PQ Scope Picture & Question Follow up

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iwire

iwire

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Back in March I stated this thread PQ Scope Picture & Question asking about the following picture ...

PX5PQRB1.jpg


It was suggested the problem was in the meter, well it was, sort of.

I noticed while using the meter that while on internal battery the current looks smooth but when I plug in the external power supply the current sign wave gets all 'jiggly'.

Anyone have some easy fixes or ideas about this?

Thanks, Bob
 
mxslick

mxslick

Senior Member
Location
SE Idaho
Hi Bob, hope you're doing well.

Two ideas:

One, the external power supply is bad...use a standard scope to look at its output to see if there is anything other than DC coming out of it.

Two, the connector for the external power supply (either the cord end or the internal one to the PQM) has a bad/loose connection.

Either one would impose "noise" on the meter's operating power that could cause this problem.

A less likely possibility is a ground loop caused by the external power supply (if it has a grounded AC cord set) to test that out, use a ground lift adapter as a temporary test.

I don't think a MWBC has anything to do with this one either. :)
 
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MichaelGP3

Senior Member
Location
San Francisco bay area
Occupation
Fire Alarm Technician
Filter capacitors in the external power supply going/gone bad. If you are inclined to peek under the hood, see if they are bulging or leaking. Of course, they can look ok externally, and still need to be replaced.
 
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Volta

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Location
Columbus, Ohio
Is it always the negative "C" leg in the display, or is it always during the negitive peak of the leg that happens to provide the external power?
 
mivey

mivey

Senior Member
Thanks for the follow-up. Can you lift the ground? What kind of external supply is it: 2-wire or grounded? Ours is powered from the load, so I'm thinking ours is isolated. Not so with my analog scope so I have to use isolation transformers with it.

How about an isolation transformer for your power supply?

What did you find out about the load?
 
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gar

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Ann Arbor, Michigan
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EE
100801-2019 EST

iwire:

Internal to the meter there is certainly a common reference point for the electronics, and in particular an analog signal ground. Ground does not mean it is connected to earth, except maybe when you plug in the AC power cord. This analog ground is probably connected to the internal chassis and any shielded areas.

The signal displayed on the screen is almost certainly from a single A/D converter with an analog multiplexer. The multiplexer may have differential inputs or single ended ones common to the analog common.

Whatever problem you have originates before the multiplexer. If the current transducers are normal magnetic core current transformers, then these can be essentially an ungrounded signal source prior to entering the meter. Are there only two leads from each current transformer? Is there a shield? Is there any grounding at the current transformer or an electrostatic shield in the current transformer ( a Faraday shield )?

If you have a UPS, then power it, pull its plug, and it should still produce output. Some UPSs won't produce output if off and no AC input and then switched to the on position.

Use this isolated UPS as mentioned in an above post to get AC power to your instrument but with no connection to the normal AC line. If this eliminates the problem, then there is a meter related problem. This would be in the analog area relative to the current transducer signals.

.
 
steve66

steve66

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Location
Illinois
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Engineer
My best guess would also be a filter capacitor. But I'm not 100% sure its one of the big power supply caps. It might be one of the smaller caps, like one right at the power input pin to a IC chip.

If the meter is pretty easy to take apart, you might start by looking on the circuit board for a bad solder connection. You also might try touching or pushing slightly on some of the caps while the unit is on (keeping safety in mind) to see if you can find a bad connection.

Do you have a schematic in the owners manual you could post??
 
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