Picolog huge voltage offset with 4444.

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Joined: Sat Feb 16, 2019 12:42 pm

Picolog huge voltage offset with 4444.

Post by DwightG »

Hi all, I've sent through an email to support on March 8th but not yet received a reply. I'll summarise here if anybody can help.
First some info;
a). I use my Picoscope mainly for low voltage monitoring of IGBT firing etc but mainly high voltage measurements. That means 3 phase power to 480VAC and DC to 872VDC, (at this stage I split a battery in two about a neutral point so we're really looking at +/- 436VDC).
b). My set up consists of;
PIcoscope 4444.
2 x TA301 current probes.
2 x Picoconnect 442 differential probes.
Connection points.jpg
Referring to the above attached drawing, (Connection points.jpg) -
Current probe 1 was clamped at A.
Current probe 2 was clamped at C.
Voltage probe 1 was connected at A and B.
Voltage probe 1 was connected at B and C.

Picolog is configured such that;
Channel A, (blue), is battery volts +Bus.
Channel B, (red), is battery volts -Bus.
Channel C, (green), is battery current +Bus.
Channel D, (yellow), is battery current -Bus.

I've added some maths channels;
Channel "E", I've called Delta and it just monitors the difference between the voltage magnitude of the channels. So it is just Channel A minus channel B.
The problem here is that with nothing connected it still registers some millivolts of course but the maths channel only sees the figure and doesn't see the difference between 400V and 400mV.
You can of course predict what happens when I use a maths channel to keep a monitoring eye on the power being consumed.

In addition to the above, the scope would not read correctly. Two multimeters registered about 409 volts but Picolog was seeing 509 volts and this error grew slightly strangely enough as the voltage dropped.

What is odd is that closing Picolog and opening Picoscope with those same probes connected generated no such errors.

The current readings were spot on.

Lastly, I reckon Picotech have missed a trick with the current probes.
Not sure what cable you guys may be using but I typically use double insulated rubber cable such as H07 or Type 4C. Now the current clamp is rated to 2000A and has a jaw opening of 32mm. Even a single insulated Tri rated cable, (H05V2-K or H07V2-K), of 400mm2 will fill these jaws completely yet pass way less than 2000A. A double insulated is even more restricted. What cables are they designed to measure?

Any help much appreciated.

Posts: 936
Joined: Mon Aug 11, 2014 11:14 am

Re: Picolog huge voltage offset with 4444.

Post by Gerry »

Hi DwightG,

I see that your Support Request is being handled by one of my colleagues.


Technical Specialist

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