RMS and Average Current

[John Larkin]

I think you should stop using this phrase, Don. You've worn it out, and it
has lost its impact.

He was, in fact correct in saying that Phil was not even wrong (since
Phil was right.) Maybe he meant it as a compliment.

John

 

[Phil Allison]

"Don Lancaster"


** Snip pile of OT drivel.

What you are alluding to is all well known to me.

But, has NOTHING whatsoever to do with the post you plonked it under.

Go away.



.......... Phil

 

[Phil Allison]

"Winfield Hill"



** Snips people's posts to pieces, ignores the awkward bits and then post
a pathetic troll.

What a sickening example of a usenet fake.





......... Phil

 

[Winfield Hill]

Phil Allison wrote...

"Winfield Hill"



** Snips people's posts to pieces, ignores the awkward bits
and then post a pathetic troll.

What a sickening example of a usenet fake.





........ Phil

That was a convincing bit of oratory, NOT.

 

[Phil Allison]

"John Larkin"
" Win the Fake "

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.

** Thank you, John.

At least you take the time to read posts properly instead of just knee jerk
reacting.

The " rectangular current pulse in a diode" problem has an even simpler
measurement solution than others here realised.

Even some with *very highfalutin* opinions of themselves.




......... Phil

 

[Phil Allison]

"John Larkin"

One might also note that it's rare to have a ammeter that actually
indicates RMS current on a DC range,

** Err - all "moving iron" ammeters do exactly that.

You see them on car battery chargers a lot - being used to wrongly
indicate rate of charge.

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.

** It is horses for courses, but where the power delivered by a DC supply
is to be found or the rate of charge going into a battery - Iav is the one
to know.



.......... Phil

 

[Phil Allison]

"Don Lancaster"

Actually, just about all but the cheapest of new instruments now routinely
provide RMS capability. The cost of doing so has dramatically dropped.

Primarily because average responders were so ludicrously bad.

** You are just plain WRONG - Lancaster.

Back to Basic Electricity 101 for you.




........... Phil

 

[Phil Allison]

"Don Lancaster"

It indicates an incompetent individual deluding themselves while making a
measurement.

** You are just plain WRONG - Lancaster.

Back to Basic Electricity 101 for you.



.......... Phil

 

[Phil Allison]

"Don Lancaster"

I seriously doubt that Fluke makes ANY average responding meters any more
because they were so bad.

** You are just plain WRONG - Lancaster.

Back to Basic Electricity 101 for you.




.......... Phil

 

[Phil Allison]

"The Phantom"
"Phil Allison" :

Actually, the power delivered by *any* current waveform (for which an
"average" exists) into a constant voltage drop can be measured by an
average
responding meter.

** Of course - an rms responding meter reads wrongly (in excess) in these
cases.

Now just convince that Lancaster cretin and Win and we will all be so much
happier.




......... Phil

 

[Phil Allison]

"Winfield Hill"

Phil Allison wrote...

That was a convincing bit of oratory, NOT.

** Win - if you wish not to be considered a *duck*

- then do not walk like one and quack like one.




......... Phil

 

[The Phantom]

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)

And you would have to use a peak responding meter to get E and I and some
other method to determine n. For the pulse type waveform described, knowing E
(peak) and the average value of e(t), you could determine n. Or use a scope to
do it all.

since

Iavg = Ion * n


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

Not necessarily. How will you know the diode conduction voltage (which we're
assuming is constant)? Suppose you don't have a scope, just meters. The
voltage across the diode varies with time with the same waveshape as the current
through the diode, so the conduction voltage can't be measured with an average
responding or RMS responding meter; you'll have to use a peak responding meter
which is somewhat uncommon. If we don't have a peak responding meter, then
what? We can measure the RMS voltage across the diode and the RMS current
through the diode and multiply them.

An RMS ammeter is appropriate if an RMS voltmeter is also used.

Phil presupposes that you know the diode conduction voltage, which is not
constant when a pulse current is applied; some sort of device to measure the
peak voltage across the diode would be needed. If you have a DVM which can
measure RMS (AC+DC) voltage, it almost certainly will be able to measure RMS
(AC+DC) current, and that's all you need. No need to make peak measurements.

 

[Winfield Hill]

Phil Allison wrote...

"Winfield Hill"

** Win - if you wish not to be considered a *duck*
- then do not walk like one and quack like one.

HEY. I simply said, "OK, I'm sure you're right, what
are some good examples?"

What's the matter with you man, have you gone totally
daft? The topic was/is "RMS and Average Current," and
that's what I invited you to discuss for the last four
posts. IIRC, you were strongly arguing for the value
of average measurements (BTW, which I agree with for DC
and low-frequency measurements - as well argued by John),
and I suggested you might have some good examples for us?
Come on, I wasn't interested in talking about the Nazis
and the Jews in WW2, or whatever other issue you raised.

 

[Phil Allison]

"Winfield Hill"

Phil Allison wrote...

HEY. I simply said, "OK, I'm sure you're right, what
are some good examples?"

** Go to Google Groups .

Maybe you cannot see all my posts on your server.

What's the matter with you man, have you gone totally
daft? The topic was/is "RMS and Average Current," and
that's what I invited you to discuss for the last four
posts.

** Not interested - ducky.

I am prepared to defend what I have posted.

That is all.

You are just trolling and bullshitting.




......... Phil

 

[Phil Allison]

"The Phantom"


(snip piles of drivel)

Suppose you don't have a scope, just meters.

** Why not suppose the earth is flat too ??

You seem like a likely inhabitant of such a place.




