Friday, 16 June 2017

Mooshimeter Mod

Last year I bought a Mooshimeter wireless 2-channel multimeter. It is a multimeter front end that links to your mobile phone with Bluetooth and displays the results in an app.

It is handy because you can read it remotely, it can measure voltage and current simultaneously and display power, it can log to an SD card and graph the results. It can also speak the results.

Despite all these good features my "go to" multimeter is still my EEVBlog branded Brymen BM235. So my Mooshimeter sits in a drawer for most of its life. When I get it out it usually wants to do a firmware update, which needs fresh batteries. Because it has no off switch the best you can do is put it into shipping mode. It still flashes its LED occasionally, so the batteries run down over a period of several months and are then not up to doing a firmware update.

Devices with no proper off switch are a pain if you only use them rarely because the batteries are always flat when you come to use them. This is particularly a problem because modern Duracell batteries seem to leak and corrode as soon as they are flat. This didn't used to be the case. I found some very old ones that I had abandoned in outdoor devices that I expected to be corroded to hell but in fact they were not corroded at all, despite being well past their use by date. In contrast I have had many corrode recently that were flat, but still well within their use by date. I have stopped buying Duracell and now use Costco's own Kirkland branded ones. It is too early to say if they corrode or not.

So I normally remove the batteries from devices I use rarely but with the Mooshimeter this involves removing the casing that is held together by two screws. I decided to add a switch to it but it has a cat III safety rating and cutting a hole in the case would void that.

I had two ideas to get around this: the first was to put a normally closed reed switch in series with the batteries and 3D print a cradle with a magnet in it to turn it off. My second idea was to use a mercury tilt switch to turn it off when placed upside down. I ordered both but as the mercury switches arrived first I implemented that and it works well.

I decided the easiest point to break the battery circuit was the link between the two cells. For some odd reason that is a copper fill rather than just a track. It is on the top side of the PCB so I had to desolder one of the contacts to get at it.  Fortunately the battery contacts have thermal relief connections so I just cut two of those to isolate the pad.

I then made an insulating washer out of Kapton film. I used that because it is very thin and can handle soldering temperatures. I stamped it out with a hole punch but I found it very difficult to get the hole concentric. This is my third attempt that was just good enough:

And here it is in place:

I reinserted the clip over the top and resoldered it. I then soldered the tilt switch between the two now isolated battery terminals.

When upright the contacts are bridged by the mercury, which is very low resistance. When I turn it upside down the mercury flows to the top of the bulb and isolates the batteries.

So all I have to do is remember to place it upside down in its case. If you want to attach the meter to something moving then the reed switch idea is the one to go for. I think it can probably be mounted in the same place if you use a neodymium magnet. You can get it nearer the back of the case by mounting it on the other side of the PCB but I don't know if that affects clearance distances for class III.

Sunday, 3 April 2016

Beware fake wire!

I bought some test leads with banana plugs and alligator clips for £0.99 on Ebay from Hong Kong. They were described as "Alligator Probe Test Leads Clip Pin to Banana Plug Cable for Digital Multimeter GF". I don't know what the GF means.

Very cheap and what could possibly be wrong with them? Well actually, almost everything!

The first time I used them to connect a regulator to a bench PSU they got hot and dropped several volts. Without needing to do any sums with wire gauge and current I felt the resistance must be too high, so I measured it to be about 1.4Ω for the round trip. Way too high as multimeter leads are generally about 0.2Ω.

I unscrewed a plug and found this: -

The screw bites down on the soft insulation and that presses the folded back strands against the barrel. Not the best way to make a connection as you want the screw biting down directly on the strands, or better still a ferrule.

At the clip end it was more of the same: -

The strands are trapped between metal and plastic again instead of being soldered through the hole. The crimp is there just for mechanical strain relief, not the electrical connection.

I removed all the connectors (the clips pull off really easily due to not being soldered) and measured the resistance of the wire on its own. Still 1.4Ω, too much I felt for that gauge of copper wire 2m long. The simple explanation is that it isn't copper.

The fact it sticks really well to magnets leads me to believe it is copper plated steel. That might be OK for measuring voltage but no good for measuring or carrying current. I can't see any reason for using it other than it must be cheaper.

I replaced it with 32/0.2mm copper wire half the length and got a total resistance of 20mΩ. Much more suitable for hooking up PSU test circuits but a bit less flexible than ideal for multimeter leads.

So basically I got usable connectors for £0.99, which is still probably cheaper than I could buy them for in the UK. The wire and the construction were junk.

Tuesday, 29 March 2016

Beware of cheap Kelvin clip LCR meter test leads

When I first got my Elektor LCR meter I didn't have any four wire test leads for it so I made up a pair from the ends of an old VGA monitor cable and some miniature crocodile clips.

These worked well enough for measuring normal components but they are not true Kelvin leads because the wires meet at the crocodile clips, not at the device under test. The steel clips add about 3mΩ and 200nH. True Kelvin clips insulate the two jaws from each other and one wire goes to each jaw. They also have non-ferrous jaws, so are less inductive and more conductive.

I found some suitable clips at RS for £9 and was going to buy a pair of those and make up my own leads until I found I could buy them ready made on eBay for only £6.70 from Hong Kong, bargain! So I purchased a set : -

N.B. they had red and black sleeving when I bought them. This picture is after I fixed them!

When I tried them out I found that the readings were very inconsistent and the open circuit capacitance increased when the wires were moved close to each other. That made me think the wires were not screened cable so I opened one of the BNC connectors to check.

What I found was the cable is actually screened, in fact it has both a copper braid screen and an inner aluminium foil screen like digital TV coax. However the screen is not connected to to the BNC outer contact, it is cropped back to the insulation. It was the same in all four BNC plugs so I had to remake all those connections.

Then I found that the screens were shorted to the inner core at the clip end! I had to cut off the red and black sleeving and completely disassemble the clips.

It is as if the person making them didn't understand the concept of screened cable and just used it like normal single conductor wire. Much easier and faster to assemble that way of course.

So I had to remake all four of those connections as well, stripping back the screen and covering it with heat shrink sleeving.

Another problem is the insulation tape that prevents the jaws being shorted together by the spring that holds them closed is not tough enough and was already starting to fail. 

To fix this I added some heat shrink sleeving to the spring.

I then hot glued the wires in place as there isn't much strain relief and replaced the outer sleeving with 19mm heat shrink. That is why mine ended up grey instead of red and black.

They now work well but remaking all eight ends was a lot of time and materials. The seller was apologetic and gave me a refund but I wonder how many other ones like this are out there. I also ordered Kelvin tweezers for surface mount devices and a 4 terminal test fixture from China, so it will be interesting to see if they are built properly.