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'simple D/A for 500W AC?'
1996\04\05@180721 by Tom Sgouros

flavicon
face
I don't know if this is appropriate to this list, but the device that
needs to do it is controlled by a PIC16C65...

Does anyone know of an easy way to control the voltage on a 120VAC line? I
need to vary the light from some lightbulbs, and I don't want to use the
standard SCR solutions because they produce artifacts that seem to wear
the bulbs out faster.

I understand how to do D/A to get a variable DC voltage up to some small
voltage. How can I get from there to the 120VAC?

A cryptic remark from someone has led me to believe that there may be a
solution involving a PWM signal and some power MOSFETs. I think I can see
how that would work, but has anyone done anything like it who could
reassure me that I'm not going to fry myself or my PIC?

-Tom

p.s. Many thanks to all for the help with my crystal questions. The
crystal was bad and I misread the capacitance of the capacitors, and I
didn't realize how much my scope probe would confuse things. It's all fine
now...

---------------------------------------------------------------------
spam_OUTtomssTakeThisOuTspamids.net - 401-861-2831 - 42 Forge Road, Potowomut, RI 02818 USA

1996\04\05@180721 by Tom Sgouros

flavicon
face
I don't know if this is appropriate to this list, but the device that
needs to do it is controlled by a PIC16C65...

Does anyone know of an easy way to control the voltage on a 120VAC line? I
need to vary the light from some lightbulbs, and I don't want to use the
standard SCR solutions because they produce artifacts that seem to wear
the bulbs out faster.

I understand how to do D/A to get a variable DC voltage up to some small
voltage. How can I get from there to the 120VAC?

A cryptic remark from someone has led me to believe that there may be a
solution involving a PWM signal and some power MOSFETs. I think I can see
how that would work, but has anyone done anything like it who could
reassure me that I'm not going to fry myself or my PIC?

-Tom

p.s. Many thanks to all for the help with my crystal questions. The
crystal was bad and I misread the capacitance of the capacitors, and I
didn't realize how much my scope probe would confuse things. It's all fine
now...

---------------------------------------------------------------------
.....tomssKILLspamspam@spam@ids.net - 401-861-2831 - 42 Forge Road, Potowomut, RI 02818 USA

1996\04\05@205304 by Andrew Yalowitz

flavicon
face
  There are some next generation theatrical dimmers that use IGBT
(insulated gate bipolar transistors) devices to control AC power.
Apparently, the dimming system works the opposite of SCRs and Triac dimmers,
in that the dimmer conducts at the start of the AC wave and shut off after
the appropriate amount of power has been delivered to the light. The system
is more resistant to dirty AC power, since it is not dependent on zero cross
detection. The dimmers use mathematical intergration to determine when the
correct amount of energy had been delivered.

  I was reading the lit (the company is Rosco - Entertainment Technologies)
and the IGBT method of dimming is much more efficient and produces much less
in the way of RF harmonics than does the SCR/Triac method. The dimmers also
apparently run much cooler than traditional models do.



At 06:01 PM 4/5/96 -0500, you wrote:
{Quote hidden}

**********************************************
* Andrew Yalowitz
* Theatrical Assistant
* University Unions and Student Activities
*
* ayalowitspamKILLspamvt.edu

1996\04\05@205304 by Andrew Yalowitz

flavicon
face
  There are some next generation theatrical dimmers that use IGBT
(insulated gate bipolar transistors) devices to control AC power.
Apparently, the dimming system works the opposite of SCRs and Triac dimmers,
in that the dimmer conducts at the start of the AC wave and shut off after
the appropriate amount of power has been delivered to the light. The system
is more resistant to dirty AC power, since it is not dependent on zero cross
detection. The dimmers use mathematical intergration to determine when the
correct amount of energy had been delivered.

  I was reading the lit (the company is Rosco - Entertainment Technologies)
and the IGBT method of dimming is much more efficient and produces much less
in the way of RF harmonics than does the SCR/Triac method. The dimmers also
apparently run much cooler than traditional models do.



