Review and tear-down of UNI-T UT612 LCR meter

[BravoV]

Wytnucls, thanks for the close up shot.

Yeah, don't think that kind of modding is necessary, but as a tinkerer this is an option for the future just in case if needed. 

[Wytnucls]

The long leads you can compensate for, their resistance is constant. Kelvin connections are critical once contact resistance between the probe/clip and DUT becomes significant. This contact resistance often varies between measurements, and may be dependent on force. Kelvin connections can compensate for this variation.

Quite correct Alm. So, it is very important to have a good electrical connection to the DUT with a 2-wire system to measure small resistors of less than 1 Ohm. Whether the 2 connecting pressure plates on the UNI-T adapter are sufficiently adequate, I don't really know. Contact resistance is usually about 1mOhm.
I did run a test with a 30cm length of wire on the UT612, with the ends pushed through the blade connectors after calibration, which gives me repeatable measurements of 5milliOhms in Rs 1KHz mode. I don't have the instrument to measure the actual resistance of that wire, unfortunately.

Here is a sample of small 10% inductors measurements taken both with the UNI-T and the CEM 9935, using the pressure plates and also a sample of 1% resistors using a 1KHz frequency and short leads and clips (resistor legs are too thin to push through the plates), with the UT71D DMM as an extra reference:

[Hydrawerk]

Here is a better PCB picture, with a close-up of the jacks. At first glance, it seems possible to modify for a proper 4-wire measurement system. You would need to investigate further, before you start hacking at your meter.

There is no doubt that UT612's PCB is better designed than DT-9935.

[Wytnucls]

Here is a selection of small caps measured with both LCR meters again, using short test leads and clips and also some electrolytics:

[Wytnucls]

Some inductor measurements, with short test leads and clips:

[nadona]

Thank you very much for the awesome review.

You created the spreadsheets the same way I would have, with all of the parameters/data desired.
The photos are also very useful; they are taken in a way that I can grasp the real-life size of the parts.

After reading your review, I made one click on eBay without a second thought, and purchased one for $153 including shipping to California.

Regards.

[Wytnucls]

Thank you for the kind words. I sure hope you won't be disappointed.
Unfortunately, I don't own shares of the company (UNI-T is a privately-held company, with the owner based in Hong Kong). Let us know later if the meter lived up to your expectations.
If enough people are interested, PM Franki first, as he can probably beat that price with direct sales to members of this site.
http://stores.ebay.com/99centhobbies

[Wytnucls]

I had a look at the IET logging software. It is more sophisticated than the UNI-T program, with mostly better control of the graph display scales and a proper manual trigger control on key F12.
It is freely available on the IET site, if anybody wants to have a trial run. And, no, it is not compatible with the UT612. 
Of course, it comes at extra cost, as it needs the dedicated USB LCR meter hardware interface connector to work properly.
Incidentally, IET doesn't give you proper 4-wire Kelvin clips with their meter either. You will have to buy those separately or modify the standard crocodile ones. SMD tweezers are also available, but I don't think they are proper 4-wire Kelvin to the tip, only to the handle.

[PA4TIM]

Just install the UNI-T software that came on the included CD (unlike the IET)

My IET came with manual and a CD with software, but I have bought it including the USB interface, tweezers and psu.

The IET had several connectors. The blade connectors are true 4 wire. the adapter you can push in there forthectweezer or croc leads are using that 4 wire connection. The adapterd uses 4 guarded wires ( technical speaking the IET is 5 wire)  to go to the croks, but the croks are not 4 wire ( they are however one best made croks I have ever seen)  the tweezers are 4 wire upto the start of the tips, so the last cm or so is not 4 wire but that is more then good enough. The guarding is more important in my opinion as that last cm ( doing L and C measurements)

I found one strange thing in the IET, the banana jaks are true 4 wire jacks, so you could use 4 wire if you had special bananas ( that AFAIK do not excist) but in that case the blade connectors make still contact and so cancel your 4 wire connection. The strange thing is non of my bananas fit, the holes are just smaller in diameter. So they made special 4 wire bananas that can not be used for 4 wire technic because there are no bananas that fit and if there were bananas, the blade connctors would short out the 4 wire connection.

