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54615B Agilent Scope Amplitude Low When Measuring 120V AC
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Bostonman
I have an Agilent 54615B scope that works fine, except, the other day I measured the AC line voltage (119.8V on a meter) and got 318V p-p on both channels.
I used another oscilloscope and confirmed the amplitude to be approximately 340V p-p. At low voltages such as the probe calibrator of 5V, the scope measures correctly. It has passed on the self calibration, and, from what I see in the specs, it shouldn't have such a huge error at high voltages. Does anyone know if this is normal of does something needed adjusting? |
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What probe are you using in each case?
What model is the other 'scope? AC or DC coupling? The 54615B AC coupled LF 3dB point is 10 Hz and accuracy only 2% so a 10% under-read at 60Hz is not unreasonable. Generally speaking it is not a good idea to measure the mains with a conventional mains powered 'scope without using specialist probes. Robert G8RPI. |
开云体育From: Bostonman I have an Agilent 54615B scope that works fine, except, the other
day I measured the AC line voltage (119.8V on a meter) and got
318V p-p on both channels. Well... Assuming a good undistorted sine wave (unlikely but later...) 119.8 x Sqrt(2) = 169.42V Pk.?? (Was the meter a "True RMS" type or just any-old meter, DMM or moving needle!? Plus what is it's supposed and/or proven accuracy.)x 2 = 338.84V pk to pk.?? BUT!?? If the mains waveform is at all distorted (Highly likely) it is probably "flat topped" (because of the thousands of low power non corrected rectifier/capacitor input power supplies hung off the line, not just in your premises. So you indication of 318V p2p is in the ball park.?? Only some 6%
adrift.?? Not to bad in truth for a mains power reading taken with
two different instruments, without knowing the distortion factor /
harmonic content due to poor power factor loads elsewhere, and how
said instruments perform in the presence of such distortion. 73. Dave B (G8KBV)
-- Created on and sent from a Unix like PC running and using free and open source software: |
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A suitable probe is No matter how careful you are about personal safety, even normal surges and spikes on the mains supply cn be high enough to damage normal probes and your scope. |
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Robert makes a good point. Totally isolated equipment is safer for line voltage measurements.? On Mon, Nov 15, 2021 at 1:01 PM Robert G8RPI via <robert8rpi=[email protected]> wrote:
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Jeremy Nichols 6. |
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Bostonman
As far as incorrect voltage, the 340V is pretty much spot on with the other oscilloscope, and the meter is a true RMS that is reading 120V AC.
So you're thinking that this scope has an acceptable error at this high voltage? This makes sense, and at least helps to know the scope isn't actually broken. Isolated inputs is something I've never tinkered with, but have heard they should be used. Isolating equipment from the outlet is a concept I never grasped, and always been afraid of messing with it. Measuring line voltage was just for fun and not something I do often, but I guess it isn't the smartest thing to do. Does a more affordable solution exist rather than $1k on that probe in the link? |
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You don't necessarily need isolated probes, you just need to have isolation between you and the test setup. Please learn from my mistakes where I have fried probes and oscilloscopes. Here are some hard learned lessons: 1. If you aren't sure that your oscilloscope inputs are isolated then assume they are not. This is the correct assumption 99.99% of the time. 2. The ground lead on that probe is there protect you 3. The ground lead on the oscilloscope is also there to protect you 4. Resist the temptation to circumvent ground with isolation transformers for the oscilloscope power or by cutting the ground path. Making a high fidelity high voltage measurement takes a bit of thought. The crudest method is to use 2 probe channels to make a crude differential probe. You connect the probe grounds together and not to the DUT. 1 probe goes to your signal and the other to your DUT ground. Then you subtract the two channels. Better yet you can get a proper differential probe ($$) or a high voltage rated oscilloscope probe meant for this type of work ($$$). I personally prefer another option, using a battery powered oscilloscope ($-$$$) instead. These can be purchased with the same type of safety ratings as a DMM which a regulator oscilloscope could never achieve. But to reiterate: the operator danger is when you are touching any part of the DUT or test setup. Avoid this when you can but setting up the measurement beforehand and then switch the DUT on safely. Cheers, Neil On Mon, Nov 15, 2021, at 9:41 PM, Bostonman wrote:
