UNI-T UT204 Clamp Meter review and tear-down

[Wytnucls]

This is the cheapest True RMS clamp meter of the low cost UT200 range. It comes in a simple soft nylon case with an English operating manual, 2 adequate short plastic probes (marked CAT I 1000V CAT II 600V 10A), with unmarked PVC AWG20? leads, and a lanyard. It feels quite light at 260gr.
I bought this one in Hong Kong recently, for 390 HK$ (USD 50.00) and it came with a 3 year warranty.
It has a 4000 count DMM all-in-one ADC and is meant to be used indoors only.
Data is updated 3 times per second on a small LCD display, which lacks a backlight.
The meter is powered by an easily replaced 9V battery, through sturdy spring PCB connectors, which gives about 150hrs of normal operation. The sliding battery cover is held in place with a self-tapper screw, which could work itself loose eventually.
Do not confuse this meter with the UT204A, which is not True RMS and sells for about USD 10.00 less.

[Wytnucls]

The ergonomics are well thought out for one-handed operation (right hand), as the design, for the most part, follows the industry standard for these clamp meters. The clamp jaw is moved with the index finger and the rotary range selector is turned easily with the thumb. The 3 selector buttons are also accessible with the same thumb. Unfortunately, it has no screen backlight and no forward illumination of the work area. It looks like the LCD could be easily modified to accept 2 LEDs, as it has some cut-outs for that purpose already. There is a small finger guard to avoid getting too close to the action and a low battery indicator on the LCD to prompt for battery replacement.

The principle of operation for DC current measurement is the Hall effect . http://en.wikipedia.org/wiki/Hall_effect

The ranges are:
40A/400A DC (If + reading, conventional current is from bottom up).
40A/400A AC
AC Volt 600V (40~400Hz)
DC Volt 600V (square wave average DC measurement is accurate, unlike the UT71 or 61E)
Resistance 400~40MOhm

The continuity is not latched and a bit slow, with 10 Ohms detection and a soft tone buzzer (2.7kHz).
Diode test at 1.48V with no illumination or forward bias voltage measurement of LEDs.
Frequency 1Hz-10MHZ
Duty Cycle 0.1%~99.9%
It is rated CAT III 300V CAT II 600V, with a 600V PTC protection.
It also has a Relative mode, Hold, auto/manual ranging and an auto sleep mode (15 minutes) which can be turned off.

You would have noticed that capacitance and temperature measurements are not available on this particular meter.

[Wytnucls]

Inside, there are 2 PCBs, linked by a 6 pin header connector, which are populated with:

U1: Fortune FS9721-LP1 DMM ADC covered with a blob of black gunk
U2: NJU7211V30 Voltage regulator (-3VDC)
U3: True RMS to DC converter AD737J
U4: ST EZ11P TI 27L2 Rail-to-Rail Output Op Amp
U5: HEF4053BT triple 2 channel analogue multiplexer/demultiplexer
U6/7/8: 3 Texas Instruments precision Op Amps OP07C

There are 7 accessible trim pots:
VR1: DC Volt adjustment (10K)
VR2: AC Volt adjustment (2K)
VR5: Hall voltage balance adjustment (50K)
VR6: Input Offset Voltage Null adjustment for opamp U6 (5K)
VR7: Amps adjustment (400A range) (1K)
VR8: Amps adjustment (40A range) (1K)
VR9: Input Offset Voltage Null adjustment for opamp U7 (50K)

It looks like VR3 and VR4 are nonexistent on this clamp meter PCB.

Limitations: 1% deviation if the lead is not in the middle of the jaws and the maximum conductor diameter is 26mm.

Best accuracy:
DCV 0.8%+1
ACV 1%+5 (400Hz max +4% if peak factor 3)
Resistance 1%+2
Frequency 0.1%+3 (30VRMS max)
DC Amps 2%+5
AC Amps 2.5%+8 (50-60Hz)

[Wytnucls]

It is quite important to keep the lead in the middle of the jaws for accurate AC and DC currents measurements.
The recommended procedure for DC is to clamp the cable, remove the clamp, zero the reading with the Relative button and remeasure the current with the conductor in place again.
In my experience, measuring DC milliamps is quite difficult, with a fair amount of hunting around the real value. AC readings seemed easier, with more accuracy and less fluctuations.

I did adjust the DC voltage through the VR1 trim pot on the lower board, as it was reading slightly below my other meters.
There are 5 trim pots on the upper board, VR7 and VR8 adjust current readings.
More expensive meters have additional features, like USB or internal logging, higher counts, wider jaws, higher currents measurements, True RMS AC+DC, Max/Min, 3-phase measurement capability, bar graph, low-pass filtering, inrush current and more, so make sure you know what you need, before you buy.

So, to conclude, I am quite happy with this purchase, but it is no real replacement for a normal DMM to measure currents below 10A, as a decent accuracy is difficult to achieve. It will be handy in some special circumstances, when the current leads cannot be connected to a normal multimeter for instance or for use in a difficult environment, where accuracy is not that important.

[Wytnucls]

Here is a short video to show the variations one can expect while measuring about 400 mA DC:
These clamp meters are very sensitive to any electromagnetic fields close by, so that could be the problem here, with a few electrical appliances around, like air-conditioning.



Here is a much steadier True RMS measurement of a 12V 620mA 50Hz AC source:

[BravoV]

Thanks for sharing this, appreciate the effort and also the short video. 

These clamp meters are very sensitive to any electromagnetic fields close by, so that could be the problem here, with a few electrical appliances around, like air-conditioning.

Don't be disappointed, even my Agilent U1213A clamp meter also has similar problem on DC current measurement, and not only from surrounding things like you mentioned, the earth magnet it self affects the reading, although only at the least significant digit.

