Author Topic: #137 BK LCR meter  (Read 9368 times)

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Offline mikeselectricstuff

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#137 BK LCR meter
« on: January 02, 2011, 12:57:13 pm »
Why would you ever need isolated USB on an LCR meter?
You're only ever going to need USB for batch testing, or maybe testing sensors - in both cases they would be standalone parts so no need for  isolation.
The only time you might need to test a component that's not isolated is in-system fault finding type applications, and even then you'd have to power off to get a meaningful reading, and anyway it's highly unlikely you'd need USB for faultfinding type applications.

Re. manual ranging and the 40pf/40uH ranges - what are those up/down arrow keys  for? I'd normally expect these to be for manual ranging. It's not uncommon for manual ranging to have more ranges than auto (e.g. the conductance range on Flukes), so maybe the 40pf/40uH are only available in manual ranging modes, if they exist...

As regards positioning of the USB connector, I suspect this is mainly ergonomic - a heavy USB lead plugged in to the top of the case will make it more likely to fall over.
 
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Offline Floyo

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Re: #137 BK LCR meter
« Reply #1 on: January 02, 2011, 02:09:34 pm »
I also couldn't think of an application that required opto-isolation, tough I think it might be more of a "better safe then sorry"  thing. One thing I can think of tough is that the usb ground is connected to the computer ground, and pc power supply's tend to be noisy, so that could possibly interfere with the measurement. But then again I do not know whether the usb power is even connected in this unit.

I myself bought the agilent u1731B (the 1732B was sold out >:(, and I needed the meter quickly). I quite like it, as Dave said the case is nicely shaped, and feels quite rugged. One thing I don't like about the Agilent though is that it does not come with the infrared-->usb cable, and that there are different types of cables for different types of Agilent meters. I also bought the u1272a Dmm (I must do a review of that sometimes, when I find out how to open it up :P), which of course needs a different cable.   


About the price, I agree it is quite a lot of money to pay for these kind of units, but then again, as a beginner hobbyist you generally do not really need an lcr meter anyway. And once you get to a more advanced level you are likely to spend more money on your hobby anyways.

Then I have a question, The Agilent meter does not have a dedicated ESR mode. But when I put the meter in impedance mode and select series measurement, will this give me the ESR. I was just wondering, because if I measure like this and I put  a 1K resistor in series with the cap it shows the impedance of the 1K resistor (and of course the "ESR" which is negligible on top of the 1K). (@mods, if you think this question ought to have its own topic, please say so.)

Btw, nice review Dave.
 

Offline Chasm

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Re: #137 BK LCR meter
« Reply #2 on: January 02, 2011, 03:38:01 pm »
Optical isolation is one of those things you don't notice until something blows up.

In this case it would not be too hard.
While the interface is USB 2.0, the communication protocol is RS232 (as UART at TTL levels) via a FT232 (or similar) chip.
USB 2.0 <-> FT232 <-> UART

Simply whack a opto isolator in between- and respect the distances in your layout - that's it.
USB 2.0 <-> FT232 <-> ISOLATOR <-> UART

It does not even have to be fast, UART is slow, in this case only 9600 Baud.
You can power both the FT232 and the Isolator directly via USB. Ok, maybe they use another bus to talk to the FT232 like SPI or I2C, but they is not that fast either. I'd say about 3-4$ if you buy a single isolator and a couple square millimeters of real estate on the board.


Interestingly the the LCR meter can be powered via the wall wart, but not by USB.
So USB power for the whole device is no argument either.
 

Offline Chris_R

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Re: #137 BK LCR meter
« Reply #3 on: January 02, 2011, 06:12:15 pm »
Can someone explain what opto isolation is and why you'd need it?  Is it isolating light or using light to isolate or do I really just have no idea at all?
 

Offline JohnS_AZ

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Re: #137 BK LCR meter
« Reply #4 on: January 02, 2011, 06:20:59 pm »
This is a much better explanation and I could type in here ...

http://en.wikipedia.org/wiki/Opto-isolator

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Offline Chasm

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Re: #137 BK LCR meter
« Reply #5 on: January 02, 2011, 06:42:35 pm »
Since I have typed my answer already I'll post it anyway. ;)

What is optical isolation?

The goal is galvanic isolation, or simply said: No electrical connection between two parts of a circuit, or different devices.