At 06:01 PM 4/5/96 -0500, you wrote:
{Quote hidden}

**********************************************
* Andrew Yalowitz
* Theatrical Assistant
* University Unions and Student Activities
*
* .....ayalowitKILLspamspam.....vt.edu

1996\04\05@231548 by terogers

flavicon
face
Tom Sgouros wrote:
>
> I don't know if this is appropriate to this list, but the device that
> needs to do it is controlled by a PIC16C65...
>
> Does anyone know of an easy way to control the voltage on a 120VAC line? I
> need to vary the light from some lightbulbs, and I don't want to use the
> standard SCR solutions because they produce artifacts that seem to wear
> the bulbs out faster.
>
> & etc.

I can give you this much of a really good design my brother & I did
years ago:

Use a cheap optically isolated scr or triac AC relay. Engineer a signal
to the micro at the AC line zero crossing. Maintain a number that
corresponds to the desired intensity level: say, the number of cycles
out of 128 that you expect to deliver power. Construct an algorithm that
turns the relay on or off at each zero crossing such the the following
two requirements are met: 1) the average voltage is zero and 2) the
number of on cycles are evenly distributed throughout the 128 cycle
period.

This will give you an outstanding control of the outout power with no RF
switching funnies and long bulb life. The light intensity won't be
linear with the control number, and there is a specific problem that is
solvable with regard to maintaining the two above requirements in the
face of changing control values. I don't know if I can give any other
details, but if it seems important enough I might be able to find the
old code, decipher it and try to determine how much is too obvious to
protect (I did the work for another company).

Good luck - Tom Rogers  VP - R&D Time Tech, Inc.

1996\04\05@231548 by terogers

flavicon
face
Tom Sgouros wrote:
>
> I don't know if this is appropriate to this list, but the device that
> needs to do it is controlled by a PIC16C65...
>
> Does anyone know of an easy way to control the voltage on a 120VAC line? I
> need to vary the light from some lightbulbs, and I don't want to use the
> standard SCR solutions because they produce artifacts that seem to wear
> the bulbs out faster.
>
> & etc.

I can give you this much of a really good design my brother & I did
years ago:

Use a cheap optically isolated scr or triac AC relay. Engineer a signal
to the micro at the AC line zero crossing. Maintain a number that
corresponds to the desired intensity level: say, the number of cycles
out of 128 that you expect to deliver power. Construct an algorithm that
turns the relay on or off at each zero crossing such the the following
two requirements are met: 1) the average voltage is zero and 2) the
number of on cycles are evenly distributed throughout the 128 cycle
period.

This will give you an outstanding control of the outout power with no RF
switching funnies and long bulb life. The light intensity won't be
linear with the control number, and there is a specific problem that is
solvable with regard to maintaining the two above requirements in the
face of changing control values. I don't know if I can give any other
details, but if it seems important enough I might be able to find the
old code, decipher it and try to determine how much is too obvious to
protect (I did the work for another company).

Good luck - Tom Rogers  VP - R&D Time Tech, Inc.

1996\04\06@005614 by John Payson

flavicon
face
> Tom Sgouros wrote:
> > Does anyone know of an easy way to control the voltage on a 120VAC line? I
> > need to vary the light from some lightbulbs, and I don't want to use the
> > standard SCR solutions because they produce artifacts that seem to wear
> > the bulbs out faster.
>
> I can give you this much of a really good design my brother & I did
> years ago:
>
> Use a cheap optically isolated scr or triac AC relay. Engineer a signal
> to the micro at the AC line zero crossing. Maintain a number that
> corresponds to the desired intensity level: say, the number of cycles
> out of 128 that you expect to deliver power. Construct an algorithm that
> turns the relay on or off at each zero crossing such the the following
> two requirements are met: 1) the average voltage is zero and 2) the
> number of on cycles are evenly distributed throughout the 128 cycle
> period.

While I acknowlege that your approach would probably work, its output would
have a significant 1Hz flicker component which could induce flicker and also
reduce bulb life.  I would think it would be preferable to modulate the bulb
at a higher frequency [perhaps from a filtered DC supply, to eliminate 60Hz
flicker/wear], with a little LC filtering to minimize RF junk.

