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'[EE]: AC Voltage Compensation'
2001\05\03@131923 by

Dear Bob,

Bob <op1cwkflashmail.com> on 2001-05-02 02:14:16 PM wondered:
>Well, a simple description of the application is that I want to calculate the
>triac firing angle (time delay after ZC), based on the incoming line voltage
>(for an open looped system), so that I get "X" amount of RMS volts going out.
...
>The PID version of this would be to initially "guesstimate" the firing angle
>(first time only), measure the SCR waveform output, and adjust the firing angle
>on a per cycle basis until it settles to the correct output voltage.
>
>Here are a list of the variables that the device has to deal with:
>1:  Incoming or outgoing line voltage (varys in magnitude and shape)
>2:  Desired user selected power level (scale is 0 to 99, or 10vRMS to 90vRMS)
>3:  User selectable load (goes to when the triac will shut off).
>4.  50 or 60hz input voltage (maybe, otherwise I'll have to make a slightly
>different circuit for each freq.)
...
>non-sinusoidal waveform
...
>Is there a way to "filter" or modify that SCR waveform

Maybe you don't need to measure the waveform at the output of the triac.

My understanding is that a triac is just a switch. It's open (letting zero
current through) until you trigger it.
After you trigger it, it shorts the line voltage from the wall directly to the
output, until the current drops to zero, and it resets back to the open state.

So if you know exactly when the triac turns on and off, and you measure the
voltage at the wall (rather than "downstream" of the triac), you can get a
pretty accurate estimate of the voltage downstream of the triac since you know
(a) triac off -- output voltage zero. (well, actually, output power is zero).
(b) triac on -- output voltage same as wall voltage.

Yes ?

>Um, according to this, I'd have to look at "both" the incoming line voltage,
AND
>the SCR output voltage.  Which begs the question:  Why not do just straight PID
>control based on just the output then?

Sampling the wall voltage at 1 KHz is more than fast enough. You wouldn't have
to worry about fast triac output voltage swings, and where exactly they fall
in-between samples.

Going even further in the direction of not measuring triac output waveform:
Perhaps you could just measure the voltage and current from the wall (even
upstream of the input filters), assume that losses are negligible (constant),
and adjust the triac firing time until the power at the wall socket looked
right.

>with an opamp (or
>something), but so that it doesn't induce much lag into it (rounds the SCR
>waveform spike out linearly, rather than just stretching it out, or clipping
off
>voltage data)???  Even a 4 or 5 cycle lag wouldn't be too bad, but 7/10ths to 1
>second is way too much (because when you add that up with the PID control
>settling lag, it could add up to something like 3 to 5 seconds of total
reaction
>time, which IS too slow for this application).  I'm shooting for under 2
seconds
>of total reaction settling time.

It sounds like your elements have a time constant of about 4 seconds or more.
One rule of thumb is to make the output sample period less than 1/10 the time
constant of the thing you're trying to control, which would be about 0.4 seconds
(2.5 Hz) in this case.

You're controlling when the triac fires, which only happens 2wice per cycle,
right ? That's your (fastest) output sample rate: 120 Hz. Just a little bit of
overkill when you only need 2.5 Hz. Lag is not a problem when the total lag is
less than 1/2 of your output sample rate (e.g., in this case 1/4 of a cycle

The rule of thumb for simple RC lowpass filters is to make the time constant (T
= RC) at least 5 times your input sample rate. At Olin's suggestion of roughly
0.5 ms sampling rate, that gives a minimum time constant of T roughly 2.5 ms,
well below 4 ms.

>triac firing angle?

Fine idea, with a sampling rate of 1 KHz or more, I suspect that the difference
between synchronizing and not synchronizing is insignificant.

>I could maybe get around that looking at the voltage on
>the output side of the transformer instead (everything is in phase there).  But
>that's kind of a hassle, as far as design and manufacturing the circuit goes.

That's really the question. What exactly is it you want to control ? The power ?
The average voltage ? The peak voltage ? The power *ratio* relative to full-on ?

It's usually a lot easier to control something when you sense as close as you
can to the thing you want to control (compensating for a few things between what
you sense and what you wanted to control), than to measure something distantly
and try to compensate for lots of stuff between the two.

