Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently May Fri 24, 2019 10:30 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 23 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: O'scope refresher question
PostPosted: Feb Fri 16, 2018 5:44 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23584
Location: Haledon, NJ, usa
Here's a question about scope basics that I think I displayed maybe a few years ago... but I'll repeat it here again because it's one of those things we do quite often without thinking ..... and then we get a surprise.

The goal in this set up was to compare the input AC sinewave to the pulsating DC wave-shape across the load.

Image

_________________
To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 6:03 am 
Member
User avatar

Joined: Jan Mon 05, 2009 4:09 am
Posts: 1322
Location: Hudson, MA
The ground of channel A & Channel B are usually connected together in the scope and would short together the two nodes being tested. A very few scopes do have isolated inputs and would not short the channel A & B returns.

_________________
Eddy


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 6:10 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23584
Location: Haledon, NJ, usa
Eddy wrote:
The ground of channel A & Channel B are usually connected together in the scope and would short together the two nodes being tested. A very few scopes do have isolated inputs and would not short the channel A & B returns.

I have never found a dual channel scope with separate grounds for the probes.

But that's the problem here, yes,... the two ground leads will actually create a dead short across D2.

_________________
To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 6:15 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23584
Location: Haledon, NJ, usa
How many of you readers caught that problem?

_________________
To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 7:57 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34274
Location: Maryland 20709, USA
Pbpix wrote:
How many of you readers caught that problem?

It's kinda obvious.

- Leigh

_________________
73 de W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 8:08 am 
Member

Joined: Feb Wed 07, 2018 6:52 pm
Posts: 433
Location: Stone Mountain, GA
The more common trap is on a non-isolated supply, hooking the scope ground anywhere in the circuit will fry things.

_________________
Scott.
Powered by infinite improbability drive.


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 8:34 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23584
Location: Haledon, NJ, usa
Leigh wrote:
Pbpix wrote:
How many of you readers caught that problem?

It's kinda obvious.

- Leigh

Only "kinda"?

_________________
To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 2:27 pm 
Member
User avatar

Joined: Jan Mon 05, 2009 4:09 am
Posts: 1322
Location: Hudson, MA
Pbpix wrote:
Eddy wrote:
The ground of channel A & Channel B are usually connected together in the scope and would short together the two nodes being tested. A very few scopes do have isolated inputs and would not short the channel A & B returns.

I have never found a dual channel scope with separate grounds for the probes.

But that's the problem here, yes,... the two ground leads will actually create a dead short across D2.


The Isolated channel scope I have used is a Battery portable, a very handy piece of test gear.

THS720P TekScope IsolatedChannel™ Oscilloscope/DMM
https://www.tek.com/datasheet/ths700-series

Tek also make a TPS2000 Quad channel Isolated
https://www.tek.com/oscilloscope/tps2000

_________________
Eddy


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 3:11 pm 
Member

Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1401
Location: Santa Rosa, CA
There is an easy way to make isolated measurments with most
good quality dual trace scopes (Tek, HP, maybe others).
Select 'A+B", then 'Invert A'. The result will be a channel A
minus channel B measurment, aka differential mode.
No need to ground the scope to anything.
However you won't be able to make a dual trace measurment.

Most of the 500 series Tek scopes have differential vertical
amp plug ins available.
Steve

_________________
'cell phones and the internet are tools, not a lifestyle'


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 3:48 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4001
Location: Sunbury, Ohio 43074
Quote:
No need to ground the scope to anything.
However you won't be able to make a dual trace measurment.

That's what the 4 channel scopes are for :lol: 8)

_________________
Preserving the history of electronics, one boat anchor at a time! :)
www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 4:35 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 25716
Location: Annapolis, MD
Leigh wrote:
Pbpix wrote:
How many of you readers caught that problem?

It's kinda obvious.

- Leigh

One man's "obvious" is another man's disaster.

I'll always remember the methods we used back in the cal lab at General Dynamics--exactly 1/2 century ago. Tek 5XX scopes on carts, other T/E surrounding the DUT.
The DUT would be grounded, and all the T/E would be on ground-breakers... Scary, but it worked.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 6:30 pm 
Member

Joined: May Tue 30, 2006 4:46 pm
Posts: 9770
Location: Santa Rosa, CA
Obvious now, but I did do something similar on my Induction Heater project (transformer-less supply), years ago. Fortunately, I only fried a diode.

In the lab at Westinghouse Semiconductor there were old scopes with the ground pin cut off the power plug, or using adapters with no ground pin.

Image

A recipe for disaster.

I got myself a 500 VA toroidal isolation transformer and never looked back again.

Assuming the "AC source" in the original question is actually a power line, why would you want to view it on a scope, while looking at the rectified load-side voltage?

Rich


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 9:31 pm 
Member

Joined: Mar Wed 16, 2011 10:44 pm
Posts: 917
Location: Peekskill, NY
One of the best ways to to do multichannel measurements is using a digital scope. Many of them
have the ability to display math functions that the scope calculates on the fly using the data from
each channel.

As an example one can measure the line-to-line voltages from a three phase source, each of which
is 120 volts above ground, using only three scope inputs.

