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Old 27th August 2004, 07:11 AM   #1
Tarzan is offline Tarzan  Belgium
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Question Scope connection on Audio Dummy load

I have a dummy load made up of various High Power resistors of 1 Ohm. I can easely handle 1500 Watts in 8 Ohm and 6000 Watts in 2 Ohm.
I like to monitor the waveform with an osciloscope but like to keep the amplifier intact after connecting the instrument.
I think to use an audio transformer to seperate the load and the osciloscope to avoid a short in the case of a bridged amplifier.
Any suggestions on what transformer to use?
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Old 27th August 2004, 11:46 AM   #2
SY is offline SY  United States
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Why not use an instrumentation amp? Simpler, cheaper, higher bandwidth.
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Old 27th August 2004, 03:49 PM   #3
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How about a power line isolation transformer for the scope?
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Old 27th August 2004, 05:34 PM   #4
Tarzan is offline Tarzan  Belgium
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Explain how to use an instrumentation amplifier?
Voltages on the load can be as high as 300 V AC.
With the latest generation of High Power amps with concequent higher voltages this can only raise.

The idea of using an isolation transformer is out of question.
You have to forget to remove the earth only once to find a burned power stage.
No No, isolate the signal towards the measerung instruments (I said scope, but this can also be a spectrum analyser...)
I my opinion the safest way (for the equipment)

Thanks any way,
Marco
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Old 27th August 2004, 06:01 PM   #5
moamps is offline moamps  Croatia
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Hi,

High voltage differential oscilloscope probe is good solution.
http://www.tek.com/site/ps/0,,51-11195-INTRO_EN,00.html

Regards
Milan
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Old 27th August 2004, 06:30 PM   #6
Tarzan is offline Tarzan  Belgium
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Euh, Yes?!
But what about the price of these?
I prefer a much cheaper solution.
Perfection is not needed, visualisation is.
Thanks,
Marco
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Old 27th August 2004, 06:37 PM   #7
moamps is offline moamps  Croatia
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This one isn't expensive.
PICO ADF25A
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Old 27th August 2004, 07:05 PM   #8
SY is offline SY  United States
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Instrumentation amps are explained in detail in any opamp text or apps book. I'd draw you one, but I'm nowhere near my scanner at the moment. But basically, it's a few opamps and resistors. Do a quick search at Analog Devices and National Semiconductor websites. Voltage range is rarely a problem since it can be scaled with resistive voltage dividers.

I'll admit to having used Bill's solution (isolation transformer on the scope) more than once, but at these voltages, it's potentially dangerous, if you'll pardon the pun.
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Old 27th August 2004, 07:09 PM   #9
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Quote:
Originally posted by Bill Fitzpatrick
How about a power line isolation transformer for the scope?
That is my best practical soulution as well and what I would do.

In case it is not obvious, the 3rd pin (earth ground) on the scope power cord (connects to scope chassis) has to be left floating. The scope chassis only gets connected to the point in the amplifier circuit under test though the scope probe shield lead along with the scope probe hot lead. Place the entire o-scope on a phone book or insulated carpet and do not allow it to touch any other equipment.
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Old 27th August 2004, 07:34 PM   #10
Tarzan is offline Tarzan  Belgium
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Thanks guys but have you ever seen an amp going up in smoke because you forgot to remove the earth lead from the test equipment?
I did and it whas an expensive experiance.

It has to be a setup without thinking twice about earthing the test gear.

To get back at the original proposal in using an audio transformer or even an opto coupler?
The latest I have no experiances with.
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