Bob <op1cwkflashmail.com> on 2001-04-25 12:52:19 PM mentioned:
>> 16 or 18 gauge cord to carry 20 amps to a load that runs at
>> only a couple of volts... I'd suggest heavier wire, or most of your
>> power is going to be wasted heating the cord.
...
>Yep, I already know that happens with the current analog circuit (you lose at
>least 1/2 of the power).  Ain't no way around it, unless you got deep pockets
>and can afford silver wire.  Imagine holding a small pen like tube, that has a
4
>foot 16 gauge cord attached to the end of it (it's already pretty bulky and
>unwieldly).

Actually, there is a way to avoid power loss in the wires. The method used when
a person wants to run large amounts of electrical power for many miles. (This
person can't even afford thick copper wire, much less silver wire, so this
method has to work with cheap, relatively thin steel wire).

If this device needs to pass FCC regulations on conducted (up the power cable)
and radiated emissions, you'll need several capacitors and maybe some ferrite
beads in the appropriate locations. They block the high-frequency parts of the
waveform, making it easier to measure the waveform on the other side of these
components.

TRIAC circuits
http://discolitez.com/circuits-triac.html
www.ednmag.com/ednmag/reg/1994/072194/15di3.htm
http://www.aaroncake.net/circuits/dimmer.htm
http://www.solorb.com/gfc/elect/constemp/
www.ti.com/sc/docs/psheets/abstract/apps/slaa043a.htm
http://www.st.com/stonline/books/toc/an/57.htm

Hope that helps. This stuff isn't nearly as complicated as most people think.

--
David Cary

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My dad used to do the thing with a light bulb in series with any loads he
was worried about in industrial equipment he built...

The trick is to use a lightbulb that is too big to even glow when under
normal operating conditions. If the circuit overloaded or shorted out the
light bulb starts heating up and provides enough resistance to keep
everything from frying, while also providing a visual indication that
something is wrong. He used to dip them in red glass stain so that the
operator would see this "burning" glow from inside the equipment... very
effective at getting them to shut down! He used to get these "Oh God Jim!
The plasmic defrabulator just burned up inside!" phone calls.

I suppose it is Rube Goldberg'ish to some degree, but it sure worked will
for him! It's a crusty old guy trick. (no offense Dave)

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{Original Message removed}

James Newton <jamesnewtonpiclist.com> wrote...

>I suppose it is Rube Goldberg'ish to some degree, but it sure worked will
>for him! It's a crusty old guy trick. (no offense Dave)

People keep telling me I'm getting crustier with each passing year,
and I'm old enough that I've done commercial products with vacuum
tubes (back when you sometimes HAD to use them; not this modern-day
stuff designed for Tube Cult audiophiles who stare in fascination at
the glowing filaments) and I got my first embedded-systems experience
on the Intel 4040 (a batch controller for a concrete block plant); so
I guess I'm a "crusty old guy".  No offense taken.

"Rube Goldberg'ish", though, is an attribute usually given to
something which is outrageously- indeed comically- complex and
roundabout.  This thing has only three working parts (and it responds
to true RMS, to boot!) so it ain't exactly complex.  Yeah, it's
unusual, perhaps you could call it unorthodox; but not "Rube
Goldberg'ish".

It's not original, either, as I've seen circuits using a CdS cell in
that fashion as part of a lamp power regulator years ago.

Cheers,

Dave

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> This thing has only three working parts (and it responds
> to true RMS, to boot!) so it ain't exactly complex.  Yeah, it's
> unusual, perhaps you could call it unorthodox; but not "Rube
> Goldberg'ish".

There was a bright young engineer named Bill Hewlett back in the 1940s (?)
who used a lightbulb in an unusual way as a voltage-variable resistor to
stabalize the output gain of an oscillator.  Too bad there wasn't a PIClist
around back then to tell him his design and his company would never work
out.  Would have saved him a lot of trouble I guess.  I wonder what ever
happened to the doomed product.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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2001\05\06@220615 by
Olin Lathrop <olin_piclistEMBEDINC.COM> wrote...

>There was a bright young engineer named Bill Hewlett back in the 1940s (?)
>who used a lightbulb in an unusual way as a voltage-variable resistor to
>stabalize the output gain of an oscillator.

Hewlett described his lightbulb-stabilized Wien bridge oscillator in
his 1939 master's thesis at Stanford.

