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RMS and Average Current

J

JH

I've not posted to this forum before, hope this is not too dumb of a
question!

Should I use RMS current or Average current in finding the power on a
diode? Say I have a pulse current waveform through the diode. The RMS
value of this waveform is different than the average value. Which do I
use? If I use the RMS value, do I also need to use the RMS value of
the voltage across the diode? If I use the average current, do I then
use the forward diode voltage with it to get the diode average power or
do I need to find the average diode voltage instead? There is some
confusion!

Thanks in advance
Jeff
 
P

Pooh Bear

JH said:
I've not posted to this forum before, hope this is not too dumb of a
question!

Should I use RMS current or Average current in finding the power on a
diode? Say I have a pulse current waveform through the diode. The RMS
value of this waveform is different than the average value. Which do I
use? If I use the RMS value, do I also need to use the RMS value of
the voltage across the diode? If I use the average current, do I then
use the forward diode voltage with it to get the diode average power or
do I need to find the average diode voltage instead? There is some
confusion!
Click to expand...

Average.

Graham
 
J

John Larkin

I've not posted to this forum before, hope this is not too dumb of a
question!
Click to expand...

There are no dumb questions... only dumb people. (Smilie)
Should I use RMS current or Average current in finding the power on a
diode? Say I have a pulse current waveform through the diode. The RMS
value of this waveform is different than the average value. Which do I
use? If I use the RMS value, do I also need to use the RMS value of
the voltage across the diode? If I use the average current, do I then
use the forward diode voltage with it to get the diode average power or
do I need to find the average diode voltage instead? There is some
confusion!
Click to expand...

Neither. A diode is nonlinear, so neither works. There's no "R" to
plug into P = I^2 * R

The power is the average of the instantaneous series of E*I samples,
which can get messy to compute.

If it's a clean rectangular pulse, measure pulse current and
simultaneous diode voltage drop, and do

P = E * I * n

where n is the duty cycle.

John
 
P

Phil Hobbs

JH said:
I've not posted to this forum before, hope this is not too dumb of a
question!

Should I use RMS current or Average current in finding the power on a
diode? Say I have a pulse current waveform through the diode. The RMS
value of this waveform is different than the average value. Which do I
use? If I use the RMS value, do I also need to use the RMS value of
the voltage across the diode? If I use the average current, do I then
use the forward diode voltage with it to get the diode average power or
do I need to find the average diode voltage instead? There is some
confusion!

Thanks in advance
Jeff
Click to expand...

That isn't a dumb question at all--probably in the 75th percentile, if we
include politics.

Electrical power dissipation in any device is the time average of V*I. For a
resistor, which is very linear, V = IR, so the power dissipation is the time
average of I^2*R. RMS is short for "root mean square", i.e. you time-average
the square of the current, and take the square root of the average value.
For a linear resistive load, the RMS value is convenient because when you
plug it into the I^2*R formula, it gives you precisely the time average of
V*I, which you can easily verify from the definition. (Reactive loads are a
bit more complicated because V and I get out of phase with each other.)

For a nonlinear device such as a diode, the power dissipation is still the
time average of V*I, but V is no longer I*R, so the RMS current will give the
wrong answer. Diodes tend to behave like an resistor in series with an ideal
diode (Vf = I*R + gamma*ln(I/Is), where gamma = 25 to 60 mV at room
temperature). To predict the dissipation accurately, you'd need to compute
the Vf waveform from your I waveform, multiply the two together, and
time-average. Of course, that would require you to have a decent idea of
what R and gamma are for your diode.

For most engineering purposes, you can assume that (to one significant
figure) a signal diode will drop 0.7 volts whenever it has forward current,
so your power dissipation will be the time average of (0.7 I), which is of
course 0.7 * I_avg. Rectifiers and high-speed switching power supply diodes
will drop more than this--sometimes more than 1 V--so you'll probably have to
measure Vf if you need any sort of accuracy.

Cheers,

Phil Hobbs
 
F

Fred Bloggs

I've not posted to this forum before, hope this is not too dumb of a
question!

Should I use RMS current or Average current in finding the power on a
diode? Say I have a pulse current waveform through the diode. The
RMS value of this waveform is different than the average value.
Which do I use? If I use the RMS value, do I also need to use the
RMS value of the voltage across the diode? If I use the average
current, do I then use the forward diode voltage with it to get the
diode average power or do I need to find the average diode voltage
instead? There is some confusion!
Click to expand...

It depends. To a first approximation the diode characteristic is a
piecewise linear V=VF+I*R for I>0, and R is reciprocal slope of the I/V
characteristic at the breakpoint. The power dissipation is then VF*I +
I^2*R and this averages to VF*Iavg + Irms^2*R over one cycle of the
rectified current. At large currents, the RMS current will be a very
significant factor in estimating the power dissipation.
 
P

Phil Allison

"John Larkin"
The power is the average of the instantaneous series of E*I samples,
which can get messy to compute.

