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Measuring Frequency,waveform and inrush current of 240v

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Hi , I am interested in comparing the frequency and waveform of 230/240 outputs from the mains supply, a petrol generator and various inverters. I would also like to measure the max start up current of a 240v aircon compressor. ( I know that the newer fluke clamp meters could do the inrush test, but as its a one off I cant justify the cost as I already have two cheaper clamp meters)

At present I dont own and have never used a scope , but am seriously considering buying a rigol DS1052E (would eventually do the firmware hack to the 1102e). Would this scope and the supplied probes be suitable for these tests or can you sugest a set of more suitable probes that could be used with this scope or a more suitable scope.

Am I right in thinking that it would be best to power the scope with a different source than I wish to measure , i.e. use battery bank via inverter to power the scope to measure the mains sockets and vice versa. The mains tests would "only" be to provide a reference with which to compare the other sources to.

These tests are not the prime reason for buying a scope , but if this cheap scope would enable me to do the above tests then that goes a long way to justify finaly buying one.


You'll need a CAT-rated voltage probe for mains measurements. Isolating scope power is not enough to make it safe.

This is because even if isolating the scope from the mains prevents direct short circuit if live is connected to scope ground, there is a big hazard for somebody accidentally touching scope ground and grounded object. Scope has only one ground so everything becomes live if single-ended probe is used.

Here is an example of suitable probe for safe mains voltage measurements:

For the currents, you'll need something like this (Fluke 80i-110s, good for 150A peak):

I have used that current probe to measure inrush current limiter effectiveness in my power amplifier project. It works quite nice, as you can see here.

Unfortunately, those costs easily more than what you probably would pay for clamp meter.


Agree 100% with Janne.

Never use a non-isolated scope (almost all of them, except things like Fluke Scopemeters, Tektronix THS7xx series or Tektronix TPS2xxx series) without proper ground connected. It's not just the BNC ground shell that would become a shock hazard, but the whole scope is designed with the idea that ground is a safe potential, so no attempt was made to isolate the user from it. The case might be connected to ground, or the isolation between the knobs/buttons and ground might be bad.

Depending on whether you want to measure the voltage or current waveform, the correct tool is a high voltage differential probe or current probe. High voltage differential probes are also made by companies like Tektronix, and also cheaper alternatives like Testec, but it's going to be expensive regardless. For current probes, you should be able to get a relatively cheap one because you only need AC, and not a lot of bandwidth. Should be possible for under $100 used, probably more when new.

As it's low frequency, a simple mains isolation transformer should be able to do the job.

The only thing is, you need to calibrate it because the turns ratio will be slightly lower than the primary/secondary voltage rating would indicate so the secondary voltage will be higher than expected.

Start by using a 12V power transformer run of the mains, which should give a fairly good sinusoidal waveform. Measure the primary and secondary voltage with a DVM and work out the turns ratio, then you should be ready to get going.

A more accurate way would be to use a special voltage measuring transformer which will have a wider bandwidth and lower distortion than a mains transformer.

For current waveform measurement, use a current transformer or a hall effect sensor.

Another option for voltage measurement is to use a current transformer or hall effect but passing the mains cable through it many times and connecting it in series with a resistor but you'll probably find the power dissipation will be unacceptable to get enough current to give an accurate result.

How about a few turns of wire around the live of whatever it is you are trying to measure the current in?


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