Re: [CAD_CAM_EDM_DRO] Re: Power Supply & Gecko's (test results)

----- Original Message -----
From: "Roy J. Tellason" <rtellason@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, August 13, 2005 1:26 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Power Supply & Gecko's


> On Saturday 13 August 2005 12:48 pm, turbulatordude wrote:
>> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Roy J. Tellason"
>> <rtellason@b...> wrote:
>> > On Saturday 13 August 2005 11:42 am, Peter Reilley wrote:
>> > > I believe that you cannot measure AC ripple with an AC meter where
>> > > there is a DC component. The meter does not filter out the DC.
>> > > Thus, it get confused. You really need a scope to measure AC
>> > > ripple.
>> >
>> > I think this depends on the meter...
>>
>> Not sure if this is 100% correct,
>> from Newport's website
>> http://www.newportus.com/manuals/ACGUIDE.htm
>>
>> True-RMS voltmeters use an integrated circuit which computes the true
>> root-mean-square value of complex signals. AC coupling allows the
>> measurement of the AC component (or ripple) from a signal with both AC
>> and DC components. DC coupling allows the measurement of total RMS, or
>> "heating value", including DC and AC components to over 30 kHz.
>> (Maximum crest factor (Vp/Vrms) is 3:1). Screw terminals allow the
>> selection of AC or DC coupling. True-RMS meters are especially useful
>> in monitoring voltage and current supplied to motors or heaters
>> controlled by an SCR or triac circuit.
>
> Yep, and "true RMS" meters are few and far between, and much more
> expensive
> than most of what we're likely to be using...
>
> It depends on the meter... :-)
>


This piqued my interest. I have the equipment to do the actual testing.
I tested;

meter 1) Fluke 87 true RMS meter about 8 years old, bought new.
meter 2) Simpson 269, the best movement meter that you could buy 30 years
ago. Bought at a flea market.
meter 3) Ramsey KD-508, a cheap Radio Shack digital meter about 10 years
old, bought new.

None of there meters have been calibrated since I have owned them.
I used a Racal-Dana signal generator that can add a DC offset to it's output
signal.
I used a scope to watch the signal to make sure that I was getting what I
expected.
All tests were done at 60 Hz.

Three test signals were measured by all meters;
Signal 1) 3 VAC with 0 VDC
Signal 2) 3 VAC with 3 VDC
Signal 3) 0 VAC with 3 VDC

Each meter read the signal at with it's AC setting and then it's DC setting.
Here are the results.

Fluke AC Fluke DC Simpson AC Simpson DC Ramsey AC
Ramsey DC

Signal 1) 3 0 3.1 0
2.7 0
Signal 2) 3 3 3.95 3.1
6.1 3
Signal 3) 0 3 3.5 3.1
5.1 3.1

I hope this chart comes out OK.

It is clear that the Fluke will read only the AC component when the setting
is AC and the
signal has both an AC and a DC component. In the DC setting the AC is
ignored as
expected.

The Simpson does the right thing on the DC setting when measuring a combined
AC/DC signal.
On the AC setting it is wrong. Movement meters like the Simpson are
average reading but RMS
calibrated. This works OK for pure sinusoidal AC but gives weird readings
for anything else.

The Ramsey does the right thing for DC but the AC reading is wrong. Cheap
digital
meters are peak reading but RMS calibrated. Thus the reading is wrong for
complex signals and
the error is different than for movement meters like the Simpson.

The moral of the story is that the Fluke is correct. You get what you pay
for.

Pete.