If it's a clean rectangular pulse, measure pulse current and
simultaneous diode voltage drop, and do

P = E * I * n

where n is the duty cycle.
Click to expand...


** The power produced by such a rectangular pulse can be measured with an
average responding meter.

P = E . I

Where E = diode conduction voltage

and I = average amps.



.......... Phil
 
D

Don Lancaster

Phil said:
"John Larkin"




** The power produced by such a rectangular pulse can be measured with an
average responding meter.

P = E . I

Where E = diode conduction voltage

and I = average amps.



......... Phil
Click to expand...

Not even wrong.

An average responding meter will ALWAYS lie like a rug.

See http://www.tinaja.com/glib/numschip.pdf for a tutorial.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
J

John Larkin

Not even wrong.

An average responding meter will ALWAYS lie like a rug.
Click to expand...

You mean, it won't indicate average? OK, what *does* an
average-indicating ammeter indicate?

John
 
F

Fred Bartoli

John Larkin said:
You mean, it won't indicate average? OK, what *does* an
average-indicating ammeter indicate?
Click to expand...

It indicates average ...on the average.
 
W

Winfield Hill

Fred Bartoli wrote...
John Larkin wrote...

It indicates average ...on the average.
Click to expand...

:) I think Don Lancaster meant, average indication isn't
very useful, in that it's rarely what you really want, so
relying on its value is likely to give you the wrong answer.
For example, an average-indicating meter may be calibrated
to show rms, but of course doesn't. But we all knew that.
 
P

Phil Allison

"Winfield Hill"
:) I think Don Lancaster meant, average indication isn't
very useful,
Click to expand...


** That is NOT what the pompous ass posted - Win.

in that it's rarely what you really want,
Click to expand...


** So said the Nazis about the Jews in WW2.


In a GREAT many, VERY useful cases the *average value* is JUST what is
needed.


so
relying on its value is likely to give you the wrong answer.
Click to expand...


** You are so full of crap - Win.

Try judging a purely technical matter on the facts alone, instead of
involving your emotions.

You would not want folk to think you were partial or foolish, now would you
??




......... Phil
 
W

Winfield Hill

Phil Allison wrote...
** So said the Nazis about the Jews in WW2.
Click to expand...

Hey, Phil, cool your jets!
In a GREAT many, VERY useful cases the *average value*
is JUST what is needed.
Click to expand...

OK, I'm sure you're right, what are some good examples?
 
P

Phil Allison

"Winfield Hill"
Phil Allison wrote...

Hey, Phil, cool your jets!


OK, I'm sure you're right, what are some good examples?
Click to expand...



** Snip to pieces, ignore the awkward bits and then post a troll.

What a sickening example of a usenet fake.




......... Phil
 
T

Tim Williams

Winfield Hill said:
OK, I'm sure you're right, what are some good examples?
Click to expand...

Well, DC consumption, with non-constant current draw across the meter. As
long as there's a good capacitor (constant voltage) in the circuit, V * I =
P.

You can calibrate the scale in square root volts and read off the average of
a numerically squared AC voltage to get RMS. ;-)

Tim
 
T

Tim Williams

Winfield Hill said:
OK, I'm sure you're right, what are some good examples?
Click to expand...

Well, DC consumption, with non-constant current draw across the meter. As
long as there's a good capacitor (constant voltage) in the circuit, V * I =
P.

You can calibrate the scale in square root volts and read off the average of
a numerically squared AC voltage to get RMS. ;-)

Tim
 
J

John Larkin

Phil Allison wrote...

Hey, Phil, cool your jets!


OK, I'm sure you're right, what are some good examples?
Click to expand...




Phil's example a few posts ago is one such:


Which corresponds to my version,

P = E * I * n (E and I being the 'on' values)

since

Iavg = Ion * n


Sort of clever of him, in fact. An RMS ammeter would be the wrong one
in this case.

John
 
J

John Larkin

Fred Bartoli wrote...

:) I think Don Lancaster meant, average indication isn't
very useful, in that it's rarely what you really want, so
relying on its value is likely to give you the wrong answer.
For example, an average-indicating meter may be calibrated
to show rms, but of course doesn't. But we all knew that.
Click to expand...

One might also note that it's rare to have a ammeter that actually
indicates RMS current on a DC range, unless you have an ancient Weston
electrodynamic thing with a mirrored scale and brass lugs. As far as
DC current measurements, average is usually more meaningful.

John
 
D

Don Lancaster

Phil said:
"Don Lancaster"




** Huh ?

What strange dialect of gibberish is this ?







** Really ?

Not like a * magic carpet * at all then.






** The correlation between " Magic Sinewaves" and " magic carpets " is
blindingly obvious - even to the congenitally tone deaf.




........ Phil
Click to expand...
Sorry about that.
The link copier apparently got off track.

The correct cite for the crucial differences between RMS and average
pulse power is http://www.tinaja.com/glib/muse112.pdf

The specific analysis of the trouble this caused with the PE "magic
lamp" is found at http://www.tinaja.com/glib/muse112.pdf



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
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