The display is set up by using three math functions calculated from inputs A, B, and C:

A - B
B - C
C - A

Of course doing this means you can own or borrow a reasonably high-end digital scope. But there are
becoming more and more common.


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 10:59 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23584
Location: Haledon, NJ, usa
Rich, W3HWJ wrote:
Obvious now, but I did do something similar on my Induction Heater project (transformer-less supply), years ago. Fortunately, I only fried a diode.

In the lab at Westinghouse Semiconductor there were old scopes with the ground pin cut off the power plug, or using adapters with no ground pin.

Image

A recipe for disaster.

I got myself a 500 VA toroidal isolation transformer and never looked back again.

Assuming the "AC source" in the original question is actually a power line, why would you want to view it on a scope, while looking at the rectified load-side voltage?

Rich


An isolation transformer or a floating AC plug would not help in my example.
Because even if the whole set up was battery powered or whatever won't matter.
... the problem is that the two channels inside the scope have their ground leads common. So the two leads would be like one long wire shorting across D2

_________________
To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 11:50 pm 
Member

Joined: May Tue 30, 2006 4:46 pm
Posts: 9770
Location: Santa Rosa, CA
Correct. It can't be done.


Question remains, why do it at all?

Rich


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Fri 16, 2018 11:55 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 23584
Location: Haledon, NJ, usa
Rich, W3HWJ wrote:
Correct. It can't be done.


Question remains, why do it at all?

Rich

Who knows.. perhaps when troubleshooting noise problems in a power supply?

But quite often the problem is that while using dual probes we may forget that the two ground clips act as a common wire and especially when using a bridge rectifier it's easy to overlook that the incoming ground is not the same as the newly created circuit ground.

_________________
To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
-Emerson


Last edited by Pbpix on Feb Sat 17, 2018 8:24 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Sat 17, 2018 8:15 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 3319
Location: Lincoln City, OR
Greetings to the Forum:

I agree with Leigh.... it is sort of obvious. I have just sort of gotten into the habit of thinking about grounding issues every time I get out the scope.... saves me a world of hurt.

One thing you could do to make the measurement desired in the first drawing (and this assumes that the AC source is the AC line) is to connect only one probe to the output of the bridge. Set the scope to line trigger and the second channel to "Trigger View".

I do use the ground isolator adapter on my scope from time to time. In particular, when I am trying to observe the output of my Swan 350 with a two-tone test signal applied to the radio.

It seems that the Swan power supply returns the filament current (as well as the speaker and the B-) from the radio to the power supply on a common ground lead. This not only results in hum in the speaker at all times, but the radio is now above Edison ground by a noticeable amount. The result, when using a directional coupler to get the scope sample out of the coaxial output line on its way to the dummy load is that a ground loop is formed through the sample coaxial shield to the scope input and then on to Edison ground. This ground loop couples enough AC into the scope input to render the scope display unusable. Since I don't own a large isolation transformer, I use the simple solution - float the scope Edison ground.

Don't ask how long it took stupid me to figure out what was happening... I don't want to talk about it! :lol:

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Sun 18, 2018 1:02 am 
Member

Joined: Aug Mon 04, 2014 9:42 pm
Posts: 86
I just happened to be finishing aligning a Tek RM502a, and decided to post a picture. As mentioned above, having a differential input is handy, having more than one is handier. This is a picture of the rectifier section of a EUW-15 (an educational Heath power supply), which is normally set-up as a voltage doubler

Attachment:
y.jpg
y.jpg [ 76.38 KiB | Viewed 1070 times ]

Upper trace is the AC input, 100v/div with x10 probes. Lower trace is across a pair of rectifiers and a cap, 5v/div.
There's a few combinations of input modules with Tek 500 and 560 plug-in series that yield two differential inputs with dual beams o'scopes (555, 556 or 565); with single beam 500's you'll need a 1A4 and for 560's a 3A3

Here's a nice exercise: Distortion of the AC input goes away when disconnected from the caps+rectifiers - why?
Below is a picture of the waveform between the two rectifiers that are in series; each rectifier in this standard full wave voltage doubler is actually two diodes in series; bottom trace is the differential signal at the midpoint between the two


Attachments:
x.jpg
x.jpg [ 132.62 KiB | Viewed 1070 times ]
Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Sun 18, 2018 11:25 am 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 11138
Location: Powell River BC Canada
In the early days (no money) I lifted grounds by using repeating coils, for demos
involving low frequency signals.

A good four channel scope will provide two differential channels. Now you have a +
and a - probe tip, per channel, and you do not use the grounds to the scope at all.
Just tape them up and forget them.

We scoped 600 volt paper machine drives this way. Then we used a bezel Polaroid
camera to save trace.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: O'scope refresher question
PostPosted: Feb Mon 19, 2018 4:38 pm 
Member

Joined: Aug Mon 04, 2014 9:42 pm
Posts: 86
Keep in mind that while common mode rejection ratio (CMRR) of differential inputs can be vary large, they're not infinite, and while you do not have to connect a ground, you still have to be aware of the voltages on the inputs relative to ground. It is possible to fry the inputs at worst or maybe just get a waveform that doesn't mean anything because the inputs have been driven into saturation.


Top
 Profile  
 
Post New Topic Post Reply  [ 23 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Pierrot du 82 and 6 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB