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Author Topic: Oscilloscope  (Read 14112 times)

lendusaquid

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Oscilloscope
« on: December 07, 2008, 09:57:45 PM »
I have just been given an Oscilloscope and have not a clue how to use it.Out of curiosity i would like to see the wave form from my gen.How do i set it up?

compig

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Re: Oscilloscope
« Reply #1 on: December 07, 2008, 10:20:46 PM »
What is the make and model of your Scope ?  Did you get probes with it ?  If so , how are they marked ?  Basically you use the vertical amplifier to feed the signal to and use the horizontal frequency control to synch the input signal and display the waveform. You need an attenuated probe to reduce the 240 volt's you will be using as the input and start with the vertical input at max attenuation and then reduce to get a good signal amplitude on the display.
I can go through a detailed sequence if you need more info.
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RhodesRoundtable

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Re: Oscilloscope
« Reply #2 on: December 07, 2008, 10:31:49 PM »
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lendusaquid

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Re: Oscilloscope
« Reply #3 on: December 07, 2008, 10:48:00 PM »
Its a Hitachi Model V-212 and the only lead i have has a couple of crocodile clips on the end of it.I shall take a look at those web sites.

Wizard

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Re: Oscilloscope
« Reply #4 on: December 07, 2008, 11:34:36 PM »
Get scope probe or two.  Not very much (around 20-50-60 each depending on bandwidth and brand type).

MUST, must get isolation transformer and "break" the ground on scope's side.

Cheers, Wizard

rcavictim

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Re: Oscilloscope
« Reply #5 on: December 09, 2008, 01:05:59 PM »
Get scope probe or two.  Not very much (around 20-50-60 each depending on bandwidth and brand type).

MUST, must get isolation transformer and "break" the ground on scope's side.

Cheers, Wizard


Just to help clarify the good advice given here by Wizard.  What you will be doing is hooking up the two scope leads to live AC power for some of your measurements.  Since the shield/ground terminal on the scope input (the outer ring of the BNC scope probe input receptacle) is attached to the metal frame and cabinet of the scope that entire instrument will become 'hot' with mains power during some tests if the shield lead is connected to a hot terminal.  It is for this reason that extreme care must be employed when doing such measurements.  The scope itself has to have isolated power. Power it from an isolation transformer rather than hooked directly to the mains supply.  Make sure the third power cord pin (ground) is not connected to anything so the scope case can 'float'.  Place the scope on a dry piece of plywood or other suitable insulator when using it for these measurements.  Make sure you are wearing dry, rubber soled footwear and are also standing on an insulating surface.  Avoid touching the case of the scope when making knob adjustments.  This assumes the scope has insulating plastic knobs!  Keep one hand in your back pocket so you cannot accidentally rest it on a grounded surface and electrocute yourself.   BE AWARE AND BE CAREFUL.  THINK THROUGH YOUR ACTIONS BEFORE MAKING THEM.......ALWAYS.

Limit your waveform tests to 120 volt circuits or less.  Higher voltages start to create unexpected undesireable and not necessarily intuitive behaviors to the novice, plus are increasingly dangerous.

Even with a X1, X10 scope probe you will need to reduce the voltage to the probe with an additional series resistor placed in series with the probe input and the AC power terminal being examined.  I suggest obtaining a 10 meg ohm, 1/2 watt resistor.  This will reduce the voltage that the scope probe is subjected to saving it from possible burnout.  Note that you will lose absolute voltage alibration accuracy but that is not what you are doing with the scope anyhow, you are more interested in the waveform shape which will not be compromised.  We have voltmeters for voltage measurements.

If you are a novice in working with electricity try to find a experienced person who can mentor you in person or walk you through these tests the first time.  Good idea to always have a buddy present when working with this stuff too! 
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compig

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Re: Oscilloscope
« Reply #6 on: December 09, 2008, 01:20:41 PM »
Have to admit I was being a bit casual in my information , the result of years of abusing safety rules by a practicing technician !!
« Last Edit: December 09, 2008, 10:53:29 PM by compig »
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lendusaquid

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Re: Oscilloscope
« Reply #7 on: December 09, 2008, 10:38:31 PM »
If say i plugged in a 12 or 24 volt transformer to my gen supply and tested the output, would i get an accurate picture or would the transformer alter the wave in some way ?.

compig

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Re: Oscilloscope
« Reply #8 on: December 09, 2008, 10:54:29 PM »
Depends on how good the TX is , but most will affect the shape of the waveform to some extent.
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AdeV73

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Re: Oscilloscope
« Reply #9 on: December 09, 2008, 11:03:35 PM »
 :o

I've been prodding around inside switching PSUs with my 'scope (looks similar in functionality to the OPs - mine is a Telequipment D54, I think), with readings of over 400v, with no isolation transformer or grounding of any particular description. and yes, I would have been twiddling some of the adjustment knobs while plugged into such voltages.


