RMS approximation of Square/PWM signal

[Jamie]

You know the duty cycle. Lowpass filter the load voltage, measure that
average voltage, do the math.

John

No wait John, I'm being blasted by yours truly for suggesting the same
thing.

wait your turn, I'm not done getting blasted yet!


http://webpages.charter.net/jamie_5"

 

[Phil Allison]

Jamie" = Maynard A. Philbrook KA1LPA

A fuckwit, asshole radio ham and bad code scribbler by trade.

Radio hams don't even know the significance of rms values.


You are an absolute FUCKWIT Philbrook.

FOAD you retarded, autistic POS cretin.




....... Phil

 

[Jamie]

"Jamie" = Maynard A. Philbrook KA1LPA

A fuckwit radio ham and bad code scribbler by trade.

Radio hams don't even know the significance of rms values.


** You are an absolute FUCKWIT Philbrook.

FOAD you retarded, autistic cretin.




...... Phil

Yawn!
Same boring shit from demented Phill/Phillis, different day.

I knew he/she didn't have anything else to offer..


http://webpages.charter.net/jamie_5"

 

[Phil Allison]

" Terry Given "


** Hey sheep fucker.

You and the Maynard A. Philbrook septic, autistic cretin ought to get on
real well.

You have a lot in common.

Same genetic mental defect for a start.

Plus you are both complete arseholes.




...... Phil

 

[Phil Allison]

"John Larkin"

But he's controlling it... and he doesn't know it?

** You are the one to say how he knows it.

Cos that is your claim.




....... Phil

 

[Fred Bartoli]

Terry Given a écrit :

how wide a range do you need to regulate over? average the square wave,
then run it thru a suitable nonlinear amplifier to get a crude
approximation.

or: use a pair of multipliers. square the sensed voltage & average
(LPF), then square the setpoint prior to your controller, IOW control
Vrms^2 not Vrms. squaring is a lot easier than sqrting

Squaring is dead easy and you don't need multipliers.
Just double integrate the sensed voltage (it's roughly a constant) then
average the sampled value at the turning off time. Obviously reset the
integrators at the turning on point.
With enough luck (I didn't check) and the right values this might be as
simple as a pair of switches and a pair of LPFs.

 

[Fred Bartoli]

Terry Given a écrit :

Cute, Fred!

a boost PFC using V^2 rather than V is a linear control system. no
small-signal perturbation required. this is, IIRC, called "input
linearisation" and is the easy way of building nonlinear controllers
(thanks mr Slotine)

you must have read some of Keyue Smedley's & Dragan Maksimovic's
one-cycle control papers

Yes I remember I did (Maksimovic for sure, others don't sound familiar)
but I didn't though about them. Rather I have a nice collection of old
70's semiconductors appnotes (you know the spirit: do everything with
nothing) that I have pretty well assimilated when young (in French I'd
have said 'integrated' but I don't know if English carries the same
meaning) which bent my mind quite a bit make all this natural for me.

Now, how do you as easily invert some value?

 

[JMini]

"JMini"



** A moment ago you thought a small lamp did not respond fast enough ?

Got a clue how slow that R plus NTC combo would have been ?

BTW- what sort of mad design is this ?

A tiny 30 x 6mm cavity for a complete 20 amp PWM regulator ????

But the lamp is right next to it too.



...... Phil

Yeah. I figured the response of the resistor/NTC would be worse. I was really
just thinking of anything that could read an RMS value.
It's a design to fit inside a customized D cell maglite. Some of the real
wacko designs run as high as 200+ watts (10-10.5A) The battery is no further
than 1.5 inches away, and the bulb is only about 1-1.5 Inches away. I can use
an RMS converter that requires only one external cap and a few external
voltage divider resistors. I was hoping someone could come up with an idea
like "Oh sure, you use an op-amp and a cap like in the attached schematic!
F*wit newbie!" There are some great thoughts in the thread, but most have a
parts count higher than what I'm using now. I guess I'll get into the shop
and stat fiddling with the RMS converter feedback to the Ti TL5001.

 

[JMini]

If he's generating it digitally, like from a uP PWM controller, he
knows. If the system is all analog, it's still not hard to measure the
duty cycle, but the math gets hard to do.

John

That's just it. I'm using a TL5001 PWM controller, Not a uC. I just don't
have the skills to implement a uC. I'd love to learn how. The AVR Tiny85
looks pretty nice. But I have no idea how to program it or how to write code
to do regulation with PWM.

 

[JMini]

"JMini"



** A small size ( 1 or 2 watt) lamp will light up way faster than the main
20 amp load one - plus the PWM needs to ramp up from zero to avoid a large
surge in current at start up.

If your PWM drive snaps on there will be a 200 amp plus surge due to the
tempco of tungsten.