1996\04\06@005614 by John Payson

flavicon
face
> Tom Sgouros wrote:
> > Does anyone know of an easy way to control the voltage on a 120VAC line? I
> > need to vary the light from some lightbulbs, and I don't want to use the
> > standard SCR solutions because they produce artifacts that seem to wear
> > the bulbs out faster.
>
> I can give you this much of a really good design my brother & I did
> years ago:
>
> Use a cheap optically isolated scr or triac AC relay. Engineer a signal
> to the micro at the AC line zero crossing. Maintain a number that
> corresponds to the desired intensity level: say, the number of cycles
> out of 128 that you expect to deliver power. Construct an algorithm that
> turns the relay on or off at each zero crossing such the the following
> two requirements are met: 1) the average voltage is zero and 2) the
> number of on cycles are evenly distributed throughout the 128 cycle
> period.

While I acknowlege that your approach would probably work, its output would
have a significant 1Hz flicker component which could induce flicker and also
reduce bulb life.  I would think it would be preferable to modulate the bulb
at a higher frequency [perhaps from a filtered DC supply, to eliminate 60Hz
flicker/wear], with a little LC filtering to minimize RF junk.

1996\04\06@075528 by terogers

flavicon
face
John Payson wrote:
>
> > Tom Sgouros wrote:
> > > Does anyone know of an easy way to control the voltage on a 120VAC line? I

>
> While I acknowlege that your approach would probably work, its output would

> & more stuff..

Actually, no. There is a considerable amount of thermal inertia in even
small incandescents, and the practical lower illumination limits are at
high enough numbers that the flicker is not any more noticible than in
your monitor or conventionally dimmed lights. The trick is to evenly
distribute the half cycles while maintaining zero DC offset and also do
this smoothly in the presence of changing control values. In fact, we
designed this as a heater controller but ran light bulbs during debug
(old trick); the result was surprising to both of us. It ended up being
used elsewhere in a light controller.

Of course, higher frequency or DC control will do a better job, but at
the cost of complexity and (perhaps) noise. Our design did a really nice
job of controlling higher wattage floodlamps (for us higher wattage: 300
- 500) in a way that would have worked well on stage. In fact, I assumed
a similar scheme was being used commercially in some cases, because I
have seen a similar flicker signature in watching stage lights dimmed to
dark. You don't notice the flicker if you don't look at the lamps, and
you don't see it at all in high end controls.

-- Tom

1996\04\06@075528 by terogers

flavicon
face
John Payson wrote:
>
> > Tom Sgouros wrote:
> > > Does anyone know of an easy way to control the voltage on a 120VAC line? I

>
> While I acknowlege that your approach would probably work, its output would

> & more stuff..

Actually, no. There is a considerable amount of thermal inertia in even
small incandescents, and the practical lower illumination limits are at
high enough numbers that the flicker is not any more noticible than in
your monitor or conventionally dimmed lights. The trick is to evenly
distribute the half cycles while maintaining zero DC offset and also do
this smoothly in the presence of changing control values. In fact, we
designed this as a heater controller but ran light bulbs during debug
(old trick); the result was surprising to both of us. It ended up being
used elsewhere in a light controller.

Of course, higher frequency or DC control will do a better job, but at
the cost of complexity and (perhaps) noise. Our design did a really nice
job of controlling higher wattage floodlamps (for us higher wattage: 300
- 500) in a way that would have worked well on stage. In fact, I assumed
a similar scheme was being used commercially in some cases, because I
have seen a similar flicker signature in watching stage lights dimmed to
dark. You don't notice the flicker if you don't look at the lamps, and
you don't see it at all in high end controls.

-- Tom

1996\04\06@134733 by John Payson

flavicon
face
{Quote hidden}

Tom suggested in his post that the wear on the lights from an "ordinary"
dimmer was producing excessive wear (since I don't know the thermal mass/
emissivity (is that the word?) of the lights he's using.  I don't know how
much effective "intertia" they have, but was assuming that if it was a
problem at 120Hz it would also be a problem at 1Hz.

> Of course, higher frequency or DC control will do a better job, but at
> the cost of complexity and (perhaps) noise. Our design did a really nice
> job of controlling higher wattage floodlamps (for us higher wattage: 300
> - 500) in a way that would have worked well on stage. In fact, I assumed
> a similar scheme was being used commercially in some cases, because I
> have seen a similar flicker signature in watching stage lights dimmed to
> dark. You don't notice the flicker if you don't look at the lamps, and
> you don't see it at all in high end controls.