[iloveelectronics]

Great review! Thanks!

[Wytnucls]

Thanks Bravo, I feel better about it now 
Quite a beast you've got there:

[BravoV]

Yeah, but it cost me arm & leg    , did a tear down while ago here -> Agilent U1213A Clamp Meter, Teardown

[Wytnucls]

Sorry, I missed that one somehow. Nice review. I like the beefy jacks and the double-sided range switch! 

[Wytnucls]

Great review! Thanks!

My pleasure Franky. Another candidate for your online megastore... 

[peter.mitchell]

Here is a short video to show the variations one can expect while measuring about 400 mA DC:
These clamp meters are very sensitive to any electromagnetic fields close by, so that could be the problem here, with a few electrical appliances around, like air-conditioning.

You know that actually looks pretty damn useable for lower current,  dispite how much it floats around,  for non contact current measurement over such a wide range,  AND for its price,  that's pretty impressive, you could almost use it instead of the 10A range on cheaper meters!

[SeanB]

Not a bad performance for such a low current. It is hard to get good stability with such a low magnetic field with all the sources of stray magnetic fields around. Most steel has a small bias in it.

[Jay_Diddy_B]

Hi,

Interesting review of the clamp meter!!

Just a couple of comments.

1) Since these are usually used by electricians working close to the electrical panel, I don't think that the CAT rating is high enough. I would want CAT IV

2) When measuring small currents you can increase the resolution by passing the wire through the clamp multiple times.


Season's Greetings !!

Jay_Diddy_B

[Wytnucls]

1) Since these are usually used by electricians working close to the electrical panel, I don't think that the CAT rating is high enough. I would want CAT IV
2) When measuring small currents you can increase the resolution by passing the wire through the clamp multiple times.

1.UNI-T makes more expensive and sturdier CAT IV clamp meters, like the new UT221/222 (CAT IV 600V) (AC and DC) and some older models rated CAT IV 300V, like the UT233 (AC only).
2. Yes, you're quite right, I forgot to mention that if the conductor lead has some extra length, it can be looped several times around the jaws and the reading, when divided by the number of loops, should give you a more accurate figure.

Now, whether the following clamp meters live up to their CAT ratings, is another matter.

[Wytnucls]

I'm trying to understand how the thing works:
There are 8 traces disappearing into one of the sealed jaw leg, by way of a flexible connector, with the Hall effect sensor right at the tip, I presume. There is also an unknown transistor underneath the upper PCB, possibly also involved in Hall effect measurement (actually, Z1 is just a Zener diode 1.2V reference).
I found a closed loop measurement procedure, but that only accounts for 6 traces.
There are also 2 trim pots missing somewhere (VR3 and VR4), possibly hidden in the jaw leg itself.
Quite puzzling so far. 
Maybe the last 2 traces are used for the AC pickup only.

[SeanB]

Open loop means you need a linear sensor, but it can have a fast response to transients. Closed loop places less importance on linearity, but it must have a stable zero and low drift. Has better linearity as the feedback reduces the influence of the core material and the sensor on the transfer function. I think that the extra 2 wires are for a thermal sensor to compensate for temperature of the coil and magnetic sensor. The 2 trims are possibly on the hall sensor for zero setting and the coil itself to set a fixed resistance, so that during manufacture the parameters are set before the unit is sealed.

[Wytnucls]

That's quite plausible, Sean; I'll have to crack it open to find out. 

Here is a picture of a ramp wave being measured, to test the UT204 True RMS capability on voltage:

[SeanB]

Remember that often a sealed assembly is not going to be possible to put back together, especially if it was designed never to be repaired. Often you need a bigger can for the worms.

[Wytnucls]

I was only kidding Sean, I'm not that desperate to find out. 
I'll take a picture of it on the X-ray machine screen at the airport, if they let me! 

[SeanB]

Standard got rid of the Xray unit as well, there goes my teardown ideas............. Bit far to go to Mike to use his......

[Wytnucls]

I did some consumption measurements on the clamp meter, testing the battery first, which is supposedly good for 150 hrs of operation.
Low battery voltage warning comes in at 4.7V and least voltage for power up is 4.5V, which points to a 3.3V voltage regulator for ADC operation.
The power hog is the continuity test, jumping from 1mA when idle to 33 mA, with the buzzer active.
Next, unsurprisingly, AC/DC amperage clamp energizing, draws about 7 mA.
AC voltage measurement stands at 1.4 mA and DC voltage, a meager 0.9 mA.
All other functions draw about 1 mA each.

[Wytnucls]

I thought it would be interesting to see what other manufacturers have to offer at around USD 50.00 for a similar AC/DC True RMS clamp meter:
Mastech 2108 USD 70.00 (extra features: 6600 count, Max/Min, Backlight, Clamp light, Capacitance, Inrush current).
http://www.p-mastech.com/images/SPEC/ms2108%20ms2108a.pdf
That's really the only one I can find in that price range. If you know of other true RMS clamp meters that would qualify, please post your information over here.
I found 2 C.E.M True RMS clamp meters which may qualify, although I haven't seen any prices quoted yet:
C.E.M. 332T/333T

[Wytnucls]

Updated the review with 2 PDF files: UT204 Schematics and Fortune FS9721 DMM ADC:

[Huluvu]

Hi all,

I bought a Uni-T 204 a couple of weeks ago but it was very disappointing from my point of view.
The theoretical resolution below the 1 Amp range is not really usable due to the high fluctuation around the last (2) digits.
I was not able to measure a DC current around 100mA  and sent back the unit 

Any recommendation for a decent clamp meter that can measure down to 10mA without having the feeling just to read some random numbers on the display...

Regards Arne