ETCR 007AD Current Transducer Teardown

       I bought this from a chinese online store as I sometimes find a need for a hall effect type current transducer for my oscilloscope. I got it for $62.80 with free shipping.

       This was the cheapest I could find with decent specs so I took on the chance.

       These are the specifications it came with:

Function Measurement of AC/DC current, leakage current
Power Supply Zn-Mn dry battery, 6RL61, 9V. Continually using for 100 hours, connecting external power supply is also available for long time using.
Test Mode Clip-on CT, non-contact measurement
Clamp Size Φ7mm
Input Range 0mA-60.0A AC/DC
Output Voltage 10mV/A;100mV/A(corresponding)
Output Range 1V peak max
Resolution 1mA AC/DC
Accuracy ±3%FS(23C±5C, below 5%rh)
Phase Error ≤3°(AC 50Hz/60Hz; 23C±2C)
Calibration Adjust ZERO to calibrate, eliminate external electric field interference
Output Wire Core wire: signal positive output; shielding wire: signal ground
Lead length 2m
Dimension LWH: 168mm×65mm×34mm
Frequency AC: 45Hz-400Hz
DC: DC-100kHz
Test Position Tested wire in the jaw center
Line Voltage AC600V
Weight 170g (Including the battery)
Working Current 5mA
Working Temperature and Humidity -10C-50C,below 80%rh
Limit Temperature Error -10C-0C and 40C-50C, within 2%rdg
Storage Temperature and Humidity -10C-60C,below 70%rh
Insulating Strength AC3700V/rms (between the alloy of the clamp and the housing)
Applicable safety rules IEC1010-1, IEC1010-2-032, Pollution level 2, CAT III(600V)
Accessories Sensor: 1piece; Battery(6F22,9V): 1piece; User Manual/Warranty card/ Certification: 1 copy

Here is the unit.

With the clamp opened, we can see the hall sensor elements and the exposed core faces.

The clamp spring is pretty strong.

The zeroing knob.

A small"zero" text should have been fine, but noooo, they think a big font is better!

Back of the unit.

With the battery door opened. The screw holding the battery door goes into a threaded brass bushing.

Plus points there.

Back cover removed. The PCB has a bunch of parts not mounted.

The zeroing control uses a multiturn pot. One of those Bourns 3296 knockoffs.

These are not rated for a lot of operations but atleast it is an easy replacement if needed.

The knob has a metal blade that fits in the adjustment screw slot.

Metal blade = stronger. Another plus.

With the board removed, We can see why there are a bunch of unmounted parts.

The manual also has drawings of a version with an LCD display readout so they just used one case and PCB and left the unnecessary parts out.

This is the other side of the board where the LCD display goes.

Here is the PCB marking. It is FMA6210V1.2

That PCB footprint appears to be a mini USB connector.

Circuitry is rather simple. A TL064 quad op amp, an IC with the part number rubbed off and a bunch of other support components.

The switch has been manually soldered and there are some resoldered parts. Flux residue was not cleaned.

Power LED was a boring red.

So I changed it to an ice blue one.

The indicator LED just indicates power. It does not indicate (turn off) when the battery runs low.

Connected it to my Fluke 115 to check its accuracy.

With the 10mV per amp range, it reads close to my ammeter.

And here it is on 100mV per amp range.

A little work with the sandpaper and the annoyingly large "zero" text is removed.

We can also see two points where a lanyard could be attached. One on the left and the other on the right in the picture.

       Seems to be a decent current transducer for the price. I don't have an immediate project that I could test the AC frequency response but the DC accuracy seems ok.

       Current consumption was a quoted 5mA but actual measurement showed it to be 9.7mA and contrary to the specifications, there is no input for an external power supply.

09 Jan 2016:

I did a little reverse engineering and the whole thing runs off + and -3.3V. The 9V from the battery is dropped by the SOT-89 part which is a Holtek HT7133 3.3V 30mA CMOS LDO. The SOIC8 chip is a MAX755 DC-DC converter which generates the -3.3V rail.

The TL064 quad op amp runs off the + and -3.3V rails.

Since everything runs off the 3.3V linear low dropout reg, that meant the whole adapter could operate down to 4V or less battery voltage.

I had the thought of modifying this to have some sort of low battery indicator.

I found the LED anode is connected to the +3.3V rail, cathode goes to the 0 ohm jumper R8 then 1K resistor R9, both above the blue trimmer.

Some creative thinking, testing and trial and error, I ended up with this.

As long as the battery voltage is above 6V, The zener conducts allowing a small voltage to go to the logic FET which lets it conduct. This steals the base current so the transistor is off keeping the red LED off and the green illuminated.

Once the voltage drops to about 5V, the FET loses its gate voltage so it turns off. The transistor conducts and the red LED illuminates. Since its Vf is lower than the green GaN based LED, the green LED turns off while the Red is lit.

Here is the original 1K limiting resistor mounted skewed and directly mounted to the ground pour to free up the pads. The pad has tracks going to the ASIC pads which is where the rest of the circuitry will be placed.

The green LED is mounted across the original pads. The red LED diagonally beside it.

The transistor placed to reach the +3.3V pad and the red LED anode.

The base is connected to the freed up pad on the other side.

The zener and its 47K 0805 resistor is conveniently mounted to the unpopulated R3 and R4 positions respectively (below the SOT89 regulator chip). This conveniently goes to a pad to the unpopulated ASIC.

The last three components are mounted on the unpopulated ASIC near the upper right of the pic.

Close up of the 3.3V zener and 47K resistor on R3 and R4 positions.

The 47K 0603 resistor is mounted bridging the FET gate terminal and the pad that leads to the zener/resistor junction. The drain terminal goes to the +3.3V pad of the ASIC chip and source goes to the pad that leads to the transistor base. A 39K 0603 resistor is placed on this pad and a scraped ground pour to complete the circuit.

With the battery voltage at 6V or greater, the LED indicator is green.

At about 5.6V, the green LED fades, and the red LED faintly comes on so it turns orange-ish in color.

When the battery voltage drops to 5V or lower, the LED indicator is red.

And now to test the response of the Hall effect circuitry.

The aluminum box is a high current inductor tester with a 0.5ohm resistor on its output to produce a sharp high current step from zero to about 17A. For the higher sensitivity range, I used a 10ohm resistor.

The inductor tester uses a 10mOhm shunt to measure the current and the scope vertical scales are selected to show direct 1V = 1A conversions.

This is on the 100mV/A range and a 10ohm resistor load. The response is pretty quick with a bit of ringing before settling on the actual value about 500us later.

With the higher current range, we can see slew rate limiting. It settles at about the same time.

Here, it shows the trailing edge to be about 17.4A before going back to zero with the clamp adapter exactly the same scale as the shunt resistor.

Here is the slew rate being 1mV/us which is very slow. At full scale of 1Vpeak, that would mean a high frequency limit of 150Hz on a sine wave!

For the quoted upper frequency limit of 100kHz and 1mV/us slew rate, that would mean the output would only be 1.5mV!!!

Well, that's disappointing. Fortunately, my main use for this is for the wattmeter mod and for my Fluke 115 which has min/max/average modes since my two clamp meters do not have that feature.

Page created and copyright R.Quan ©09 Sep 2015.