Well, it probably would be easier to keep the RF noise in check if you
limitted switching times (both on and off) to the zero-crossings.  Or if,
as someone suggested, you use an IGBiT and let lights have the "early"
part of the cycle (in which case your turn-on is clean, and your turn-off
can be cleaned up a little with a capacitor if you drive the lights with
full-wave rectified unfiltered DC).

By the way, why the concern about maintaining zero DC offset?  I can think
of a few ways to do this (some very easy) but am not quite clear of the
purpose.  The simplest way I can think of would be something like this:

Ph0:    ds      1
Ph1:    ds      1
Val:    ds      1

Init:
       clrf    Ph0
       movlw   $80
       movwf   Ph1
       retlw   0

HalfCycle:
       movf    Val,w
       addwf   Ph0

       btfss   C
        bcf    Output
       btfsc   C
        bsf    Output
                       ;       Ph0     Ph1     W
       movf    Ph0,w   ;       Ph0     Ph1     Ph0
       xorwf   Ph1,w   ;       Ph0     Ph1     0^1
       xorwf   Ph1     ;       Ph0     Ph0     0^1
       xorwf   Ph0     ;       Ph1     Ph0     0^1

       retlw   0

1996\04\06@134733 by John Payson

flavicon
face
{Quote hidden}

Tom suggested in his post that the wear on the lights from an "ordinary"
dimmer was producing excessive wear (since I don't know the thermal mass/
emissivity (is that the word?) of the lights he's using.  I don't know how
much effective "intertia" they have, but was assuming that if it was a
problem at 120Hz it would also be a problem at 1Hz.

> Of course, higher frequency or DC control will do a better job, but at
> the cost of complexity and (perhaps) noise. Our design did a really nice
> job of controlling higher wattage floodlamps (for us higher wattage: 300
> - 500) in a way that would have worked well on stage. In fact, I assumed
> a similar scheme was being used commercially in some cases, because I
> have seen a similar flicker signature in watching stage lights dimmed to
> dark. You don't notice the flicker if you don't look at the lamps, and
> you don't see it at all in high end controls.

Well, it probably would be easier to keep the RF noise in check if you
limitted switching times (both on and off) to the zero-crossings.  Or if,
as someone suggested, you use an IGBiT and let lights have the "early"
part of the cycle (in which case your turn-on is clean, and your turn-off
can be cleaned up a little with a capacitor if you drive the lights with
full-wave rectified unfiltered DC).

By the way, why the concern about maintaining zero DC offset?  I can think
of a few ways to do this (some very easy) but am not quite clear of the
purpose.  The simplest way I can think of would be something like this:

Ph0:    ds      1
Ph1:    ds      1
Val:    ds      1

Init:
       clrf    Ph0
       movlw   $80
       movwf   Ph1
       retlw   0

HalfCycle:
       movf    Val,w
       addwf   Ph0

       btfss   C
        bcf    Output
       btfsc   C
        bsf    Output
                       ;       Ph0     Ph1     W
       movf    Ph0,w   ;       Ph0     Ph1     Ph0
       xorwf   Ph1,w   ;       Ph0     Ph1     0^1
       xorwf   Ph1     ;       Ph0     Ph0     0^1
       xorwf   Ph0     ;       Ph1     Ph0     0^1

       retlw   0

1996\04\06@220655 by terogers

flavicon
face
John:

DC offset contributes to uneven emission of from the filament and early
failure. In heaters (this started as a heater controller) there is the
added detriment of uneven power distribution. (This from our heater
manufacturer's engineer; we called him to discuss an observed uneven
temperature distribution).

I'm still looking for my copy of the code. I recall the final result
being fairly trick, but I don'e think it's unique enough to deserve
protection.

-- Tom

1996\04\06@220655 by terogers

flavicon
face
John:

DC offset contributes to uneven emission of from the filament and early
failure. In heaters (this started as a heater controller) there is the
added detriment of uneven power distribution. (This from our heater
manufacturer's engineer; we called him to discuss an observed uneven
temperature distribution).

