Sunday, March 18, 2018

UNI-T UT33C multimeter: modifications (part 2)

Modding the UT33C for increased robustness

Disclaimer: Changing the circuit of the multimeter is inherently dangerous. Even a small change can lead to serious injury or death. It is done here only for educational purposes. I take no responsibility for the consequences or for any loss or damage suffered by anyone who uses this information. Even if I modified this meter, it will never be used for measuring high energy circuits, only signals up to 24 V and currents up to 2-3 A. If a meter is engineered poorly, it will probably remain that way!

In this post I'll discuss about a few modifications that should increase the robustness of the UT33C multimeter. The changes are crude and will most likely not improve the safety, however it might save the meter from some ESD events. Input protection for test equipment is serious business and should be done at factory!

Adding a fuse for the 10 A range

First, I added a fuse for the 10A current range. There is a place on the PCB where it can be mounted, but UNI-T didn't bother to add it. This is one dangerous decision because it's possible to have the probes connected in the wrong socket while measuring something and things could go wrong  very fast. I scraped off the resist to make soldering possible.

Good thing they at least made the PCB tracks for the fuse
Then I cut the trace that was bypassing the fuse:

Lastly, I have added a piece of kapton tape (to protect the PCB if the fuse ruptures) then soldered the 2 pieces of the fuse holder. Finding a fuse holder like the original proved difficult so I mounted the ones I had sideways, otherwise the case wouldn't close all the way. The type of fuses I used are fast, glass type, filled with sand, one 315 mA and another 10 A. High Rupture Capacity ceramic fuses would have been much better but their cost is close to that of the meter.

Fuse added on the 10 A range, heat shrink tube added over PTC

Adding a varistor on the V/Ohm/mA connector

The next modification consisted in adding a (m)etal (o)xide (v)aristor for the V/O/mA connector which should in theory stop voltage spikes and transients (ESD for example). The MOV is connected between the exit of the PTC and COM. Since the leaflet that came with the meter said that the maximum voltage (including transient overvoltage) between any terminals and grounding is 500V RMS, I went with a 420V AC, 560V DC B72210S0421K101 MOV made by EPCOS. A lower value (385 VAC / 505V DC) could be better for protection but it would also decrease the maximum AC voltage that the multimeter can measure. Modified schematic (added components are highlighted in red):

You can find the whole schematic at ElektroTanya:

Luckily there is enough space in the case of the multimeter to hold the MOV without any problems.

That's about it with this multimeter.

Thanks for reading!

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