>Too bad there wasn't a PIClist
>around back then to tell him his design and his company would never work
>out.  Would have saved him a lot of trouble I guess.  I wonder what ever
>happened to the doomed product.

H-P finally gave up on it, after selling only a few hundred thousand
of the things.

DD

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> There was a bright young engineer named Bill Hewlett back in the 1940s
> (?) who used a lightbulb in an unusual way as a voltage-variable
> resistor to stabalize the output gain of an oscillator.  Too bad there
> wasn't a PIClist around back then to tell him his design and his
> company would never work out.  Would have saved him a lot of trouble I
> guess.  I wonder what ever happened to the doomed product.

I dunno if this was a pun (missing smiley indicator) so I'll just say that
I have a Wien bridge audio oscillator that is stabilized by a 30mA bulb. I
found this out when the bulb burned out and I had square wave output
instead of sine ;-) I didn't know that Bill Hewlett invented this.

Peter

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> > There was a bright young engineer named Bill Hewlett back in the 1940s
> > (?) who used a lightbulb in an unusual way as a voltage-variable
> > resistor to stabalize the output gain of an oscillator.  Too bad there
> > wasn't a PIClist around back then to tell him his design and his
> > company would never work out.  Would have saved him a lot of trouble I
> > guess.  I wonder what ever happened to the doomed product.
>
> I dunno if this was a pun (missing smiley indicator) so I'll just say that
> I have a Wien bridge audio oscillator that is stabilized by a 30mA bulb. I
> found this out when the bulb burned out and I had square wave output
> instead of sine ;-) I didn't know that Bill Hewlett invented this.

This was Hewlett Packard's first product.  Their first customer was the Walt
Disney Company, which used these oscillators to produce special audio
effects in Fantasia.

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinembedinc.com, http://www.embedinc.com

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On Tue, 8 May 2001 08:03:08 -0400 Olin Lathrop
<olin_piclistEMBEDINC.COM> writes:
>
> This was Hewlett Packard's first product.  Their first customer was
> the Walt
> Disney Company, which used these oscillators to produce special
> audio
> effects in Fantasia.
>

The light bulb is really an interesting device. I believe they were used
in radios in the 1930's as part of a noise reduction system. At the
station the audio would be compressed. At the receiver, a light bulb
across the audio would expand the audio. Low level audio would cause the
lamp to have a low resistance, causing an even lower output due to the
low lamp resistance. High level audio would cause the lamp to have high
resistance making the output audio even higher in level.
The light bulb is also an interesting current limiter or constant
current device. Anyone remember "ballast tubes"? My employer built an
electric car, putting 11 12V batteries in a Fiat X19. Our charger design
consists of three diodes and a light bulb. The neutral of the incoming
three phase Y goes to the battery negative terminal (the HV battery
circuitry is not connected to the frame of the car... it floats). Each
incoming phase goes through a diode and is then combined giving us three
phase half wave rectified DC. This goes through a light bulb (12V 250W,
as I recall) to the positive side of the battery. The light bulb adjusts
its resistance to nearly constant current charge the batteries. It also
works as an indicator. If the lamp has gone out, the batteries are
charged!
So, what else can you do with a light bulb?

Harold

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

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>        So, what else can you do with a light bulb?

Break the bulb and use the (intact!) filament to build a thermal anemometer.

newell

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At 11:09 AM 5/8/01 -0500, you wrote:
>>        So, what else can you do with a light bulb?
>
>Break the bulb and use the (intact!) filament to build a thermal anemometer.

Or a Pirelli vacuum guage.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
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At 11:09 AM 5/8/01 -0500, you wrote:
>>        So, what else can you do with a light bulb?
>
>Break the bulb and use the (intact!) filament to build a thermal anemometer.

Or a Pirelli vacuum guage.
^^^^^^^
Of course that should read "Pirani"; Pirelli makes tires (tyres). ;-)

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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>>Break the bulb and use the (intact!) filament to build a thermal anemometer.
>Or a Pirelli vacuum guage.
>     ^^^^^^^
>Of course that should read "Pirani"; Pirelli makes tires (tyres). ;-)

Taires too? :oD

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> Or a Pirelli vacuum guage.

Pirani.

Peter

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