But we still need to transmit information, that is our goal after all.
So we use light as a medium for a (small) part of the distance. Thus optical isolation. The classic part is the  optocoupler.
With "some" more parts you can transmit digital information very successfully. Here a link to a RS232 UART optical isolater build out of "standard" parts.
OR you could buy a part like the ISO7721A that has the whole stuff inside a convenient DIP8 or SO8 package. (TTL levels on both sides, 4000Vpeak Isolation)

You can get those parts for various speeds and isolation voltages. The higher the voltage the larger the distance between sides, up to parts that use fiberoptics between them, but then you need "some" distance at 500KV or more.  ;)


It is also possible to use magnetic isolators. They are used when you also have to transmit energy.
The transformer is the most obvious example. =)
The drawback is that you can only transmit AC signals and that their isolation voltage is limited.


And there is of course always a Plan B, or in this a "high tech" approach: Use a Bluetooth module, no stinking cables on the bench. ;)
(There are some cheap and simple to use BT modules available. You can use them like a RS232 interface. Quite popular, say the BTM-222.)
 

Offline sonicj

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Re: #137 BK LCR meter
« Reply #6 on: January 02, 2011, 07:29:19 pm »
bluetooth spp is sweet! with a RN-41 you can effortlessly convert anything with serial i/o into a bt enabled device.
-sj
 

Offline sonicj

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Re: #137 BK LCR meter
« Reply #7 on: January 02, 2011, 07:56:48 pm »
dave, you trashed the LCD quality with the protective vinyl film still stuck to the face! lol!  :D
-sj
 

Offline zaoka

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Re: #137 BK LCR meter
« Reply #8 on: January 02, 2011, 08:12:49 pm »
BK needs better display and better colors + protection against charged capacitors!!!

I have seen capacitors, especially in older audio amps, once discharged after a while it accumulates energy again... (could somebody explane why???) this can damage meter :(


Im thinking to purchase BK over Agilent because of ESR testing.


 

Offline DJPhil

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Re: #137 BK LCR meter
« Reply #9 on: January 02, 2011, 08:55:00 pm »
I have seen capacitors, especially in older audio amps, once discharged after a while it accumulates energy again... (could somebody explane why???) this can damage meter :(
It's a phenomenon known as dielectric absorption or dielectric soakage. The best short explanation I know of is in Art of Electronics, but there's a great article from Bob Pease about it as well.
The effect is worst with large electrolytics, which can bounce back up to 20% of their voltage or more after a complete discharge, but all capacitors have this effect to some extent. Repeated discharge occasionally helps, and leaving a discharge resistor in place for several minutes can as well.
In the end it's best to test with a voltmeter to be sure.

Hope that helps some. :)
 

Offline EEVblog

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Re: #137 BK LCR meter
« Reply #10 on: January 02, 2011, 09:41:52 pm »
Why would you ever need isolated USB on an LCR meter?
You're only ever going to need USB for batch testing, or maybe testing sensors - in both cases they would be standalone parts so no need for  isolation.
The only time you might need to test a component that's not isolated is in-system fault finding type applications, and even then you'd have to power off to get a meaningful reading, and anyway it's highly unlikely you'd need USB for faultfinding type applications.

Isolation can sometimes be required for complex automated system testing.
Long cable and hydrophone capacitance testing are examples that I have delt with.
I've also had to use exotic techniques like Open Short Load Compensation for impedence.
Mains earth grounded gear may play havoc with the results depending upon peculiarities of your system configuration.
It's not for everyone, but if you need it then you really need it.

Dave.
 

Offline mikeselectricstuff

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Re: #137 BK LCR meter
« Reply #11 on: January 02, 2011, 10:41:45 pm »
It's not for everyone, but if you need it then you really need it.
..and can probably afford it, but I can't see that the cost would ever be justified in this level of instrument.
Of course as USB power is available, you don't have the cost of an isolated DC/DC, so the cost wouldn't be much more than the isolator, but you know how manufacturers tend to add premiums for this sort of things... Probably more than the cost of a standalone USB Isolator
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Offline Chris_R

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Re: #137 BK LCR meter
« Reply #12 on: January 02, 2011, 10:43:16 pm »
This is a much better explanation and I could type in here ...
http://en.wikipedia.org/wiki/Opto-isolator
That's what I thought.  Obviously wikipedia was my first port of call when I looked it up!
I presume not all USB connections should be isolated and they only are because this is sensitive equipment.  Is that correct?
Also, why would you use opto isolation for this connection and not magnetic isolation?
 

