<Test equipment

UNI-T UT203 Current Clamp analog output modification

The UNI-T UT203 is a cheap current clamp meter featuring 40 A and 400 A ranges and reasonably good accuracy (though not as good as the Fluke i310S I had used in a previous job). But for its price, the UT203 is quite ok. It can measure both AC and DC, and while the DC reading always has quite a large offset error, it can be zeroed with the touch of a button. (The device also has voltage, resistance and frequency measurement, but since I'm not short on multimeters and oscilloscopes, I'm only interested in the current clamp function.)

But I wanted an analog output that could be connected to an oscilloscope, and the device does not have that! That's how I used the i310S earlier—in fact, it did not even have a digital display like the UT203. It was simply a non-contact current-to-voltage transducer, equipped with nothing more than an on/off switch and an offset adjustment pot—simple and stupid, just the way I like it. So... Modify the UT203! I found its schematic at ElektroTanya already before I went ahead and bought the device. The CAD-drawn circuit diagram, converted first to JPG (blech) and then to PDF, was barely clear enough to make sense out of. It does contain some obvious errors, and all resistances are labeled either in kilofarads or kilojoules (wtf?), but I'm not complaining.

Here's an excerpt from the circuit diagram, showing the two Hall sensors HL1 and HL2 (the round things on the left) and their related conditioning electronics. The output of the U8 op-amp (top right, wired as a differential amplifier) is the analog signal of interest. The range switching is handled by two sections of U5 (the boxy things), which is a CMOS 4053 (triple 2-channel analog mux) in the feedback path of op-amps U6 and U7 (output amplifiers for the two Hall sensors), and chooses between two sets of feedback resistors (whose values in the schematic are wrong, by the way—both the 40 A and the 400 A ranges seem to have identical feedback resistor networks, which obviously can't be correct).

However, I hit an unexpected snag when I opened up my UT203—mine seems to be a newer revision, with a different circuit. The PCB on mine is labeled "UT203/204 REV.3 2018.08.15", and at the very least it:

  • has different op-amps (two GS8552 rail-to-rail low-voltage dual op-amps, whereas the schematic has OP07 single op-amps)
  • has no 4053 (unless it's on the reverse side of the board, which I did not remove for inspection)
  • has a 24LC04B eeprom chip, for whatever reason (factory calibration data, I'd guess)
  • seems to have a different central IC as well, at least based on its pin count.
Nevertheless, I think the analog front end of the Hall sensor read-out circuit must be essentially the same as what is shown in the schematic. I just need to find the correct pin on the correct op-amp to get the output signal I want.

So I looked up the datasheet for the GS8552, and armed with its pinout, I switched the instrument on and began probing the two op-amps IC6 and IC7 with a multimeter, measuring against the COM terminal of the UT203 (which was just a lucky guess as to what might serve as "ground"). I first measured the supply voltages. Then I measured each output pin with zero current, +4 A current, and −4 A current passing through the clamp. (At this time I paid no attention to whether the wire is properly centered and perpendicular to the clamp, as it should be for accurate results.) I observed that:
  • on either amps range, both op-amps are fed with +1.95 V (Vdd) and −1.09 V (Vss). The battery terminals, at the same time, measure +7.99 V and −1.09 V relative to COM.
  • on the volts and ohms ranges, the op-amps are still powered, being fed with +1.11 V and −1.82 V, whereas the battery terminals are +7.51 V and −1.82 V.
  • on the Hz range, battery negative measures −1.46 V, and on the diode test range −0.37 V.
  • on the 40 A range, the outputs A and B (pins 1 and 7) of IC6 and IC7 measured:
    No current   +4 A current   −4 A current  
    IC6 output A   −164 mV−176 mV −155 mV
    IC6 output B   −164 mV−155 mV −172 mV
    IC7 output A   +263 mV+262 mV +264 mV
    IC7 output B   −2 mV+21 mV −18 mV
So it seems like COM is at a virtual ground that is adjusted differently for different measurement ranges. On the amps ranges it is some 1.09 V above the battery negative terminal, and on other ranges within reasonable (and safe) limits not to wreak havoc with any op-amp I choose to use. The two op-amp ICs seem to be powered through a 3 V regulator, likely IC3 which is marked "SD30L".