Also, a CdS cell plus lamp ain't very big - a tad larger than a 1W
resistor.

Time you came up with more details - pal.

What is the DC supply voltage and source ?

Will the 20 amp lamp be very close to the PWM circuit or not?

What duty cycle will be typical ?



..... Phil

OK. It's a PWM regulator for a flashlight. It has to be small to fit inside.
Voltage range at input will be between 6 and 30V depending on battery pack
chosen. Output voltage will be between 7.2 and ~25V RMS. Duty cycle will vary
widely. The PWM controller I'm using is the Ti TL5001. It has a built in
programmable soft-start.

 

[Paul G.]

** Oh no - not another one with no clue what " rms value " is.

The only time " rms " and average value are the same is for a DC voltage.

and if it's a square wave - with 50% duty cycle, you'll get 1/2 the
power, or equivalent .707 times the peak voltage. For the RMS (root of
the mean of the square), you'll get v^2, the mean is d*V^2 (d=duty
cycle), and the root is sqrt(d*v^2) or sqrt(d)*v.
For a 50% duty cycle, 70.7% of the square wave top.
Mea culpa!

-Paul

 

[Phil Allison]

"JMini"

Yeah. I figured the response of the resistor/NTC would be worse. I was
really
just thinking of anything that could read an RMS value.
It's a design to fit inside a customized D cell maglite. Some of the real
wacko designs run as high as 200+ watts (10-10.5A) The battery is no
further
than 1.5 inches away, and the bulb is only about 1-1.5 Inches away.

** The whole thing must get very hot - or is all the IR light reflected
away by the mirror?

I can use
an RMS converter that requires only one external cap and a few external
voltage divider resistors. I was hoping someone could come up with an idea
like "Oh sure, you use an op-amp and a cap like in the attached schematic!
F*wit newbie!" There are some great thoughts in the thread, but most have
a
parts count higher than what I'm using now. I guess I'll get into the shop
and stat fiddling with the RMS converter feedback to the Ti TL5001.

** Why on earth have you selected 40kHz as the PWM frequency ????

With no LC filtering to consider - you should be using a frequency more
like 400 Hz.

Then, even the most basic ( ie low supply current) ) RMS to DC converter IC
s will work like a charm.



...... Phil

 

[Phil Allison]

"Phil Allison"

"JMini"


** Why on earth have you selected 40kHz as the PWM frequency ????

** I see it is a limitation of the TL5001 that has you stuffed - it was
never intended for what you are doing,

Better go find another PWM IC that will run at low frequencies.



...... Phil

 

[Phil Allison]

"JMini"

OK. It's a PWM regulator for a flashlight. It has to be small to fit
inside.
Voltage range at input will be between 6 and 30V depending on battery pack
chosen. Output voltage will be between 7.2 and ~25V RMS. Duty cycle will
vary
widely. The PWM controller I'm using is the Ti TL5001.

** When the duty cycle is low - you need to watch out how high the pulse
current becomes.

Example:

V batt = 30 volts, V out = 7.2 at 100 watts

Duty cycle = ( 7.2 / 30 ) squared = 0.0576 or 5.76%

I peak = V batt / R lamp = 30 / 0.52 = 57.8 amps.

That is way too high.


...... Phil

 

[JMini]

"JMini"


** When the duty cycle is low - you need to watch out how high the pulse
current becomes.

Example:

V batt = 30 volts, V out = 7.2 at 100 watts

Duty cycle = ( 7.2 / 30 ) squared = 0.0576 or 5.76%

I peak = V batt / R lamp = 30 / 0.52 = 57.8 amps.

That is way too high.


..... Phil

An excellent thought. Generally though, 7.2V bulbs are pretty much done at
50W. Not that your point isnt valid. In application, the Vbatt will not be
all that much higher than Vbulb. I can make the stipulation that the Vbatt to
Vbulb ration should not exceed 2:1.
The switching FET I plan on using for really high outputs is the IRF2804S.
I've used it in a PWM softstarter design I built to run 600W labp at 30V.

 

[JMini]

"JMini"


** The whole thing must get very hot - or is all the IR light reflected
away by the mirror?




** Why on earth have you selected 40kHz as the PWM frequency ????

With no LC filtering to consider - you should be using a frequency more
like 400 Hz.

Then, even the most basic ( ie low supply current) ) RMS to DC converter
IC s will work like a charm.



..... Phil

The TL5001 won't run under 40kHz. It has a wide operating voltage, a built in
softstart, and will go to 100% duty.

I've begun looking at other controllers that can get near, but not quite,
100% duty. I'd love to run this thing at 400Hz. Less switching loss.
The TI TPS40200 is looking interesting. Your thoughts?

The electronics are under a reflector, ceramic fiber insulation, and an
aluminum cover.