Can I directly measure 220V AC with a oscilloscope? - diyAudio
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Old 19th September 2010, 09:40 PM   #1
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Default Can I directly measure 220V AC with a oscilloscope?

Recently, I suspect my turntable motor is producing hum at 45 rpm (there is no hum at 33 rpm) therefore I wish to know if I can directly measure AC voltage from the speed controller's output with a oscilloscope to find out if there is any hum from it?
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Old 19th September 2010, 10:20 PM   #2
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Yes. Just be careful in case there is no power line isolation. Best to use an isolation transformer or, in any case, don't connect the 'scope's ground lead unless you are sure of what you have.
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Old 20th September 2010, 12:24 AM   #3
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And as long as your oscilloscope is rated for it. Not all are, although a standalone conventional instrument probably is. If it's a USB PC scope though, then it probably isn't.

w
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Old 20th September 2010, 02:52 AM   #4
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Most scope probes I have been thinking about buying now are rated 320 VDC or less, so watch this, it is marginal.
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Old 20th September 2010, 04:11 AM   #5
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I found the manual, it says maximum input voltage 400V Vpp. Does this mean I am save to measure 220V AC?

Manual page:
Atten Electronics : Test & Measurement Manufacturer, OEM, ODM, Spectrum Analyzers, Digital Oscilloscope, Power Meters, Power Supplies, Soldering Stations, Rework Stations, RF Microwave, Training Kits

My model is ADS1062C
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Old 20th September 2010, 07:52 AM   #6
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220V rms has a p-p of 2.828 times 220 so yes, it's not safe.
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Old 20th September 2010, 08:59 AM   #7
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Are you sure that the speed controller is insulated from ac mains? If this is not true, you are in danger of electrocution by touching the scope. Your ac protection (differential switch) could trip. Do't know in Hk, but ac mains in Europe can be up to 252V rms (230 + 20%), thus the peak to peak value will be up to 726 Volts.
To display this on your scope you'll need a 10X probe, that must be rated for this voltage (752 V) dc+peak ac, and most aren't.
A solution could be to use an insulation transformer, with primary (220) connected to the motor, and the secondary (low voltage, e.g. 12 or 24 V) connected to the scope.
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Old 20th September 2010, 09:36 AM   #8
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Oh, too bad. Is there any way to measure the AC in this respect? I just want to see if the wave form is distorted or the frequency is shifting from the speed controller?
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Old 20th September 2010, 09:55 PM   #9
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you could always buy a 100:1 probe, they have 100M input impedance and so usually have a higher voltage rating.
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Old 21st September 2010, 01:35 AM   #10
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Or use a voltage divider. At these frequencies you don't have to worry about compensating a divider. Just watch out for ground; an isolation transformer is needed if the voltage you are measuring isn't isolated.
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