It’s the same as measuring any other AC signal. If the scope is rated for 120 VAC (most are), just place the AC across the vertical input. An isolation transformer on the scope is a good idea, and it’s sometimes necessary for certain testing.
I’ve used AC line voltage as a 60Hz reference for a Lissajous pattern, since the frequency supplied by the power company is accurate and stable.
Be certain your test leads are in good condition. Be careful!
I’ve used AC line voltage as a 60Hz reference for a Lissajous pattern, since the frequency supplied by the power company is accurate and stable.
Be certain your test leads are in good condition. Be careful!
First you'll have to determine if your inverter's output is common grounded or floating. Then you'll pick the appropriate probes:
Passive High Voltage Probes | Tektronix
or
Differential Probe - High Voltage | Tektronix
Passive High Voltage Probes | Tektronix
or
Differential Probe - High Voltage | Tektronix
with a dual trace scope place each probe on one leg. Invert one input if need be, place the scope in the appropriate mode to add the two inputs together (assuming it does this). You may put the ground let on the ground of the AC.
Of course USA 120vac has one leg that is ground. BUT you had DARN WELL BE CERTAIN that you KNOW which prong that is, and then put the scope ground on that leg. IF YOU MESS THIS UP, POWWW!
You can also lift the SCOPE using an iso transformer, but then you may put ur scope CHASSIS at 120vac above ground and risk an electric shock to yourself.
EXTREME CAUTION AND CARE IS REQUIRED
Of course USA 120vac has one leg that is ground. BUT you had DARN WELL BE CERTAIN that you KNOW which prong that is, and then put the scope ground on that leg. IF YOU MESS THIS UP, POWWW!
You can also lift the SCOPE using an iso transformer, but then you may put ur scope CHASSIS at 120vac above ground and risk an electric shock to yourself.
EXTREME CAUTION AND CARE IS REQUIRED
That is correct, an engine generates ac, then rectifies to dc, then passes to an inverter to produce 120vac. It's supposedly more efficient for the engine and the inverter produces a cleaner signal.
I didn't make much of a distinction between measuring the generator or house mains - I thought both would require the same procedures and safety precautions.
this is never EVER a good idea! testing for Darwin Awards is that "certain" testing
always keep the scope grounded and float the other thing with an isolating transformer is the correct way. by attching the probe ground you then establish the new ground reference point. this is for testing only and never leave the bench unattended without safety precautions.
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Hard to float the mains...
The inverter, may or may not be isolated from a real ground, and as far as sitting on the outlet side of it, that makes no difference when you get an electric shock.
In the USA you could test from either prong and put the scope on the "center pin" aka "safety ground" and look that way. One prong is ground, and the other hot, so the scope probe can only be on the hot or ground, the scope ground can never then be on HOT.
An iso transformer will have the effect of a LPF (low pass filter). This is a valuable feature, since this is a good way to get rid of all sorts of transient and HF crud from the line. So, measuring the AC run through an iso transformer, after the xfmr won't tell you much about crud that is before the transformer...
_-_-
The inverter, may or may not be isolated from a real ground, and as far as sitting on the outlet side of it, that makes no difference when you get an electric shock.
In the USA you could test from either prong and put the scope on the "center pin" aka "safety ground" and look that way. One prong is ground, and the other hot, so the scope probe can only be on the hot or ground, the scope ground can never then be on HOT.
An iso transformer will have the effect of a LPF (low pass filter). This is a valuable feature, since this is a good way to get rid of all sorts of transient and HF crud from the line. So, measuring the AC run through an iso transformer, after the xfmr won't tell you much about crud that is before the transformer...
_-_-
true but that isn't the topic of this here thread, besides most are concerned about their output quality not the quality of the power companies input . my attitude is If someone has to ask about measuring the input mains for purity / integrity then they probably shouldn't be the one to do it anyway. BTW a transformer acts more or less as a bandbass filter.
I know most of you guys got thrown by the incorrect title , so choose that way out. I reckon more than 50% of the DIYA thread titles are wrong, ppl rarely ask the right question at the start.
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A transformer is indeed a bandpass filter.....AND an absolute necessity to avoid frying stuff, including the user.
The transformer is DESIGNED to pass the line frequency fundamental at 50 or 60 Hz, so the low frequency rolloff is not an issue.
A purpose built ISOLATION transformer, especially a large one with an laminated iron core will exhibit a HF rolloff as low as a few KHz. It may not show the residual high frequency switching noise present on the AC output of the inverter. Use this only if there is no other alternative. Just connect the scope to the transformer's output, and connect it's input to the generator. The scope MUST be capable of accepting 120 VAC.
A purpose built ISOLATION transformer, with a TOROID core will exhibit a HF rolloff as in the 5KHz to 20 KHz range. It will work better than a laminated core transformer. Just connect the scope to the transformer's output, and connect it's input to the generator. The scope MUST be capable of accepting 120 VAC.
A small low voltage transformer, like a 6.3 or 12.6 volt heater transformer with a laminated core will usually go beyond 5 KHz and the output is at a safe voltage level. These work best when a resistor is connected across the secondary (100 ohms or so, not critical).
A small low voltage toroidal power transformer is a better choice, and the one to use, Again loaded with a resistor, unless you have....
I use an OPT from a vacuum tube amp. Just plug the primary into your AC source, including a US spec 120 volt AC outlet, load the secondary with a resistor that's reasonably close to its rated load impedance, and connect your scope, distortion analyzer, or other measuring equipment across the load resistor. You will get about 3 watts of 60 Hz "output" from this with a typical 5000 ohm OPT on 120 VAC. This can be done on 50 Hz 240 VAC, but you need to make sure your OPT and load is rated for the power it will pass.
I could watch the distortion on my household AC line go from about 4% in the middle of the afternoon to over 8% with visible transformer saturation (their pole transformer) around 6PM when the power demands increased.
Hi all. I really appreciate all the time and advice but my head is swimming. I thought there would be a simple answer and consensus since I'm not trying to do anything that countless others haven't already done. I also don't understand the comments about the incorrect title and asking the wrong questions. And suggestions that violate diya safety rules is definitely a cause for concern.
So...maybe I'll sit this one out and not even attempt it.
Again I do appreciate your efforts.
So...maybe I'll sit this one out and not even attempt it.
Again I do appreciate your efforts.
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your title says "measuring mains power" which is to say the fixed wiring in homes and business supplied by the power company. When you really wanted know more about inverter generator measurements. Since some (most) other members saw the title their reply's are concerning how to measure mains using an O-scope powered by the same source. (not your topic) causing you even more confusion. Title should of said 'generator power' not 'mains'. but anyway your topic is now OT haha, so time to get out (kidding)
Ahh I see now. I didn't know it would make a difference where the 120V was coming from. Two different problems, I get it now.
Please forgive my ignorance, it's all part of my learning curve.
I like your idea about using tube amps OPT.
also you can get other indications of poor AC power easily with an DMM w/ AC true RMS, usually from flattened peaks due to worse PF (what you see = distortion) and IR drops . BTW Transformers don't saturate with increased current demands <full stop> / the power companies don't operate their stuff close to saturation to ever be a worry esp. in hot areas. IMO High Freq noise is becoming a bigger concern and is outside the bandwidth of using the regular transformer methods.
I concur with your transformer bandwidth statement but noting the inverse is desirable for actual use in equipment. Best using an EI core with a split bobbin will give better mains filtering over a toroid due to high inter-winding capacitance.
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