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One thing about Neils post,
the "earth" lead on the 'scope probe is not their for safety. It's just for signal fidelity. The resistance and current rating of the lead is not adequate to provide protection against shocks or fire. I've seen more than one 'sope lead literally smoking over the years when somone (not me) made a bad connection. Robert G8RPI. |
开云体育If you use standard X10 'scope probes, one way to measure something like AC mains is to use both 'scope channels, set to ADD with one of the channels inverted.? This then gives a single trace on the 'scope which is in effect a differential measurement of the signal between the 2 probes.? With one probe on each side of the AC signal and the 'scope set to 5V/cm (and the probe earths 'earthed'), the sensitivity with a X10 probe is therefore 50V/cm, so 400V total, top to bottom of the screen. ? With 120V AC, I would expect the P:P signals to be ~340V, within the 400V total range of the 'scope screen. ? Just my 5p worth! ? 73, Mike, Mike Stevens, G8CUL/M0CUL/F4VRB. ? From: [email protected] On Behalf Of Robert G8RPI via groups.io
Sent: 16 November 2021 10:32 To: [email protected] Subject: Re: [HP-Agilent-Keysight-equipment] 54615B Agilent Scope Amplitude Low When Measuring 120V AC ? One thing about Neils post, |
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Bostonman
All very interesting input.
I've only measured the hot side of an AC plug a few times in my years. This most recent time was to show someone what 60Hz from an outlet looks like for real. I don't remember every connecting the ground on the scope probe as I just leave it dangling. Most likely this would all make more sense if I someone was physically showing the connections and/or how one could get injured. Personally it seems like very little can go wrong providing one doesn't slip with the probe and short both hot and neutral (or hot to Earth ground). The other way to get injured would be to unplug the scope probe and accidentally touch the end while the probe is connected to the hot. In any case, I don't plan to measure the hot directly again, but sense this scope has a roll over or something causing the higher voltage measurements to show lower than they are. |
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I used to work in the EE lab of a university, can't tell you how many times students would bring us a scope probe with a burned up ground wire.? We had the option of an isolation xformer or even use a differential input. If we knew the student was measuring in the area of 120, 240, or 208 three phase, we would offer assistance, but we usually heard a bang as they blew a ground. Don't ever recall any actual damage to a scope though. ? |
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As long as one side of the mains is nominally at "earth ground," you can simply connect only the tip of a suitably rated attenuator (10x or 100x) probe to any point. Use DC coupling to avoid any LF rolloff issues. The line side should show the full line voltage waveform. The neutral side should show the neutral/ground junk, which should be nearly zero in the scale of things. Do NOT connect the ground clip from the probe - it's actually best to remove it from the probe if it clips in, or tape it over if not, to avoid inadvertently making a connection. At low frequencies the probe ground doesn't matter.
As others have mentioned, the line waveform is remarkably distorted, so don't bother comparing DMM, true RMS etc measurements with the time domain scope display. The scope will show what it actually looks like, as accurately as the probe/scope combo can. Ed |
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Bostonman
This is all useful information about safety. I don't want to deviate too much, but does seem the original question is somewhat answered.
Ignoring the safety aspect of measuring 120V, is it safe for the scope to be measuring 120V or should some soft of device be used to reduce the voltage? I have measured 10s of thousands of voltage using a high voltage probe, but that was with a DMM. As I stated, I need to do more research on the safety aspect. I've never heard of anyone blowing up a scope or a probe, however, I remember blowing a fuse at an old job. |
开云体育Completely neglecting any powerline considerations, most scopes
have a maximum vertical input voltage rating, typically a bit more
than the peak to peak input at the maximum volts/division.? This
does not consider the probe. I'd feel reasonably happy measuring low frequency waveforms with an X10 probe, 5 volts (50 volts/div scaled) at about 200 volts plus. Probes have a maximum voltage limit. Scope inputs have a maximum voltage limit. All of those go down as the frequency goes up. That's on a 1 meg input impedance. For a 50 ohm impedance, don't even think it. Then again, I have a high voltage scope probe, too. Harvey
On 11/18/2021 10:06 PM, Bostonman
wrote:
This is all useful information about safety. I don't want to deviate too much, but does seem the original question is somewhat answered. |
Jeremy Nichols