I purchased the PRV240FS proving unit around seven weeks ago now to go with the Fluke T110 voltage tester that replaced my old Kewtech unit that decided to exit the tool bag in two halves detailed in a previous blog;
My new voltage probe has arrived
Up until last week, both Fluke units have been working fine, but last week when I went to test for dead, the proving unit did not work. This was quickly diagnosed as a set of flat batteries and the issue was solved by installing a new set. A seven week battery life though seemed a little short. The instruction leaflet that comes with the proving unit details a battery life of 1100 tests of a 10 second duration, and it certainly hasn't done that much work, closer to 200 tests as an estimate. My old Kewtech proving unit was seven years old, and has only had two sets of batteries over that time period.
The batteries supplied with the proving unit were four Toshiba AA Alkaline batteries, so they are form a renowned battery manufacturer. Each battery measured in the region of 1.030V on no load using a DC voltmeter, so it looks like the whole pack has been drained and not just an individual weakened battery.
For a little further investigation, I hooked the proving unit up to a 6V DC power supply to measure the current drawn.
1) On initial power up the unit draws circa 16nA
2) Using the finger test button test draws around 105mA
3) Testing the T110 on the proving draws around 160mA
4) After the first test, the unit drops back down to 16nA
5) After a second test, the unit drops back down to 4.5mA
| {gallery} PRV240FS Current Tests |
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The Toshiba batteries don't have an Ah rating on them, but an internet search suggests that AA Alkaline batteries have a rating between 1700mAh and 3000mAh. The intermittent 4.5mA drain observed would therefore fit in with draining the AA batteries over a seven week period.
Proving unit power consumption tests video
As a proving unit, this isn't much use to me and I will look for a more reliable unit to replace it with, or stick with my old Kewtech proving unit that I used with the old probe, although this only puts out DC, so does not work with non-contact voltage pens that I desired.
I would be interested to hear from anyone else who has purchased one of these units, if they have had any issues with battery life.
Update 12/04/2018
Working with the idea from Gene in the comments below, I have dismantled the proving unit to see what the possibility would be of adding in a small switch. Mechanically, as the pictures show, there would be space at either end, to add in a small switch. Electrically, there are some issues with interrupting the power.
| {gallery} PRV240FS Dismantled |
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PRV240FS opened up |
Board in top part of enclosure with base removed |
Component side of board |
Underside of board showing battery connections |
Field sense end of case |
Transformer end of case |
Case Length |
Bottom case height |
Battery positive terminal |
Component side of battery positive terminal |
Battery negative terminal |
Component side of battery negative terminal |
From the photos, it can be seen that the battery terminals are soldered directly to the PCB and make electrical connections on both sides of the board. However, I believe that the positive terminal just goes to a test point pad on one side and then SNBL1 component on the other side.
It may be possible to cut the track to the SNBL1 component and wire the switch in-between that and the battery connection.
A little bit more investigation required to find a suitable switch and identify the SNBL1 component.
Update 17/04/2018
Following on from the useful comments made by members of the element14 community, I had a go at installing a switch into the proving unit that broke the battery circuit and stopped the current drain.
I don't have a huge stock of electronic parts as I am more electrically biased, but managed to find a suitable slide switch that could be mounted in the front case. There is a thin internal web that I drilled to provide the mounting of the switch as seen in the first photo. The switch was a little wide, so I had to carefully cut back a small piece of the ridge, seen in the second photo, to enable the switch to sit correctly and go through the front case enough to allow it to be operated. The cut out can be seen from the front in the third photo and is reasonably unobtrusive, but hopefully prominent enough that I don't forget to use it.
I only had veroboard which seemed to be to thick to put between the one of the batteries and its terminal. I eventually settled on a small plastic piece cut from an old credit card that had just expired. It pushed neatly into the clips that hold the battery terminal in place. It did bend them slightly, but nothing to excessive as seen in the photo below on the left. This held the first wire in place behind the battery contact. The second wire for the switch was held in place using some aluminium foil tape seen in the second photo. I then trimmed down the aluminium foil with a craft knife.
The board sits very close to the back of the battery case, so I had to cut out a small section of the case to allow the wires to pass out of the battery case as seen in the third picture above. I then secure the wires with a little hot glue in the cut out and along the inside of the case to the switch.
The switch was successfully mounted and secured with two screws into the internal web. A slight downside is that the wire crossover form the bottom half of the case to the top half now, so the unit does not dismantle as easy as it did before as seen below.
The case was then reassembled and then using my current favourite insulation tester, I conducted a 20V insulation test with the switch open, which gave a 4GOhm reading and then a 200mA resistance test with the switch closed, which gave a 0.1Ohm reading.
I have a Brother P-touch printer, so made a little label up to finish off the modification shown in the first photo below. The work went reasonably well, the opposite battery terminal is now pretty much crushed up in comparison to the other terminals as can be seen in the second photo, the top battery contains the switch circuit, where as the battery below is unmodified. This may mean that the foil tape is damaged when the batteries are replaced, but with the wires glued into position, it should just mean stick on a new piece of tape when required.
Many thanks to genebren, mcb1 and jw0752 for their suggestions and comments for ideas to resolve this, much appreciated.
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