The two halves of IC6 seem to do the job of op-amps U6 and U7 in the old schematic, i.e. they amplify the Hall sensors' outputs—note how they act as "mirror images" of each other with positive and negative current through the clamp. The "B" half of IC7 seems to be the differential amplifier (U8 in the old schematic) and its output is the analog signal that's proportional to the measured current. Its output is very close to 0 V at zero current, which confirms that the COM terminal is indeed at the correct virtual ground here. The "A" half of IC7 is probably the constant-current supply for the Hall sensors, as it never changes at all.

To connect my own circuit to the instrument, I needed access not only to the analog signal from IC7, but also to the virtual ground, and on/off-switched power supply voltages (VccB and VssB in the old schematic). I did not want to remove the circuit board in order not to disturb the connections to the buttons and LCD (sometimes reassembly can be a pain, so I try to minimize disassembly). Therefore I chose locations that are easily accessible to the soldering iron on the front side of the PCB:
VccBpositive leg of C16 (gray)
VssBnegative leg of C14 (white)
GNDupper end of LC1 (black)
Signal  pin 7 of IC7 (purple)

I breadboarded a circuit using an LM358 op-amp and started testing. I chose the LM358 just because I happened to have it immediately available in shelf. It is unity-gain stable, and its input and output voltage ranges go within 20 mV of the negative rail, so it will work just fine with a virtual ground 1.09 V above negative rail. The positive end of its output range is more limited, some 2 V below the positive rail, but that's ok as I'm using the unregulated battery voltage, not the regulated 3 V, as the LM358's positive rail. The output range (which needs to be symmetric) is, of course, limited by the negative supply to ±1 V, but if I aim for 10 mV/A sensitivity (the same as on the Fluke i310S), the full-scale output will be just ±0.4 V, which is ok.

With wires soldered so I don't have to hand-hold the multimeter probe, I was able to more carefully measure the analog voltage with respect to current, this time with the wire properly positioned in the current clamp's jaw (not that it seems to matter much). The analog signal from IC7 turns out to be 6.53 mV/A on the 40 A range, and 0.638 mV/A on the 400 A range. (Rather strange values, but if the instrument is factory calibrated into the eeprom chip, the exact magnitude of the analog signal doesn't really matter. However, if you do the same modification on your own instrument, you'll likely get different values, and will need a different gain—probably the gain potentiometer in my circuit will accommodate it, though.)

The output circuit I designed could hardly be simpler—it is just a non-inverting amplifier (with gain that can be set with trimmer VR2 from about 1.2× to 1.7× to get the desired 10 mV/A sensitivity) with an offset control. Note that the analog output's offset is not affected by the instrument's own zeroing function, and likewise the digital readout is unaffected by the offset potentiometer—the two are completely independent. The voltage from the divider formed by R1 and VR1 is about ±1 V relative to the virtual ground. That becomes about ±60 mV in the following voltage divider formed by R2 and R3. The impedance of that divider is about 20.6 kΩ, and together with R4 and VR2, the feedback network gives the upper half of op-amp U1 the desired gain range. Note that the input signal connects to the op-amp positive input only—that input is high-impedance, so my circuit does not load the signal in any significant way, and should therefore not affect the internal workings of the UT203 at all. Also, since no feedback resistors connect to the input signal, there's no way that my circuit, which runs off a higher voltage, could damage the low-voltage IC7. Since the offset voltage is derived directly from the unregulated battery voltage, it will drift slightly as the battery wears down, but that's not really a big deal compared to other drifting in a cheap instrument like this. The output impedance of my circuit is 10 kΩ, which is high enough to make an inadvertent short-circuit a non-issue, yet low enough not to skew the voltage at an oscilloscope's input, which is typically around 1 MΩ.