I'm still looking for my copy of the code. I recall the final result
being fairly trick, but I don'e think it's unique enough to deserve
protection.

-- Tom

1996\04\07@124531 by Bill Cornutt

flavicon
face
----------

>Use a cheap optically isolated scr or triac AC relay. Engineer a signal
>to the micro at the AC line zero crossing. Maintain a number that
>corresponds to the desired intensity level: say, the number of cycles
>out of 128 that you expect to deliver power. Construct an algorithm that
>turns the relay on or off at each zero crossing such the the following
>two requirements are met: 1) the average voltage is zero and 2) the
>number of on cycles are evenly distributed throughout the 128 cycle
>period.
>
>Good luck - Tom Rogers  VP - R&D Time Tech, Inc.


A easy way to determine which cycles to turn on is to use 1 to 255
for the intensity level.  One being the least, 255 being the most.
Then at the start of each cycle, add the intesity level to a register.
If there is a carry, use that cycle.

Bill C.

1996\04\07@124531 by Bill Cornutt

flavicon
face
----------

>Use a cheap optically isolated scr or triac AC relay. Engineer a signal
>to the micro at the AC line zero crossing. Maintain a number that
>corresponds to the desired intensity level: say, the number of cycles
>out of 128 that you expect to deliver power. Construct an algorithm that
>turns the relay on or off at each zero crossing such the the following
>two requirements are met: 1) the average voltage is zero and 2) the
>number of on cycles are evenly distributed throughout the 128 cycle
>period.
>
>Good luck - Tom Rogers  VP - R&D Time Tech, Inc.


A easy way to determine which cycles to turn on is to use 1 to 255
for the intensity level.  One being the least, 255 being the most.
Then at the start of each cycle, add the intesity level to a register.
If there is a carry, use that cycle.

Bill C.

1996\04\08@164510 by Hugo Ahrens

picon face
>John:
>
>DC offset contributes to uneven emission of from the filament and early
>failure. In heaters (this started as a heater controller) there is the
>added detriment of uneven power distribution. (This from our heater
>manufacturer's engineer; we called him to discuss an observed uneven
>temperature distribution).
>.......

Unless there is an inductive effect in the heater due to the winding of the
heater wire, there should be no difference in AC powered average or DC
powering of a heater. BUT, the reason power people are unhappy about DC
loading of an AC system is due to the effects on the transformer feeding the
load. DC loading of a transformer will partially saturate the transformer
core and cause distortion on the load-side voltage waveform. This of course
will affect any other load on the same transformer secondary.

Hugo

1996\04\08@193303 by Mike Keitz

flavicon
face
>>John:
>>
>>DC offset contributes to uneven emission of from the filament and early
>>failure. In heaters (this started as a heater controller) there is the
>>added detriment of uneven power distribution. (This from our heater
>>manufacturer's engineer; we called him to discuss an observed uneven
>>temperature distribution).
>>.......
>
>Unless there is an inductive effect in the heater due to the winding of the
>heater wire, there should be no difference in AC powered average or DC
>powering of a heater.

True.  Unless you are supplying AC at a frequency high enough to cause
standing waves in the wire, a heater or filament is going to heat up
uniformly and evenly either DC or AC.  I would think that any shortening of
the life of bulbs with a conventional Triac phase slicer would be due to
mechanical vibration (sometimes the bulbs can be heard to rattle or buzz).
Also, halogen bulbs should not be operated at less than 90% of rated voltage
as the life prolonging "halogen cycle" won't work properly.  This is going
to be a problem no matter what waveform is used, but it is probably only a
concern for very prolonged operation in the range just below 90%.

>BUT, the reason power people are unhappy about DC
>loading of an AC system is due to the effects on the transformer feeding the
>load. DC loading of a transformer will partially saturate the transformer
>core and cause distortion on the load-side voltage waveform. This of course
>will affect any other load on the same transformer secondary.

In Europe, non-linear loads of any sort are discouraged, thus the rush of IC
manufacturers to produce "Unity Power Factor Controllers" to prevent uneven
loading from a bridge rectifier fed switching power supply.  The bridge
rectifier flattens the peaks of the sine wave and causes harmonics to form.
Drawing only one polarity of current (a DC component) is even worse.  I
think you can still get away with it in the US, though it is something to
consider when thousands of watts are involved.

