Re: RMS and Average Current

On Wed, 4 Jan 2006 17:29:46 +1100, "Phil Allison" <philallison@xxxxxxxxxx>
wrote:

>
>"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.
>
>
>

Before I respond to specific items, let me say that all my comments have
assumed that we were dealing with line frequency currents. The OP didn't say
explicitly that he was concerned with line frequency rectifiers, but he *was*
concerned with power dissipation in a diode. Typically, signal rectifiers
aren't dissipation limited. It's usually line frequency rectifiers or,
nowadays, diodes in switching power supplies where power dissipation is a
concern. To keep things simple, I assumed line frequency. You may not think it
was a reasonable assumption; if so, then we will have to differ.

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

Sure it can. Have a look at the URL John Larkin posted:
http://us.fluke.com/usen/products/prodSelection_Grouping.htm

All of the DMMs there have a MIN-MAX function which can easily get the diode on
voltage at line frequency. As I said in a posting, you need a peak responding
meter, and that's what MIN-MAX does.

>
>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.

I can't comment on your experience, but my own differs. Every DMM I have used
on the job has had substantially greater bandwidth than a few KHz. I personally
own three DMMs. The two handhelds, an HP 974A and a Fluke 189 both have 100 KHz
bandwith. My bench meter is a 20 year old HP 3468 with 300 KHz bandwidth. The
lowest bandwidth shown on the Fluke site referenced above is 20 KHz.

But this isn't relevant to my postings since I was assuming line frequency.

>
>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.

John Larkin first posited "...a clean rectangular pulse..." in this thread,
something that will scarcely ever be seen in practice. In line frequency
applications, one usually sees currents looking like pieces of sinusoids, or
rounded pulses as in a capacitor input filter. In switchers, the pulse currents
in diodes range from triangle waves to trapezoids, but are rarely flat topped.
So neither method will work well in practice to determine diode dissipation. As
I mentioned in another post, one would have to use a wattmeter (good to low
audio frequencies, if it's an old fashioned dynamometer type), or a scope with
trace math, or some other purpose-built piece of equipment.

But John's hypothetical is a simple case which you used to point out that the
power delivered to a constant voltage by a varying current is proportional to
the *average* value of the current. This is quite true, and, I dare say, often
overlooked.

I merely pointed out that since the voltage across the diode isn't absolutely
pure DC, but is a pulsed waveform like the current, the measurement can be made
by measuring the true RMS voltage and current and multiplying the two.

This is an idealized situation and my comments were intended to be taken as
applying to an ideal where accurate RMS measurements could be made, much as your
method only works for the ideal situation where the pulse is flat topped.

John's description led me to think that his "clean rectangular pulse" might be
coming from a pulse generator, and so I assumed that there was no reverse
voltage across the diode. If there is, then it can be blocked by another diode;
and if that diode's forward drop causes too much error, it can probably be
compensated.

*Of course* it's true that if the measurements are in error because the
waveform is at a frequency beyond the capability of the DMM, then this method
won't work. I never claimed otherwise.
>
>
>
>........ Phil
>
>

.

Relevant Pages

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