Looking at output from 12-->120v inverter with o-scope?

Discussion in 'Test & Measurement Forum' started by MikeA, Mar 12, 2018.

  1. MikeA

    Thread Starter Active Member

    Jan 20, 2013
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    I would like to look at the output waveform. How would I hook up the oscope to get a good accurate reading?

    Step down isolated transformer?
     
  2. Danko

    Member

    Nov 22, 2017
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    See right by oscilloscope. 50V/div setting, and 1:10 probe.
     
    Last edited: Mar 12, 2018
  3. Reloadron

    Distinguished Member

    Jan 15, 2015
    3,286
    1,450
    For a US standard outlet:
    Outlet Typical.png

    You will need as mentioned a 10:1 probe. The above outlet is just a typical example of a 120 VAC US standard wall outlet. Should your inverter (UPS) use this type place your probe ground on the Neutral side and the probe to the Line or Hot side. Large blade hole is Neutral and the smaller blade hole is the Line or Hot. Again, you will need a 10:1 probe for your scope. Keep in mind a scope displays a peak to peak waveform so for example a standard 120 VAC sine wave is an RMS value is about a 340 Volt peak to peak signal.

    Additionally lower cost inverters and UPS units output a MSW (Modified Sine Wave) so unless you have a TSW (True Sine Wave) inverter you will see a modified sine wave output. Some simply output a square wave so know your inverter type so you know what to look for. Consider also if you measure the output voltage of an inverter using an AC voltmeter that if the waveform is anything other than a true sine wave you need a good true RMS responding meter. The cheaper average responding RMS indicating meters will not give you an accurate indication of the output voltage.

    Ron
     
  4. MikeA

    Thread Starter Active Member

    Jan 20, 2013
    159
    21
    My o-scope does have the standard switchable 1:1 and 1:10 probes.

    My concern is grounding the neutral. I don't know what kind of funny business is going on inside the inverter.

    Are isolation transformers used for these/similar types of high voltage AC measurements?
     
  5. Reloadron

    Distinguished Member

    Jan 15, 2015
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    If the inverter is totally stand alone and not, as in the case of a UPS (Uninterruptible Power Supply) grounded to system mains then measuring the output is not a problem. If the inverter has a battery and is supplied by system mains power then you want to view the signal as I mentioned. Normally Ground and Neutral are bonded (tied) where power comes into a residence. The outer shell of a scope vertical input, unless using a differential probe or scope with differential vertical input are tied to ground. This is why you do not want to try and connect a scope probe ground to an outlet Line or Hot side. You connect the vertical input outer shell to Neutral and the probe center to the Line or neutral.

    Can you incorporate an isolation transformer? Yes, as long as the inverter output is a good true sine wave. However, if it is a square wave or MSW (Modified Sine Wave) you are not likely to get a good rendition of the inverter output waveform. Do you know what you have? The inverter should have a data sheet describing the output waveform.

    I have an APC 1500 Watt inverter sitting here and ground and neutral, per my posted outlet drawing, are not bonded but as soon as I plug it in than the neutral and ground become bonded at my service entry. If the unit is plugged in to mains, using a 3 wire grounded cord then neutral and ground are at the same point and bonded. If you connect scope probe ground to neutral and the probe tip to Line or Hot side you should see your waveform just fine.

    Ron

    Ron
     
  6. ebp

    Well-Known Member

    Feb 8, 2018
    1,401
    464
    Most two channel scopes can be set for "differential input." To do this it is necessary to have a channel summing (A plus B) function and a channel invert function. In the old days these were switches, in digital scopes they will be somewhere in menus. Typically the trace is moved vertically with the control for the non-inverted channel.

    Use 10:1 or 100: 1 probes. Set both channels for the appropriate deflection factor. Set for summing and inversion of one channel. Connect one probe to each side of the AC output. You may or may not get cleaner traces by connecting the probe ground leads together and may or may not get further improvement by connecting them both to the chassis of the device under test.

    Any time you are in doubt about ground issues with a scope, check first. Don't connect the probe ground lead to anything. Set the scope for the highest voltage you might encounter and carefully probe the place to which you would like to be able to connect the scope ground lead. If that point is "floating" you may see a voltage into the tens of volts, but since you are using a high-impedance probe it doesn't necessarily mean anything. If you measure a few volts or tens of volts, very carefully connect the scope ground to point you looked at through a resistor. Select a value that will survive if the voltage difference is "real" - e.g. 56k if there might be 120 V across it and you have a 1/4 W resistor. Now check the end of the resistor connected to the circuit under test with the scope probe. If the point was "floating" you would now see only a tiny voltage. If there is still significant voltage there is current flow, which you can calculate. If you measure NO voltage even with the scope gain cranked up it should be OK to connect the scope ground to that point, but if any doubt remains use a meter to check for current between the two. Use a series resistor to protect your meter from overcurrent.

    You can use a meter instead of the scope for all the preliminary tests if it is more convenient.

    You must approach this testing with great care, since anything that might damage the scope will probably damage you.
     
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