I just made a small alu box with 4 banana jack, 4 bnc sockets and a lemo connector and a guard connector. All parallel so can be used as in or output. I have two hirshman probes that could be opened. I made 5 connctions using 2 lemo multiconductor wires from a lemo plug to both probes. Only the last 2 cm is one wire. I used these because they are low thermal and the tips are a mix of bananajack and probetip. So multifunction.

I have good kelvinclips but not ith guarded wires. I will replace the wires with 4 x RG 179 and bnc so it will be true 5 wire.
The banana jack will be used to connect to a 4 wire multimeter, and i make a cable that goes from a bridge 3 wire system to the IET 5 wire system.
To connect it to the IET i am still in doubt. I think I make a small adapter using pcb like the original adapters and the 4 guarded wires to the lemo socket or to the bananas. Or ad a lemo socket to the IET adpter and mount the existing crock wires to a lemo plug. Then I can use a lemo-lemo cable to connect the adapter to my 4 bnc adpter. I must do some tests what gives best results.

[Wytnucls]

Thanks for that information about the IET LCR meter, PA4TIM. It seems that IET didn't take that 4-wire measurement very seriously either.
As Alm pointed out, a major benefit of the 4-wire system is to cancel out most of the errors generated by contact resistance between DUT and test gear. So croc pincers or tweezers with only 2 contact points don't really qualify as precise 4-wire systems.
At least, with the DE 5000, you have the option to improve on the standard IET gear, by making your own Kelvin clips or tweezers, using the 4-wire blade connections.
As far as I understand, the guard connection is only necessary for resistance measurements above 10 MOhms and may have little effect, if any, on L and C measurements.

[PA4TIM]

The IET makes a very good job out of the 5 wire technic. Only strange the bananas are added. You can easy exand the 5 wire conection by altering the crocs adapter. That s what I probably will do. For a DC uOhm meter the adapers like they are now, would not be the way to go. But this is a LCR meter.
The resistance added by the crocks themselve or the last cm of the tweezers is neglectable if you consider the accuraty and resolution. I did buy this for LC measurements. ESR is a non accurate parameter ( it is instable, every measurement will differ more as the R residu from the adapters)
And for mOhm I use my GR1608, Boonton or 7,5 digit multimeters. For high Resistance measurement
Guarding has an advantage too, but then shielding is often enough. I have a POhm meter that uses a guard driven sense input,

I made a 4 wire adapter yesterday that uses 4 bnc and optionl guard to connect to lcr meters, bridges and multimeters. Also made rg179 wires to my kelvin clips and rebuild two hirshman probes. The difference between the two is 4-8 mOhm. Easy to zero out and in most modes in the error zone ( seebeck, EMC, accuracy error, ect)

More important is the fact they use 4 wires to go to the crocs and tweezers and use a guard signal for the shield. The shield is called the 5th wire. This is important for capacitance meaurement because it minimises capacitance between the wires. By driving the shield with a guard signal there is no voltage difference between the sense line nd shield, hence no capacitance. Ths technique was common on lab grade bridges. They most times use 3 wire because reistance is not as important there. ( ESR measuring pF or aF is in the order of kOhms, so a few mOhm in the wire is noproblem, but upto 100 pF is. The guard driveh coax minimises this stray to under 1 pF after 1 meter of coax. On my GR1620 in a guarded farraday cup it s in the fF region.

GR has a few experimenters written about guarding ad multiwire Measurements.

[BravoV]

At least, with this meter, you have the option to improve on the standard IET gear, by making your own Kelvin clips or tweezers, using the 4-wire blade connections.

Ahh.... finally, actually I've been waiting and expecting to hear these words straight from you. 

Btw, thanks again for those extra measurement results with pics, well done and really like the presentation ! 

