RMS approximation of Square/PWM signal

[JMini]

Does anyone have a way to approximate and RMS voltage from a PWM signal.
I want to regulate the RMS voltage sent to a lightbulb, but do not have the
room for the inductor and capacitor required to carry 20+ amps of current. I
need to feedback a voltage to the PWM controller that is proportional to the
RMS voltage of the output. I don't need an EXACT RMS conversion, rather a
ratio would be fine since I can adjust the actual voltage fed back to the PWM
controller via a resistor divider. For this current design, it will be
operating at about 40kHz.

 

[JMini]

square-wave, duty cycle D = Ton/Toff

RMS = peak*sqrt(D)

Cheers
Terry

I know how to calculate it on paper. I'm trying to electronically approximate
it to use as a feedback voltage for a PWM controller/regulator. I'm trying to
avoid using an RMS converter chip.

 

[Phil Allison]

"JMini"

I know how to calculate it on paper. I'm trying to electronically
approximate
it to use as a feedback voltage for a PWM controller/regulator. I'm trying
to
avoid using an RMS converter chip.

** RMS to DC converters probably lack sufficient bandwidth for 40 kHz PWM
anyhow.

A small lamp connected across the PWM output with a CdS cell sensing the
light level would give you a feedback mechanism that could then be trimmed
to give the desired rms voltage output value.

BTW with 40 kHz PWM at 20amps - I hope your cable runs are short.



....... Phil

 

[[email protected]]

square-wave, duty cycle D = Ton/Toff

This is wrong - duty cycle is D= Ton/(Ton+Toff)

Ton/Toff is the mark to space ratio (MS) which isn't immediately
useful for this kind of calculation. Obviously, it can be greater than
one.

D= MS/(MS+1/MS)

Happily, if you define D correctly, Terry's conclusion is correct.

RMS = peak*sqrt(D)

If the pulse width modulation was very slow - slower than the thermal
time constant of the filament - the situation could be complicated by
the fact that the filament would start off with a relatively low
resistance,which would rise to normal operating levels (ticket output)
during the duration of the "on" pulse, but this is a couple of orders
of magnitude slower than 40kHz.

 

[Phil Allison]

"Jamie" = Maynard A. Philbrook KA1LPA

Fuckwit radio ham and bad code scribbler by trade.

Use a R and C network to yield you the DC Loop back voltage.

** Gives the average value - not the rms one the OP is after.

At 50% duty cycle. the output is 50%.

** ROTFL

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




...... Phil

 

[Paul]

Does anyone have a way to approximate and RMS voltage from a PWM signal.
I want to regulate the RMS voltage sent to a lightbulb, but do not have the
room for the inductor and capacitor required to carry 20+ amps of current. I
need to feedback a voltage to the PWM controller that is proportional to the
RMS voltage of the output. I don't need an EXACT RMS conversion, rather a
ratio would be fine since I can adjust the actual voltage fed back to the PWM
controller via a resistor divider. For this current design, it will be
operating at about 40kHz.

It's a square wave? If it is, and it's unipolar (0-110v), then
average and RMS should be the same. Sample your voltage with a voltage
divider, and put in a RC timeconstant to average the signal. Choose a
reasonable timeconstant, say 1/10 sec (lightbulb itself isn't all that
fast, so there's no point putting in a short time constant).
If the drive is "bipolar" (AC square wave), you can rectify the
sample voltage, just make sure that the charge/discharge times in your
diode RC circuit are similiar so you get an average and not a peak.
If by PWM you mean a chopped up sine wave like you get from an SCR
or TRIAC controller, then put in a small (1/2 w) load resistor, and
strap a thermistor to it. Use the thermistor in a voltage divider from
DC, and effectively use your controller to regulate the temperature of
the resistor! You will need another similiar circuit to make a full
wheatstone bridge to reduce the effects of ambient temperature. This
circuit is truly RMS, and will take all kinds of frequencies, crest
factors, etc.
You could use a photocell in the above circuit instead, thereby
regulating the light level!
-Paul

 

[Jamie]

I know how to calculate it on paper. I'm trying to electronically approximate
it to use as a feedback voltage for a PWM controller/regulator. I'm trying to
avoid using an RMS converter chip.

Use a R and C network to yield you the DC Loop back voltage.

At 50% duty cycle. the output is 50%. if external loading is going
to be an issue that may cause this to drop even more then Use the
RC network to drive an ADC input on the Uc. Since this same Uc knows
the current duty cycle, you should be able to determine the actual
voltage on output and loading..

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

 

[Phil Allison]

It's a square wave? If it is, and it's unipolar (0-110v), then
average and RMS should be the same.