Offline EEVblog

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Re: #137 BK LCR meter
« Reply #13 on: January 02, 2011, 10:55:51 pm »
dave, you trashed the LCD quality with the protective vinyl film still stuck to the face! lol!  :D
-sj

It makes no difference with it off.

Dave.
 

Offline williefleete

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Re: #137 BK LCR meter
« Reply #14 on: January 04, 2011, 03:42:03 am »
in my understanding an opto-isolator can be used to separate different circuits of either impedance or voltage differences for example feedback in a SMPS, control and/or ring signal detection on old tele-com modems basically anywhere that you dont want (potential) high voltage leaking into another part of  a circuit or device. they are similar to a transformer which can match impedance and provide some isolation from different circuits however they are inductive and use AC signals, with isolators you can use DC signalling and serial data
i have used some in a slightly kluged PS1 emulator controller which converts the non multiplexed button pad lines to a standard multiplexed keyboard controller, so the computer picks it up as a standard USB keyboard and the PS1 "controller" is just assigned to a key on the keyboard, arrows and 10 letters for the other buttons
 

Offline mkissin

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Re: #137 BK LCR meter
« Reply #15 on: January 05, 2011, 03:30:57 am »
It's not for everyone, but if you need it then you really need it.
..and can probably afford it, but I can't see that the cost would ever be justified in this level of instrument.
Of course as USB power is available, you don't have the cost of an isolated DC/DC, so the cost wouldn't be much more than the isolator, but you know how manufacturers tend to add premiums for this sort of things... Probably more than the cost of a standalone USB Isolator

Possibly, but the Agilent meter (the U1700 series, that Dave showed briefly in the video)  has optical isolation, and the dongle is only $31 US, which is comparable to the dongle you showed there (30 pounds).

As a matter of course, I generally try to make sure that all of my test equipment is isolated, but that's mostly just me being OCD.
 

Offline zaoka

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Re: #137 BK LCR meter
« Reply #16 on: January 05, 2011, 06:24:32 am »
I have seen capacitors, especially in older audio amps, once discharged after a while it accumulates energy again... (could somebody explane why???) this can damage meter :(
It's a phenomenon known as dielectric absorption or dielectric soakage. The best short explanation I know of is in Art of Electronics, but there's a great article from Bob Pease about it as well.
The effect is worst with large electrolytics, which can bounce back up to 20% of their voltage or more after a complete discharge, but all capacitors have this effect to some extent. Repeated discharge occasionally helps, and leaving a discharge resistor in place for several minutes can as well.
In the end it's best to test with a voltmeter to be sure.

Hope that helps some. :)


Here is another good article:

Quote
Dielectric Absorption

This is the phenomenon where after a capacitor has been charged for some time, and then discharged, some stored charge will migrate out of the dielectric over time, thus changing the voltage value of the capacitor. This is extremely important in sample and hold circuit applications. The typical method of observing Dielectric Absorption is to charge up a cap to some known DC voltage for a given time, then discharge the capacitor through a 2 ohm resistor for one second, then watch the voltage on a high-input-impedance voltmeter. The ratio of recovered voltage (expressed in percent) is the usual term for Dielectric absorption.

The charge absorption effect is caused by a trapped space charge in the dielectric and is dependent on the geometry and leakage of the dielectric material.
http://wiki.xtronics.com/index.php/Capacitors_and_ESR
 

Offline Floyo

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Re: #137 BK LCR meter
« Reply #17 on: January 06, 2011, 07:07:03 pm »
Thanks for posting the link to that article :), it coincidentally had a (very simple) formula in it that lets me calculate ESR from the Q factor and the impedance of a cap. I did some experiments, but Ill post them in a separate thread.

The phenomenon of dielectric absorption is something to watch out for in high voltage circuits, I did some experiments with tube amplifiers and those caps are charged to a voltage of roughly 400 volts, I noticed that once I had discharged them they would build up charge again, sometimes to as much as 50 volts, and that could give one quite a nasty shock.
 

Offline zaoka

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Re: #137 BK LCR meter
« Reply #18 on: January 07, 2011, 06:20:08 am »
I just got Sencore LC103 I will make small review, it test for dialectric.

 


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