[robrenz]

More important is the fact they use 4 wires to go to the crocs and tweezers and use a guard signal for the shield. The shield is called the 5th wire. This is important for capacitance meaurement because it minimises capacitance between the wires. By driving the shield with a guard signal there is no voltage difference between the sense line nd shield, hence no capacitance. Ths technique was common on lab grade bridges. They most times use 3 wire because reistance is not as important there. ( ESR measuring pF or aF is in the order of kOhms, so a few mOhm in the wire is noproblem, but upto 100 pF is. The guard driveh coax minimises this stray to under 1 pF after 1 meter of coax. On my GR1620 in a guarded farraday cup it s in the fF region.

I hesitate to differ with you on any topic but aren't you confusing a guard connection and a driven shield? The guard is the circuit common of a simple auto ballancing bridge.  (Covered very nicely in the Agilent impedance handbook starting in section 2-14)  I think the DE-5000 is only a simple guard.

[PA4TIM]

There is much confusion about guarding because the term is used not very uniform. Not only between brands but also between models from brands themselve.

The configuration is also not obvious. The main function is to shield a sensitive input from "something"
In case of the PetaOhm meter it is a driven guard to make sure the current does not leak over the shield. If there is no potential difference there will flow no current. My Tek 130 LC meter also as a driven guard. But here to minimise stray capacitance.

Some meters have active guards and separate ground connections, but with an optional shorting bar between them. Like the Fluke 845 , 8500 and my HP sourcemeter.

It can be a passive form, i have an electrometer that has a sort of grounded guard grid between inputs and the rest. The HP 4260 also uses this. This adds capacitance,(no problem for DC) between traces to decouple, but also sometimes to make sure no current leaks from the source or higher voltage traces to the input.

I have a Fluke DC standard with a guard connector ( printed on the cabinet)  and that is just connected to ground, seperated from the low terminal by a capacitor, so this is a passive ground. An other Fluke calibrator has a guard, sense, and gnd ( 6 terminal connection) the ground is the cabinet, so fluke uses guard for two different things and both instruments are from the same line/area. I also have seen a resistor between low and guard, or low and ground.

In a capacitance meter you see passive and active guards ( guarding is in fact protecting, shielding is protecting too, so that makes it difficult to see what they mean by it.). Sometimes it is not a closed loop, so the shield is connected at one end only, sometimes it is closed ( for instance when shielding for EMC or RF ( to keep signal from entering or the otherway around, preventing radiation) .  It depends on the construction. You want an as less possible stray capacitance between souce and sense. If the sence impedance and signal is very low, a passive shield can sometimes be enough. This is very nice explained in several GR experimenters ( downloadable at IET website) but sometimes it needs to be driven. It is not allways very clear what they do. For instance in the GR1620 there are 3 ways to measure and two of those are 3 terminal. One uses two shielded GR connectors, this gives allmost no strays, the other is using three banana terminals, the third is using 2 of those bananas. You need to switch to those modes. The GR1608 and 1650 use two or three wire mode but you do not have to siwitch between modes.

I made some guarded circuits, sometimes passive by using a high Ohm resistor to couple the measured signal to the shield. No current flow, just an electric field to make potentials equial. But I used driven guard to by using an opamp to drive the shield. And things like traces, sometimes grounded, somtimes driven by a resistor or opamp.

The IET can be passive, or active, i have mot measured, but it does makes a difference in result.

Most important thing is that a manufacturer uses the way that fits the design and optimizes performance and I think it is save to say that if IET choses a way they do it for the right reason because they do know how to build good LCR meters. I think IET(GR), ESI and Wayne Kerr are the brands with the most experience in builing LCR meters, bridges and standards.

Just some things i collected about guarding, upto now i have not find a uniform explanation. Every brand seems to have its own definition about guarding. The Keithly book also talks about guarding.

[Wytnucls]

The guard connection appears to be the same configuration on all four meters (DE-5000, Mastech MS5308, CEM DT-9935 and UNI-T UT612): A simple direct connection to the PCB ground plane.

