
the phase control it is also receiving a 250 Kbps datastream and scanning
4 A/D inputs.

       If you want to generate triac firing externally, you can do it
with an op-amp ramp generator (that is synced with the AC line) driving a
comparator for each triac.  You can get analog output of many of the PICs
by using pwm into a simple low pass filter.  We're doing something
similar in a product where we take the 250 Kbps datastream and pull out
48 bytes, converting them to analog signals.

       Now... speaking of AC choppers...  Anyone aware of a bunch of PWM
generators (like those in PIC chips) but all in one chip (maybe a dozen
of them or so that could be driven by SPI or I2C)?

Harold

1997\03\27@201757 by Tony Matthews

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Aaron J. Miller wrote:
{Quote hidden}

-------------------------------------------------------------------------------
> -     Aaron J. Miller           Email: milleraspamspam_OUTstanford.edu
> -     Physics Graduate Student  Web  : http://www.stanford.edu/~millera
> -     Stanford University       Phone: (415) 372-5575 (until 4/12/97)
>
-------------------------------------------------------------------------------
A diac and an optoisolator with cds out and a single pic pin PWM with a
resistor /capacitor integrator infinitely variable and you need the same
parts anyway.my $.02 Tony M.

1997\03\27@202401 by Chris Atkins

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face
I looked at doing this for a project. What really matters is
that once the tirac is on, the gate goes low before the next
zero crossing if you want it to go off. This can be done by
setting up the timer to interrupt you whenever the output
needs to change state. In the ISR to handle the timer interrupt,
you would toggle the output and immediately schedual the next event.

Like you, I wanted to use the timer to keep track of other things.
One thing I found was an I2C real time clock - Phillips PCF8593.
You could set the RTC to give you an alarm interrupt, freeing up
the timer. This was optimal for me because some of the delay times
were very long. The application might want to wait an hour and then
dim the light for 5 minutes.

I drove the gate with a MOC3033 triac optoisolator. You can
built a zero crossing detector from a comparitor to synchronize
the the gate transitions with the zero crossings.

If you want the chopping to be pasive to your PIC, why don't you
use two PICs?

Regards,

Chris Atkins

----------
From:   Aaron J. Miller
Sent:   Thursday, March 27, 1997 2:15 PM
To:     @spam@PICLISTKILLspamspamMITVMA.MIT.EDU
Subject:        Suggestions for AC chopper?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hello all,

This question is only marginally related to pic's since I am going to use
a 16C84 to control the chopper, but there seem to be quite a few bright
people out there who may be able to help...

I am trying to make an incandescent lamp dimmer from a 16C84 and planned
on doing a simple waveform chop using a triac.  I realized, however, that
since the pic is going to be doing a bunch of other time sensitive stuff
that I don't have the processor time to manually sense the zero and chop
the wave using the pic.

So, is there any easy method to passively (from the pic's point of view)
chop the AC wave?  i.e. use 8 I/O pins to a D-A to give a reference to the
chopper circuit and have it do the rest?  That way I can just output the
light level to the pins and it would stay put leaving the rest of the
processor available?

Thanks for any help,
aaron

-------------------------------------------------------------------------------
-     Aaron J. Miller           Email: KILLspammilleraKILLspamspamstanford.edu
-     Physics Graduate Student  Web  : http://www.stanford.edu/~millera
-     Stanford University       Phone: (415) 372-5575 (until 4/12/97)
-------------------------------------------------------------------------------

1997\03\28@113000 by motodog

picon face
Hi Aaron,
Don't chop! Use zero crossing detection.  Decide to turn On or Off the
lamp, and leave it alone for a half cycle (1/120s).  Chopping it causes
LOTS of grief.  Radiated noise and excessive switching currents, will be
greatly reduced if you switch when the voltage is zero.  Depending on
the inductance of the load, the current might even be near zero.  Also
use opto-isolation to protect the PIC and other circuits.  The PIC
should have time to do a calculation every 1/120S?
Good Luck
Mike

1997\03\28@145918 by peter

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face
part 0 587 bytes content-type:image/x-MS-bmp; name="POWER.BMP"Not exactly an ac chopper but it controls AC loads very well

Take a look at this circuit it's opto Isolated and you drive the
opto led with PWM at whatever frequence you program allows
(within reason, 50Hz to 10KHz)
It requires no synchronisation so only one PIC pin is needed

It's in bmp format

If you have any questions email me direct if you like

Peter Cousens
RemoveMEpeterTakeThisOuTspamcousens.her.forthnet.gr

Content-Type: image/x-MS-bmp; name="POWER.BMP"
Content-Disposition: inline; filename="POWER.BMP"

Attachment converted: wonderlandfive:POWER.BMP (????/----) (0000D6C4)

1997\03\28@163954 by Aaron J. Miller

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face
On Fri, 28 Mar 1997, Michael F. Hagen wrote:
> Hi Aaron,
> Don't chop! Use zero crossing detection.  Decide to turn On or Off the
> lamp, and leave it alone for a half cycle (1/120s).  Chopping it causes
> LOTS of grief.  Radiated noise and excessive switching currents, will be
> greatly reduced if you switch when the voltage is zero.  Depending on
> the inductance of the load, the current might even be near zero.  Also
> use opto-isolation to protect the PIC and other circuits.  The PIC
> should have time to do a calculation every 1/120S?
> Good Luck
> Mike
>

Mike,

Do you mean to just play with a full half-cycle at a time?  Meaning, do I
leave the lamp off for 5/120ths of a second and on for one, off for 5,
etc..  would this cause a flicker in the lamp at low intensities or is the
incadescent filament slow enough to take care of it?  I am just wondering
about a nearly-off lamp where I pause for, say, 12/120ths of a second and
on for 1/120.  This is effectively flickering at 10 times per second which
is noticable, is it not?

Thanks,
aaron

-------------------------------------------------------------------------------
-     Aaron J. Miller           Email: spamBeGonemilleraspamBeGonespamstanford.edu
-     Physics Graduate Student  Web  : http://www.stanford.edu/~millera
-     Stanford University       Phone: (415) 372-5575 (until 4/12/97)
-------------------------------------------------------------------------------

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