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!

Friday, March 9, 2018

UNI-T UT33C multimeter: review and teardown (part 1)

About UNI-T

Uni-Trend (UNI-T) is a relatively new Chinese company (founded in early 90's) that has gained quite a lot of market share, especially in Europe. They produce all kinds of test equipment, multimeters, oscilloscopes, thermometers, etc. which usually offer a good balance between cost and quality. Their products lack the toughness of their western counterparts but they do work and most importantly, they won't break your wallet. For the general electronics enthusiast, they are more then well suited, but only when working with low energy, otherwise from a safety standpoint they're far from my no.1 choice. All in all, many of their products are worth checking out, but do make sure to read any reviews if you want to buy one.

The UT33C

The battery from one of my first multimeters (UT33C) has gone flat so I decided to review the meter it since it has to be torn apart anyway in order to replace the battery. In the next post I'll show a couple of modifications that should (at least theoretically) improve the robustness of this meter.

General description

The UT33C is part of the UT33 family and from the first look at the dial it's somewhat similar to the DT830 which everyone knows. There are however some differences. There is a hold button, a backlight button (mechanical latching), a thermometer based on a K-type thermocouple and a continuity tester. The case is made of a thick red plastic and the meter has a protective rubbery (I don't think it's legit rubber, maybe some sort of soft plastic) holster around it. The case itself is solid and does not creak when twisted in the hands. A  good safety feature is that the cases overlap around the edges so any flames or fragments will be contained should a catastrophic failure occur while taking a measurement.

Front of the device. Note that the connectors are all the same color

Back of the device and its holster.

The LCD is the same size as the DT830 but it's well protected by a thick plastic piece. On the back of the unit there is a standing bale which looks very fragile. It could be used to angle the multimeter to see its screen, but one should refrain to press the buttons or rotate the range selector while the multimeter is standing on the bale because it will most likely break off.

To keep things short, I won't get into any details regarding the accuracy and functions, but you can find everything on UNI-Trend's website.

To take the unit apart, one must remove the protecting holster then undo the 2 self-tapping screws on the back of the case. Then gently pry apart the case starting from the bottom. The top is held in place by a plastic latch. In the back cover there is a small compartment where the battery sits and the buzzer used by the continuity function. The electrical contact between the PCB and the buzzer is made by springs.

Bottom case contains the battery compartment and the piezo buzzer

Front case with the sliding contacts

The PCB is held in place with the help of a screw located near the battery and 2 plastic clips (bottom left and right). Regarding the battery, it's the classic 9V (6F22) which sits on a piece of foam glued to the board. On the bottom there are the 3 connections for the probes which are of the split type, but that's understandable considering the cost of this thing.

Internals of UT33C. The small 8 pin chip is a LM358 used to amplify the thermocouple signal

Split connectors and PTC
On the other side of the PCB there is the LCD assembly. To get it out, first undo the screw located near the battery, desolder the backlight LED then gently bend the plastic latches on the sides of the screen assembly.

Front of the PCB

The screen assembly. LED is built in, green - yellow color
Regarding the IC that powers this multimeter, it is an AME7106 (ICL7106 clone?) which is used in most low-end multimeters (including the DT830). If the ADC gets damaged, there's no way to replace it because it's in the form of a blob.

PCB with the screen removed
Regarding the protection, here is where the UT33C lacks. It only has a fuse  and a PTC thermistor (WMZ75S, 1K, made by Sinochip). Based on this, I wouldn't trust it for anything more than 50 volts. The fuse is rated 315 mA at 250V and is the cheapest one they could find (glass, fast type). Although there is space on the PCB for a fuse for the 10A circuit, they omitted it.  Another 2 cents saved!

The probes have a length of about 1 m. The wires are covered in a rubbery plastic. The probes are made of hard plastic and have an ergonomic shape. The nice thing is that the connectors are shrouded for increased protection. The tips are not very sharp but get the job done. Rated for CAT I 1000V, CAT II 600V and 10 A although I highly doubt they can hold 10 A.

Everything plastic, only plus are the shrouded connectors

My opinion about UT33C

The good:
  • Low price
  • Thick plastic case, rubber holster
  • Has backlight, continuity test, K-type thermocouple thermometer, measures resistance up to 20 MO
  • Crisp LCD screen
  • Decent probes for the price

The bad:
  • No protection besides a PTC thermistor and a fuse (for mA range). Unfused (!) 10 A range
  • Shared Volt / Ohms and milliamp connector can lead to accidents if wrong range is selected
  • To replace the battery the case must be opened (2 self tapping screws). This can't be done too many times. 
  • Limited to 3 1/2 digits (max. display 1999) because of its outdated ADC
  • Tilting bale breaks off after a few uses