.......... Phil

 

[John Larkin]

Not necessarily. How will you know the diode conduction voltage (which we're
assuming is constant)? Suppose you don't have a scope, just meters. The
voltage across the diode varies with time with the same waveshape as the current
through the diode, so the conduction voltage can't be measured with an average
responding or RMS responding meter; you'll have to use a peak responding meter
which is somewhat uncommon. If we don't have a peak responding meter, then
what? We can measure the RMS voltage across the diode and the RMS current
through the diode and multiply them.

An RMS ammeter is appropriate if an RMS voltmeter is also used.

Phil presupposes that you know the diode conduction voltage, which is not
constant when a pulse current is applied;

Why not? Should be for reasonably short pulses, maybe not for
seconds-wide things where thermals start to matter.

Most everybody has an average-reading DC ammeter and a scope with a
regular voltage probe. What's less common is a good, calibrated
current probe for the scope, or a DC ammeter meter that does true RMS.
So the suggested combo works nicely for the original problem, to
figure the diode dissipation for rectangular pulses.

If diode drop varies significantly during the pulse on time, neither
meter-only technique is accurate.

John

 

[The Phantom]

RMS volts * RMS amps <> RMS watts in the general case. It is true
here, so long as the diode forward drop is flat during the pulses.

That's the assumption Phil made, and I made it too.

It is also important that the diode voltage drop to zero when it isn't
conducting a current. If the diode were replaced by a battery so that the
voltage was constant all the time, the RMS volts * RMS current wouldn't work.
But, *in this particular case*, it does work.

Why not?

Because by pulse current, we mean a current that is on for a while and off for
a while. That's what I mean by "not constant", not that that the voltage while
the diode is conducting isn't constant *during that time*. Sorry if there was
confusion.

Should be for reasonably short pulses, maybe not for
seconds-wide things where thermals start to matter.

Most everybody has an average-reading DC ammeter and a scope with a
regular voltage probe.

If you use a scope to measure the diode conduction voltage, then you only get
scope accuracy, not the accuracy you could get with a DVM.

What's less common is a good, calibrated
current probe for the scope, or a DC ammeter meter that does true RMS.
So the suggested combo works nicely for the original problem, to
figure the diode dissipation for rectangular pulses.

So it does. Everybody has a DVM that can measure both AC (RMS with or without
DC) and DC current these days, I think, and the true RMS of AC+DC can always be
done by hand with the calculator that everybody also has. Just measure the
voltage and current in both AC and DC modes and take the square root of the sum
of the squares of the two voltage measurements and of the two current
measurements. Then you get the result with only one meter.

If diode drop varies significantly during the pulse on time, neither
meter-only technique is accurate.

Quite right. Then we need to use the wattmeter everbody should have, or the
really neat new 4-channel scope that does trace math that everbody wishes they
had. !!

 

[John Larkin]

That's the assumption Phil made, and I made it too.

It is also important that the diode voltage drop to zero when it isn't
conducting a current. If the diode were replaced by a battery so that the
voltage was constant all the time, the RMS volts * RMS current wouldn't work.
But, *in this particular case*, it does work.


Because by pulse current, we mean a current that is on for a while and off for
a while. That's what I mean by "not constant", not that that the voltage while
the diode is conducting isn't constant *during that time*. Sorry if there was
confusion.


If you use a scope to measure the diode conduction voltage, then you only get
scope accuracy, not the accuracy you could get with a DVM.


So it does. Everybody has a DVM that can measure both AC (RMS with or without
DC) and DC current these days, I think, and the true RMS of AC+DC can always be
done by hand with the calculator that everybody also has. Just measure the
voltage and current in both AC and DC modes and take the square root of the sum
of the squares of the two voltage measurements and of the two current
measurements. Then you get the result with only one meter.


Quite right. Then we need to use the wattmeter everbody should have, or the
really neat new 4-channel scope that does trace math that everbody wishes they
had. !!

Damn, you have left us exactly nothing to argue over. By s.e.d.
standards, that's not sporting.

John

 

[Phil Allison]

"The Phantom"

If you use a scope to measure the diode conduction voltage, then you only
get
scope accuracy, not the accuracy you could get with a DVM.

** You could easily get that diode " on "voltage accurate to 1 or 2%.

Just check the scope range with a good 1.25 volt reference.

So it does. Everybody has a DVM that can measure both AC (RMS with or
without
DC) and DC current these days, I think, and the true RMS of AC+DC can
always be
done by hand with the calculator that everybody also has. Just
measure the
voltage and current in both AC and DC modes and take the square root of
the sum
of the squares of the two voltage measurements and of the two current
measurements. Then you get the result with only one meter.

** That is absolute crapology.

The diode *on* voltage is wanted - in isolation from any reverse voltage -
something a DMM cannot do.

Ergo - a scope is the way to go.

DMMs with "true rms" capability vary ENORMOUSLY in their effective
bandwidth - most are only accurate to a few kHz - making them useless
for other than AC supply related waveforms.

OTOH - even garden variety scopes are accurate to tens of MHz !!

An *average responding* amp meter WILL give the correct result since
response time becomes irrelevant.


You need to try a lot harder, Mr Phantom.



......... Phil

 

[Pooh Bear]

"Winfield Hill"

** Go to Google Groups .

Maybe you cannot see all my posts on your server.


** Not interested - ducky.

I am prepared to defend what I have posted.

That is all.

You are just trolling and bullshitting.

Yeah. Sure.

Get back on your meds.

Graham