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Old 10th Jun 2018, 2:02 pm   #1
Philips210
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Default Displaying phasors on an oscilloscope

Hi.

I was watching an interesting demonstration on an Open University programme (course T283 Ep 11) https://youtu.be/8wIbBJzEXbU?t=501 which shows phasors in an ac circuit being displayed on an oscilloscope. The line length depicting amplitude and the phase shift angle in relation to a reference phasor.
I would assume the 'scope is used in the X-Y mode but I was wondering what interface circuitry is required. I am obviously only looking at low voltage mains-isolated circuits rather than raw mains so a safe set up.
It would be interesting to build an interface circuit to achieve the results in the programme. Has anyone experimented with this type of set up on the 'scope?

Regards
Symon.
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Old 10th Jun 2018, 6:31 pm   #2
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Default Re: Displaying phasors on an oscilloscope

They probably have isolating transformers on the X and Y inputs to the scope so they can get signals from all over the circuit under test and not have to worry about the commoned and grounded inputs of the scope.


There are also some special scopes for mains powerwork with isolated amplifier inputs.

David
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Old 10th Jun 2018, 7:54 pm   #3
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Default Re: Displaying phasors on an oscilloscope

Hi Gents, the Davis books "The Cathode Ray Oscilloscope" has information on showing these vectors and may also have details so that 3 phase can also be displayed.
It would certainly use isolating transformers on the inputs.

Ed
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Old 11th Jun 2018, 2:55 pm   #4
Philips210
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Default Re: Displaying phasors on an oscilloscope

Hi.

So a main requirement would be isolation transformers as a start to constructing such a set up. Assuming we are talking low frequencies 50 to 100Hz then small iron laminated transformers should suffice. Does a 1:1 turns ratio seem reasonable or could a higher ratio be used to advantage?

I'd love to see the OU's test set up, a good circuit diagram would be good to see.
Thanks for your replies so far.

Regards
Symon
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Old 11th Jun 2018, 3:17 pm   #5
Alistair D
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Default Re: Displaying phasors on an oscilloscope

In XY mode feeding the same signal to both X and Y inputs produces a trace that is at 45 degrees to the Y axis. There must be more signal processing than just a transformer.

One possible way round the problem would be to use a scope with a round faced tube and rotate it 45 degrees.

Al
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Old 12th Jun 2018, 6:12 pm   #6
Ed_Dinning
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Default Re: Displaying phasors on an oscilloscope

Hi Symon, a transformer with various ratios would be more useful so that a range iof voltages could be matched. A current to voltage transformer would also be useful if you are looking at phase angles between V & I. There were design details for these in an article I did for Elektor about 5 years ago.
Fairly standard stuff will do for experimentation, but for accurate measurements the transformers need careful winding as they will introduce their own phase shifts.

Ed
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Old 12th Jun 2018, 10:05 pm   #7
Philips210
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Default Re: Displaying phasors on an oscilloscope

Hi Ed.

That sounds interesting, I'd like to have a read of your Elektor article. Can you recall which issue it was in so I can obtain a copy?

Re. the Davis books on The Cathode Ray Oscilloscope, I seem to be having difficulty finding any information on this. Do you have an ISBN number for this particular publication? Thanks.

Regards
Symon.
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Old 14th Jun 2018, 2:41 pm   #8
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Default Re: Displaying phasors on an oscilloscope

A current transformer could be use to drive the Y axis (naturally isolated) with a differential amplifier driving the X axis - providing you don't exceed the max difference / common mode voltage.

dc
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Old 20th Jun 2018, 7:19 pm   #9
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Default Re: Displaying phasors on an oscilloscope

Quote:
Originally Posted by Philips210 View Post
I was wondering what interface circuitry is required.
Here's a description & circuit diagram for a relatively 'simple' vectorscope:-

http://www.wireless.org.uk/XY.htm

Cheers
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Old 28th Jun 2018, 10:00 pm   #10
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Default Re: Displaying phasors on an oscilloscope

Hi Trigon.

I just noticed your post and that looks a useful set up. Thanks.

Regards
Symon.
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Old 29th Jun 2018, 11:24 pm   #11
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Default Re: Displaying phasors on an oscilloscope

Hi!

I think Ed's referring to "The Cathode Ray Tube and It's Applications" by Davie and Parr, two design engineers from Cossor's Instrument Division, published in 1959 by Chapman & Hall, (with properly drawn and stencil–lettered diagrams rather than the usual Chapman and Pitman hand–lettered scrawl these two publishers were notorious for!!) – I wholeheartedly recommend buying a copy as soon as one turns up on Abebooks, etc., although written in the valved era the basic principles are excellently covered in it!

Chris Williams
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Old 30th Jun 2018, 2:02 pm   #12
Philips210
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Default Re: Displaying phasors on an oscilloscope

Thanks for that Chris, I'll look out for a copy.

Regards,
Symon.
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