There's plenty of room inside the instrument's case otherwise, but the populated circuit board cannot be very thick. A DIP op-amp on a scrap of veroboard might just fit, but since I had SMD components handy, I played it safe and used those as much as possible. I built the circuit on a tiny 14-pin SOIC-to-DIP adapter PCB from eBay—the IC and gain trimmer fit nicely on the SOIC pads, and the two capacitors and five resistors, in either SMD or through-hole format, go between various DIP pin holes. A custom PCB might look better, but with such a simple circuit I just couldn't be bothered to fabricate one. Here is a bigger photo of this mess.
I stuck a couple of layers of electrical tape (bush-league though that is) over a suitable part of the instrument's PCB for insulation, and wired up my own PCB to sit there, with the trimmer (gain) potentiometer side facing up. Some hot glue keeps it from wandering about. Once the instrument's case is closed, it should be quite snug in there. The offset adjustment potentiometer is located near the banana jacks, where a small pot (4 mm shaft) will just fit after some grinding, but this was the tightest and most awkward bit of the whole modification. The output cable exits the instrument through a hole I ground near the lanyard attachment point, at the lower-right corner of the case. A cable tie provides strain relief. The coax shield is connected to the virtual ground at the COM jack. Click here for a bigger photo.

I adjusted the gain potentiometer by feeding 10 A current through the jaw (I looped a wire through it five times, centered it as carefully as I could, and fed it 2 A from my power supply), while monitoring the output on my oscilloscope. After that I reassembled the instrument's case and cut the offset potentiometer's shaft to its final length.

The thing works as intended, though it's quite sensitive to external interference. Adding some copper tape inside the case, grounded to the negative terminal of the battery, improved it somewhat. It still helps to switch off the fluorescent ring light in my desk magnifier lamp when using this with the oscilloscope, big deal. Or I can put multiple turns through the jaw to get more signal, and scale the results accordingly. It's accurate enough on the 40 A range (well, I calibrated it, didn't I), and the 400 A range doesn't seem to be too far off either.

Here's an example oscilloscope trace of the inrush current when switching on my 200 VA isolation transformer. The first peak is some 120 mV, which would be 12 A, but I had four turns through the jaw, so it comes to 3 A. The peaks occur at 20 ms intervals (that's 50 Hz), and the peaks are all either positive or negative, depending on the magnetization state of the transformer's core when it was last switched off, i.e. during which half-cycle of mains AC power. As there was no load connected, no current flowed after the initial inrush.

As these clamps are mostly used to measure things like household AC current, I wouldn't be surprised if the instrument's bandwidth were just a few hundred Hz. To gain some idea of the bandwidth, I fed an ordinary stereo amplifier with a sinewave from my signal generator, and connected an 8 Ω power resistor to one of the speaker outputs. I then looped the output wire through the clamp's jaw a couple of times, and increased volume, while monitoring the output on my scope. I set the cursors at the sine wave's peaks at 500 Hz, then started increasing the frequency.

The output amplitude stayed very constant up until 2 kHz, but beyond that it, surprisingly, began to increase. I switched to a square wave, and saw this waveform (the green trace is the input signal, the yellow trace is the current clamp's output). Obviously something causes a tremendous high-frequency response somewhere, and I don't really know whether it's in the stereo amp or in the current clamp. But at least up to 2 kHz the thing behaves very well.

I don't know if the same modification is applicable to the UT204, but there's every indication that it uses the same circuit board (well, the text on the PCB does say "UT203/204"...), so the same modification should probably work (unless they change the PCB revision again). The only difference I know of between the UT203 and UT204 is that the latter has true-RMS measurement for AC current. By using a true-RMS voltmeter at the end of the analog output, a modified UT203 could also be used to obtain true-RMS current readings!

Of course, I take no responsibility for the correctness of the above information, and if you decide to modify your own current clamp, you do so entirely at your own risk. Let me know how it turns out.

Antti J. Niskanen <uuki@iki.fi>