The topic of lamp dimming was discussed here last summer as well, anyone
interested would be wise to check the archives rather than go through it all
over again.

-Mike

1996\04\09@014720 by John Payson

flavicon
face
> Tom Sgouros wrote:
> > Does anyone know of an easy way to control the voltage on a 120VAC line? I
> > need to vary the light from some lightbulbs, and I don't want to use the
> > standard SCR solutions because they produce artifacts that seem to wear
> > the bulbs out faster.
>
> I can give you this much of a really good design my brother & I did
> years ago:
>
> Use a cheap optically isolated scr or triac AC relay. Engineer a signal
> to the micro at the AC line zero crossing. Maintain a number that
> corresponds to the desired intensity level: say, the number of cycles
> out of 128 that you expect to deliver power. Construct an algorithm that
> turns the relay on or off at each zero crossing such the the following
> two requirements are met: 1) the average voltage is zero and 2) the
> number of on cycles are evenly distributed throughout the 128 cycle
> period.

While I acknowlege that your approach would probably work, its output would
have a significant 1Hz flicker component which could induce flicker and also
reduce bulb life.  I would think it would be preferable to modulate the bulb
at a higher frequency [perhaps from a filtered DC supply, to eliminate 60Hz
flicker/wear], with a little LC filtering to minimize RF junk.

1996\04\09@014733 by terogers

flavicon
face
Tom Sgouros wrote:
>
> I don't know if this is appropriate to this list, but the device that
> needs to do it is controlled by a PIC16C65...
>
> Does anyone know of an easy way to control the voltage on a 120VAC line? I
> need to vary the light from some lightbulbs, and I don't want to use the
> standard SCR solutions because they produce artifacts that seem to wear
> the bulbs out faster.
>
> & etc.

I can give you this much of a really good design my brother & I did
years ago:

Use a cheap optically isolated scr or triac AC relay. Engineer a signal
to the micro at the AC line zero crossing. Maintain a number that
corresponds to the desired intensity level: say, the number of cycles
out of 128 that you expect to deliver power. Construct an algorithm that
turns the relay on or off at each zero crossing such the the following
two requirements are met: 1) the average voltage is zero and 2) the
number of on cycles are evenly distributed throughout the 128 cycle
period.

This will give you an outstanding control of the outout power with no RF
switching funnies and long bulb life. The light intensity won't be
linear with the control number, and there is a specific problem that is
solvable with regard to maintaining the two above requirements in the
face of changing control values. I don't know if I can give any other
details, but if it seems important enough I might be able to find the
old code, decipher it and try to determine how much is too obvious to
protect (I did the work for another company).

Good luck - Tom Rogers  VP - R&D Time Tech, Inc.

1996\04\09@022514 by Andrew Yalowitz

flavicon
face
  There are some next generation theatrical dimmers that use IGBT
(insulated gate bipolar transistors) devices to control AC power.
Apparently, the dimming system works the opposite of SCRs and Triac dimmers,
in that the dimmer conducts at the start of the AC wave and shut off after
the appropriate amount of power has been delivered to the light. The system
is more resistant to dirty AC power, since it is not dependent on zero cross
detection. The dimmers use mathematical intergration to determine when the
correct amount of energy had been delivered.

  I was reading the lit (the company is Rosco - Entertainment Technologies)
and the IGBT method of dimming is much more efficient and produces much less
in the way of RF harmonics than does the SCR/Triac method. The dimmers also
apparently run much cooler than traditional models do.



At 06:01 PM 4/5/96 -0500, you wrote:
{Quote hidden}

**********************************************
* Andrew Yalowitz
* Theatrical Assistant
* University Unions and Student Activities
*
* EraseMEayalowitspam_OUTspamTakeThisOuTvt.edu

1996\04\09@022712 by Tom Sgouros

flavicon
face
I don't know if this is appropriate to this list, but the device that
needs to do it is controlled by a PIC16C65...

Does anyone know of an easy way to control the voltage on a 120VAC line? I
need to vary the light from some lightbulbs, and I don't want to use the
standard SCR solutions because they produce artifacts that seem to wear
the bulbs out faster.