As I say: :o

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AdeV73

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Re: Oscilloscope
« Reply #10 on: December 09, 2008, 11:09:16 PM »
Get scope probe or two.  Not very much (around 20-50-60 each depending on bandwidth and brand type).

MUST, must get isolation transformer and "break" the ground on scope's side.

According to Tektronx, you must absolutely never use an isolation transformer ???

Of course, that may only apply to Tek 'scopes - I know they are seriously expensive, so maybe thay have excellent built-in protection.

Edit: More info -

Quote from: Tektronix
Floating An Oscilloscope: A Definition

“Floating” a ground referenced oscilloscope is the technique of defeating
the oscilloscope’s protective grounding system – disconnecting “signal
common” from earth, either by defeating the grounding system or using an
isolation transformer. This allows accessible parts of the instrument
such as chassis, cabinet, and connectors to assume the potential of the
probe ground lead connection point. This is dangerous, not only from the
standpoint of elevated voltages present on the oscilloscope (a shock haz-
ard to the operator), but also due to cumulative stresses on the oscillo-
scope’s power transformer insulation.

This stress may not cause immediate failure, but may lead to future danger-
ous failures (a shock and fire hazard), even after returning the oscilloscope
to properly grounded operation!

Not only is floating a ground-referenced oscilloscope dangerous, but
the measurements are often inaccurate. This results from the total
capacitance of the oscilloscope chassis being directly connected to the
circuit under test at the point where the common lead is connected.

Download the PDF that quote is taken from.
« Last Edit: December 09, 2008, 11:17:33 PM by AdeV73 »

compig

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Re: Oscilloscope
« Reply #11 on: December 09, 2008, 11:19:48 PM »
That probably means that an Iso TX should never be used to power the scope.
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compig

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Re: Oscilloscope
« Reply #12 on: December 10, 2008, 12:10:55 AM »
Reminds me of that saying , "If the only tool in your box is hammer , pretty soon everything looks like a nail" LOL !!
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Wizard

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Re: Oscilloscope
« Reply #13 on: December 10, 2008, 12:57:19 AM »
I'm aware of this issues, for my work where I examine stuff all the time and use scope often, my scope is just directly plugged and plug all the stuff I work on either to test or to look at through isolation transformer.

In your particular situation with checking waveforms on generators/alternator/etc, need to have scope floating or route the isolation transformer from generator output and monitor the waveforms on the output side of transformer's.  All the transformers actually reflect what waveforms is generated to mostly degree.   Even a wall wart tranformer that outputs AC not DC is excellent way to monitor the quality of waveforms and give you cheap isolation to boot.

Cheers, Wizard
« Last Edit: December 10, 2008, 12:59:51 AM by Wizard »

RhodesRoundtable

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Re: Oscilloscope
« Reply #14 on: December 10, 2008, 02:58:12 AM »
lendusaquid
I have a model V-509, which I was looking into new probes for, and it is similar to the Hitachi Model V-212.
If you need probes and/or manual, I finally found my links to these sources.
http://www.probemaster.com/shop/article_info.php?articles_id=5
http://www.probemaster.com/shop/advanced_search_result.php?keywords=4904-2&search_in_description=1&x=62&y=12
Price seems hight to me though!
Scanning the list you take the same master kit as mine apparently.
number 4904-2

0-1-2-3 pages for the manual. Have to click the numbers to start the load,
took me a sec to figure that out and I had a bit of trouble with them in firefox, but they do work.
http://www.eserviceinfo.com/downloadsm/16944/Hitachi_V211.html
They are in rar format so if you don’t have winrar, here is a 40 day trial de-compressor
http://www.download.com/WinRAR/3000-2250_4-10745708.html

I also found this interesting discussion on oscilloscope grounding.
http://www.electronicspoint.com/oscilloscope-grounding-question-t113920.html
http://www.marcspages.co.uk/pq/4120.htm
Hope some of this stuff helps.

obfu
« Last Edit: December 10, 2008, 03:07:35 AM by obfuscation »
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