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1. gar
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## Instrument, measurement, results

180507-0912

Problem:

Using various instruments and calculations, a 10 ohm resistor as a load, and rectangular wave function generator what are the results of measurements across the load?

1. Average DC volts.
2. Actual RMS volts.
3. Fluke 87 AC RMS volts.
4. Fluke 27 AC volts.
5. Simpson 260 on AC volts.
6. Simpson 260 on Output.

For the following waveforms where the frequency is high enough for satisfactory coupling and averaging of the signal for the time percentages.

1. +10 V for 10% and 0 V for 90%.
2. +9 V for 10%, and -1 V for 90%.
3. 0 V for 10%, and -10 V for 90%.

How do a Simpson 260 and a Fluke 27 measure AC and get their readings? Define the philosophy.

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Do you get to reverse the leads on the Simpson test for # 3. That there needle thing just goes into the blank white space off to the left of zero. Can I put a 9 ohm diode* across the meter?

* You know the black thingy with the white stripe in another thread.

Seriously, good post, should help some folks understand equipment. -- and entice others who do not have one to get a 'scope of some type, even a cheap hantek or similar.

3. gar
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1801037 EDT

junkhound:

No just switch the switch to -DC.

What is a 9 ohm diode? Is there a thread with a 9 ohm diode? I have many thingys with glass and a black strip, or a gray and black, and then the black with white strip, but none that are 9 ohms, or not for long. Had some marked backwards.

The waveforms I selected were designed to achieve certain results and ones where one can almost do the RMS calculation in their head.

Using a Simpson 260 in AC, a 120 V 60 Hz sine wave source, and a diode in series with a resistive load I did not get the result I expected, which is a reading of 60. It was more like the Fluke 87. I need to study this further.

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What is a 9 ohm diode? Is there a thread with a 9 ohm diode?

Tongue in cheek description.
Reference to the doorbell thread where a supposed EE describe a diode across a switch as a resistor. The OP took issue with you for calling into question his knowledge base on measurement techniques.

I'd forgotten the Simpson had a -DC switch position.

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re: Using a Simpson 260 in AC, a 120 V 60 Hz sine wave source, and a diode in series with a resistive load I did not get the result I expected, which is a reading of 60

For a calculated value get 59.7 volts expected for calculation of 1.1 * avg(1/2 wave rectified 120 vac) n e.g. =59.7

However, believe the Simpson 260 USES only a single forward CuO diode and a resistor string to the 50uA meter, so is already measuring just 1/2 wave rectified and the meter scale calibrated as such - did you read (an unexpected) 120 Vac on your test with an actual 260 ?

6. you mean like measuring the voltage of AC PWM at 20kHz and 20% duty cycle?
maybe i not understanding your question.

but how sneaky of them, adding secret functions

7. gar
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180507-1640 EDT

FionaZuppa:

The purpose of this thread was to stimulate thought and possibly get readers to think about whether a particular reading on some instrument was giving them the information they expected. For example RMS current should give a measure of the heating effect in a resistance. Does the AC current measured on a Fluke 87 do so?

All the Fluke hidden functions were interesting.

junkhound:

Neglecting diode drop the DC voltage across my 240 ohm resistor from a 120 V source should be ( (120/0.707)*0.636)/2 = 53.97 DC V avg. The calibration multiplier on this value in AC mode should be 0.707/0.636 = 1.112 . Thus, reading becomes 53.97*1.113 = 60.0 . This is intutitive because current only flows 1/2 the time.

Assuming the Fluke 27 and Simpson 260 meters use a full wave bridge to rectify the AC input and then do a DC average, then 60.0 V on the AC scale should be expected. The AC scale vs the DC scale on the 260 does some linearization as well as the 1.11 scaling. In the 40s the rectifier was CuO. Sometime around 1960 it became 4 small diodes. Possibly 1N34s.

On 260 and 270 meters we have to work on the 250 V scale. The 260 is I believe a +/-2% full scale accuracy on DC and less on AC. The 270 is 1% instead of 2. The present day 27 has an AC accuracy listed as +/-0.5% plus a count of 3.

My results were:

DC meter mode calculated 54.0 V assuming no diode drop
87 ........ 53.3 V average
27 ........ 53.4
S270-1 . 53
S270-2 . 52

AC meter mode
87 ........ 65.2 V
27 ........ 65.0 expected close to 60
S270-2 . 64.0 expected close to 60
S260 .... 60 this meter is from 1946 or 47 and was jumping on DC

.

8. The 27 & probably all VOMs takes an average reading then the scale is calibrated to read RMS. Works fine for a normal sine wave.

Amprobe model ACD-2 threw me for a loop on day reading a compact fluorescent light bulb. The reading was over double what was expected. I replaced it with an incandescent & it read as expected. I reproduced the situation for this thread.

The AMPROBE is reading is direct. You move the decimal point on the DMM two digits to the left so it is actually reading .640A or .718A. The Fluke probe is set in the 20A position or 100mV/A.

First picture the measurement is being taken on a 100W incandescent. The meters agree.
Picture 2 is an old compact Fluorescent. Rated at 120V/620mA, that's what is printed on it with the 42W. The AMPROBE has lost it's mind or has it ?

I will post back tomorrow evening on why the AMPROBE is doing this. I have to wonder what the designers were thinking when they made this meter...

9. gar
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180508-0723 EDT

SG-1:

I will let you complete your story before commenting.

Others:

Is someone willing to do the calculations for the RMS, average, and full wave rectified values of the 3 pulse waveforms I described in post #1, and describe your method for the +9,-1 V waveform?

Fluke has a discussion at
http://en-us.fluke.com/training/trai...-true-rms.html
that is poorly written, and to an extent incorrect.

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Originally Posted by gar
180508-0723 EDT

describe your method for the +9,-1 V waveform

.
to keep the interest going...

2 methods here for show and tell, others left to the students <G> :
a. cheat, use PSpice
b. 9-1 = average; Watts is average of 9V and 1V power dissipation; thus Vrms = sqrt (watts*load) (note: 200k in PSpice schematic if from Simpson user manual for 50 uA movement)

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