Author Topic: I want to use my Rigol DS1054Z to measure 72VAC & 93VDC, how to do safely?  (Read 232 times)

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Offline SpannersToSparks

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    I have a home built power supply using a rescued transformer. As the taps are set right now, its 240VAC 50Hz in, stepping down to 72VAC out, then rectifying and giving 93VDC. There is a 1500 uF capacitor after the rectifier. I want to assess how effective the smoothing is, and if there is any other interference. I know I can use a DMM set to AC to get a feel for ripple, but I would like to look at the waveform in a little more detail.

    The PSU is powering a makerspace controller project for a robot arm, and as part of troubleshooting I want to look at the waveforms being supplied to it.

    My scope is only CAT I rated, and though it says 300V max, some searching around the internet suggests that just plugging high voltages into a cheap scope is not a good idea. So I have looked at differential probes, HV probes and home built voltage dividers. I am also keenly aware that the BNC shroud is a dead short to ground, so my question has two parts.

    • Do I need a specialised probe or attachment to measure these voltages? Ideally I would prefer to buy if relatively cheap but if unavoidable am happy to build.
    • Where should the two probe elements be attached for safety? In the diagram I have attached, I have guessed for AC that point 3 would be the "hook" connection for the probe, but where is the safe connection point for the ground clip, if used at all? I am guessing that if I hooked the ground clip to point 4, that would be the dreaded ground loop, is that correct? If so, where should the ground clip go, if at all?
    • For DC, are the selected points appropriate?

If people are curious, the problem I am troubleshooting is that when switched on, the arm makes a noise like a boiling kettle and occasionally one of them makes a kind of "rubbing" sound. This is all when not moving. As the only live components are 6 industrial spec AC servomotors running on 3 phase from a set of 6 amplifiers, I am assuming they are the source of noise. I inherited a cable that had been cut from the controller, so I am flying a little blind. The cabling for the resolvers is passed through an RJ45 connector for each motor, and then there are separate UVW coils for each.

All servos came with resolver feedback and shielding, though if all shielding lines are connected to the RJ45 grounds, the arm will not operate. Some do need to be connected. Unfortunately I no longer have the original controller or detailed schematic to understand how the shields were wired in the original controller, so I am hunting down causes the hard way at the moment.

I would also like to measure the output on the motor phases, is the same issue of ground loops going to be present if I attach the ground clip across a servo phase and its ground?

« Last Edit: July 01, 2019, 08:42:44 pm by SpannersToSparks »

Offline rstofer

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According to the User Manual, with a 10x probe, the maximum input is still 300V (3,000V at the probe) and CAT II.
So, from a voltage point of view, I would just use a 10x probe and realize the scope is only seeing 7.2VDC.  IF the probe got inadvertently set to 1x, no big deal, the input to the scope would be 72V, well below the 300V limit.  But I almost always use 10X.  If I were truly paranoid, I would buy a 100X fixed attenuation probe.  No switches...

There is nothing to see on the output of the transformer that won't show up on the output of the rectifiers so why bother?

As long as your supply is EXACTLY as drawn with no hidden earth ground connections, you should be good to go on the DC side.  You will be effectively grounding the negative side (and there's doesn't seem to be a reason this won't work) when you connect the ground clip.  But you could ground the negative rail anyway and, in fact, you could do that with a piece of wire before you even begin probing.  If you do that, you absolutely don't want the the transformer secondary grounded.

If you simply must look at the transformer (and I can't think of a single good reason for doing that given that the rectifier output is correct), remove any earth grounds that may exist on the DC rails and ground one side of the transformer secondary.  This will be where you place the probe ground clip.

Basically, you don't want the scope to be providing an earth ground to anything.  It is far better if you have already grounded a DC rail or transformer secondary lead external to the measurement process.  This will eliminate potential arcs, sparks and smoked scopes.  BTW, if you do apply an external ground to the DC (-) rail, you probably don't need to use the scope ground clip.  The image will be close enough just using the probe tip and the fact that the scope is already grounded.

I don't know what you expect to see.  It doesn't seem like the kind of supply where mV of ripple are going to be a factor.

BTW, when you are looking at the DC link and you want to see ripple, choose AC Coupling and reduce V/Div so you can see the ripple riding on the DC.

Dave did a video:

It's a best seller...

« Last Edit: July 02, 2019, 01:05:51 am by rstofer »

Offline schmitt trigger

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Or even better, in the trigger menu, select line triggering.

Ripple at the powerline frequency will be perfectly still.
If there is other un-synced ripple, for instance something drawing high current pulses, it will be very obvious.

Online HB9EVI

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+1 for the fixed 100x probe

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