** 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.




...... Phil

 

[JMini]

"JMini"


** RMS to DC converters probably lack sufficient bandwidth for 40 kHz PWM
anyhow.

A small lamp connected across the PWM output with a CdS cell sensing the
light level would give you a feedback mechanism that could then be trimmed
to give the desired rms voltage output value.

BTW with 40 kHz PWM at 20amps - I hope your cable runs are short.



...... Phil

It's funny you suggest the lamp/CdS cell. I had thought of that, but thought
the response would be to slow during startup. Also, size is a huge factor.

 

[Phil Allison]

"Jamie" = Maynard A. Philbrook KA1LPA

Fuckwit radio ham and bad code scribbler by trade.

Phil, Please go back to school and learn how to read peoples request
of what they're trying to do, dick head.

** You are an absolute FUCKWIT Philbrook.

FOAD.



....... Phil

 

[Jamie]

"Jamie" = Maynard A. Philbrook KA1LPA

Fuckwit radio ham and bad code scribbler by trade.






** Gives the average value - not the rms one the OP is after.






** ROTFL

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




..... Phil

Phil, Please go back to school and learn how to read peoples request
of what they're trying to do, dick head.

And get your nose out of the text books. Because it's obvious that
most of that vile you spuke out comes from nothing but text and not
experience..

Reading about it and actually doing something with it are two
different worlds boy!


P.S.
No, I won't be all upset with your come back, It'll be the usual
Phil Vile with nothing but dog shit comments and lack of content.

Mostly old one's because you haven't an imagination to think up some
new material to keep it interesting.

But then again, with you it's always been a circus show so why should
we expect any better?

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

 

[JMini]

It's a square wave? If it is, and it's unipolar (0-110v), then
average and RMS should be the same. Sample your voltage with a voltage
divider, and put in a RC timeconstant to average the signal. Choose a
reasonable timeconstant, say 1/10 sec (lightbulb itself isn't all that
fast, so there's no point putting in a short time constant).
If the drive is "bipolar" (AC square wave), you can rectify the
sample voltage, just make sure that the charge/discharge times in your
diode RC circuit are similiar so you get an average and not a peak.
If by PWM you mean a chopped up sine wave like you get from an SCR
or TRIAC controller, then put in a small (1/2 w) load resistor, and
strap a thermistor to it. Use the thermistor in a voltage divider from
DC, and effectively use your controller to regulate the temperature of
the resistor! You will need another similiar circuit to make a full
wheatstone bridge to reduce the effects of ambient temperature. This
circuit is truly RMS, and will take all kinds of frequencies, crest
factors, etc.
You could use a photocell in the above circuit instead, thereby
regulating the light level!
-Paul

While somone else has already addressed your RMS misinterpretation, I had
thought of the resistor/thermistor idea. A 1206 SMD package resistor with a
NTC thermistor superglued to it. The problem there would be that this
regulator would be fitted inside a 30mm round x 6mm high cavity. The heat
from the D2PAK FET, just mm away, would corrupt the reading of the
thermistor. So the linger this thing was running, the dimmer the output bulb
would get because the thermistor would be getting heat from a source other
than the resistor on which it's mounted.

 

[Jamie]

** 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.




..... Phil

You're so fucking clue less. Please stop and get back
on track.
I'm getting a hernia..

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

 

[Phil Allison]

"JMini"

It's funny you suggest the lamp/CdS cell. I had thought of that, but
thought
the response would be to slow during startup. Also, size is a huge factor.

** 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

 

[Phil Allison]

"Jamie" = Maynard A. Philbrook KA1LPA

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

 

[Phil Allison]

"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

 

[Phil Allison]

"John Larkin"
"JMini"

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

** ???????????

The OP does NOT know the duty cycle at any particular time - that is a
variable in the system.



...... Phil

 

[Phil Allison]

"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

 

[Jamie]

"Jamie" = Maynard A. Philbrook KA1LPA

Fuckwit radio ham and bad code scribbler by trade.





** You are an absolute FUCKWIT Philbrook.

FOAD.



...... Phil

I feel so good that you need to address me by my real
name and not the alias we use here. So let me return
the favor and do the same for you..


Phil/Phillis = Mr/Miss. Asshole!. Depends on what day of
the week.

That is all his mother could come up with before she
through him/her out with the bath water..

In this case, A child that not even a mother could love!

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

 

[Phil Allison]

"JMini"

While somone else has already addressed your RMS misinterpretation, I had
thought of the resistor/thermistor idea. A 1206 SMD package resistor with
a
NTC thermistor superglued to it. The problem there would be that this
regulator would be fitted inside a 30mm round x 6mm high cavity. The heat
from the D2PAK FET, just mm away, would corrupt the reading of the
thermistor. So the linger this thing was running, the dimmer the output
bulb
would get because the thermistor would be getting heat from a source other
than the resistor on which it's mounted.

** 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