I understand how to do D/A to get a variable DC voltage up to some small
voltage. How can I get from there to the 120VAC?

A cryptic remark from someone has led me to believe that there may be a
solution involving a PWM signal and some power MOSFETs. I think I can see
how that would work, but has anyone done anything like it who could
reassure me that I'm not going to fry myself or my PIC?

-Tom

p.s. Many thanks to all for the help with my crystal questions. The
crystal was bad and I misread the capacitance of the capacitors, and I
didn't realize how much my scope probe would confuse things. It's all fine
now...

---------------------------------------------------------------------
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1996\04\09@082701 by Keith Dowsett

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<snip..>


>In Europe, non-linear loads of any sort are discouraged, thus the rush of IC
>manufacturers to produce "Unity Power Factor Controllers" to prevent uneven
>loading from a bridge rectifier fed switching power supply.  The bridge
>rectifier flattens the peaks of the sine wave and causes harmonics to form.
>Drawing only one polarity of current (a DC component) is even worse.  I
>think you can still get away with it in the US, though it is something to
>consider when thousands of watts are involved.

<snip..>

>-Mike

Somewhere in the back of my mind is the thought that for big electricity
consumers the electricity supply companies impose quite a hefty surcharge
for non-unity power factors. This is why large users often have their own
'capacitor farms' somewhere on site.

On a more domestic note I recall some discussion about the first generation
of low energy lightbulbs having a significant dc component. Since domestic
meters don't accurately measure dc current from the supply these bulbs were
unexpectedly cheap to run.

Just a few idle musings,

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail:      @spam@kdowsettKILLspamspamrpms.ac.uk
Phone:       0181-740-3162
Fax:         0181-743-3987

Snail mail:  MRC Clinical Sciences Centre, Cyclotron Unit.
                Hammersmith Hospital. London W12 0NN.

1996\04\09@213222 by Martin J. Maney

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On Sat, 6 Apr 1996, terogers wrote:

> DC offset contributes to uneven emission of from the filament and early
> failure. In heaters (this started as a heater controller) there is the
> added detriment of uneven power distribution. (This from our heater
> manufacturer's engineer; we called him to discuss an observed uneven
> temperature distribution).

It will also make the transformers feeding you your power rather unhappy
if, as might easily be the case for high-power loads like heaters or
theatrical lighting, this assymetrical load were a large fraction of the
total branch load.  Imagine a dimly-lit scene with a great many lamps all
pulling a single half-cycle every couple cycles - the same half-cycle if
they're all controlled by a single controller that doesn't take pains
to avoid such synchrony.  I can almost hear the oil in the transformer
bubbling from here...  :-)

1996\04\09@223411 by Ken Parkyn

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>Does anyone know of an easy way to control the voltage on a 120VAC line? I
>need to vary the light from some lightbulbs, and I don't want to use the
>standard SCR solutions because they produce artifacts that seem to wear
>the bulbs out faster.

text clipped

How about driving a 6vac to 120vac mains transformer **secondary**  via a
darlington?
This would depend on how much current your 120vac lamp pulls...but it may work
Cyrus
I don't mind dying, I just don't want to be there when it happens...

1996\04\10@090907 by terogers

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Martin J. Maney wrote:
>
> It will also make the transformers feeding you your power rather unhappy
> if, as might easily be the case for high-power loads like heaters or
> theatrical lighting, this assymetrical load were a large fraction of the
> total branch load.  Imagine a dimly-lit scene with a great many lamps all
> pulling a single half-cycle every couple cycles - the same half-cycle if
> they're all controlled by a single controller that doesn't take pains
> to avoid such synchrony.  I can almost hear the oil in the transformer
> bubbling from here...  :-)

Yup, you're right there.

About the DC offset & uneven filament & heater wear: I can imagine real
world physics reasons for the filament effect (filament ions boiling off
the surface being one of the primary sources of decay, & ions subject to
the field around the filament) but I can't come up with a logical reason
for the effect on ceramic packed metal sheathed heaters...

Oh, well, not really a PIC question anymore. I'm still looking at my
original HOTCON code, written in assembler for the 6502.  -- Tom

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