[PA4TIM]

Thanks for looking, i do not want to open my meter ( i know, the motto is " take it apart"  ;-) but that is not my motto unless it needs repair) so it looks like it is a passive guard. But in my opinion, it does not matter what kind it is, as long as it works and it must be accesible for the user. I often use farraday cups but also standards that have guard connections and then you need acces to the connection.
This is the kit I made. The lemo is for the normal probes but also to make a shieded connection to the IET. But I'm still thinking how I'm gonna do that. First I think I wil make a kelvin adapter by turning two connectors on my lath, split them and separate them by teflon make a bridge to hold them in position ( so make a connector like the double pomonas) and then to the lemo connecor. Or I maybe replace the existing croc wires with two longer ones and a lemo connector.

[dark_hawk]

Thanks for the Review, was thinking about getting one and after the review I'm going for one.

[dark_hawk]

Hi Wytnucls,

I got the UT612 mainly for measuring caps, I'm very new to electronics so I need some help please.

What are the typical values for ESR for a cap to be considered good:
Should I use this table from the old Bob Parker ESR Meter:
http://i1119.photobucket.com/albums/k636/mib_instruments/mib_instruments2/PreP1070439.jpg

Or should I use this table from peak atlas ESR meter:
http://dc338.4shared.com/img/kwg9wwYx/s3/esr_meter_table.jpeg

Or this chart from the Blue ESR Meter:
http://www.electronicrepairguide.com/images/blueesrmetertestchart.jpg


I also got these cables:

But the meter does not pass the calibration, are the cables too long? Any suggestions?

Thanks.

[Wytnucls]

Hi,
I'm no expert on electrolytic caps, but I think that the best procedure would be to work from the datasheet, if you can find one.
Most datasheets for electrolytics will give you specs based on Tan d (D).
I would first measure capacitance at 120Hz, to find out if the cap is still within the usual 20% limit of the published rating and then compare Tan d (D) with the original max figure, to see if the ESR has increased dramatically. Tan d should be no more than 150% of the max published value.
If both capacitance and tan d are found to be out of limits, the cap is probably bad.
If you can't find datasheets, those tables should help you, if you prefer to work with ESR figures. Be aware that low ESR electrolytic caps, found mostly in power supplies, will have different parameters.

Kelvin tweezers or pincers are nice, but won't give you better measurements, as this meter is not a 4-wire instrument. To pass calibration, the test leads should be shielded and equipped with a guard connection or, if not shielded, kept as short as possible. I would recommend short test leads with small croc connectors.

[dark_hawk]

Thank you for the response.

I have an assortment of caps from different manufactures and even of different series, it would take a long time to hunt every specific cap datasheet and all I need are ballpark numbers.

I don't think the leads are shielded. Will try to shorten them and see if they pass calibration.

[Wytnucls]

If you are obliged to work with ESR ballpark figures, that can be done with the dissipation factor (D) too. The advantage being that it is mostly independent of capacitance. The only variable is the capacitor rated voltage.
Here is a table listing dissipation factors for different caps from several manufacturers. You will notice that the values are not very different, even between series.
Any gross deviation from those figures for a cap under test would mean a necessary replacement.
For known Low ESR caps, any measured dissipation factor above 0.10 should be treated with suspicion.

[don.r]

Wytnucls (or anyone with this meter),

I know the maximum cap value is 20,000uF but will it measure ESR (or dissipation) for a cap greater than this?

Thanks

[jarvis]

Wytnucls (or anyone with this meter),

I know the maximum cap value is 20,000uF but will it measure ESR (or dissipation) for a cap greater than this?

Thanks

Use Rs mode to measure the capacitor. Rs mode is equivalent to ESR measurement.

[don.r]

Wytnucls (or anyone with this meter),

I know the maximum cap value is 20,000uF but will it measure ESR (or dissipation) for a cap greater than this?

Thanks

Use Rs mode to measure the capacitor. Rs mode is equivalent to ESR measurement.

Much appreciated. Still shopping and looking for any weaknesses for my uses. The other question i have is... does the lack of true kelvin probes compromise much on this meter?

[iloveelectronics]

Much appreciated. Still shopping and looking for any weaknesses for my uses. The other question i have is... does the lack of true kelvin probes compromise much on this meter?

One other major issue of the UT612 (seems worst on my unit than Wytnucls') is the LCD display. It's almost completely washed out if I look at it from a direct angle. I have to look at it at a 45 degree angle.