Fluke i50s current probe - RJ45 power supply layout

[0xdeadbeef]

While there was a power adapter included with each Fluke i50s probe to connect any +/-12V supply to the RJ45 connector, they seem to have a tendency to disappear. Neither in the manual of the probe nor in that of the according power supply I can find a layout of the connector, i.e. where the +/-12V and ground are connected. Does anybody have this information by chance?

[0xdeadbeef]

Hm, surprisingly I won an eBay auction for an i50s, so this theoretical question has become a real one. I still haven't found the layout of the RJ45 used.
The probe seems to be a rebrand of a Lem PR50. The according supply is named Lem LS50 or Fluke PSi50s.

[0xdeadbeef]

Ok, for the sake of completeness, I figured out the layout myself.

According to the RJ45 socket pinout, the connection has to be
6  GND
7  -12V
8  +12V

I built myself a little adapter using of the shelf stuff:
http://www.voelkner.de/products/501641/D-SUB-Adapter-D-SUB-Buchse-9pol.-RJ45-Buchse-1-St..html
http://www.voelkner.de/products/341629/RJ45-Netzwerk-Anschlusskabel-CAT-6-S-FTP-1x-RJ45-Stecker-1x-RJ45-Stecker-0.15-m-Grau-Flammwidrig.html

Note that with this adapter, the numbering is inverted:
P3  GND  black
P2  -12V orange
P1  +12V blue

Currently lacking a proper bipolar supply, I created +/-12V with two different (isolated) supplies connected back to back (i.e. ground of the 1st supply connected to V+ of the 2nd supply so V- of the 2nd supply is -12V).

With this constellation however the probe wouldn't work. No LEDs lit, only like 14mA flowing into the +12V supply and 300mA into/from the -12V supply.
I suspected an issue with a voltage regulator in the +12V input, took the control unit apart, didn't find anything, put it together, still wouldn't work, already had given up. However in a last attempt, I tried to connect only the +12V supply: blinking LEDs, 700mA drawn from the 12V supply, overcurrent warning. Connected also the -12V: only ~150mAp on both supplies, green range LED lit. Probe seemed to work, but with implausible values and the auto-zero didn't work. However it needed just one degauss and it showed plausible currents. Yippee Ki Yay

Anyway, as I took it apart, I can as well show some pictures. Note that you need to desolder the two TO220 +/-8V regulators (L7808 and L7908 - screwed to the case) to get access to the bottom side. Also note that the base plate of the L7908 is connected to Vin (-12V) while the base plate of the L7808 is connected to GND. Actually, the L7908 is isolated from the case by a thin transparent plastic sheet.
Well, next step is to build some kind of +/-12V supply and I finally have a state of the art 50MHz current probe in my toolbox.

[robert_]

I also came across this probe. Surprisingly cheap, as far as DC current probes go. And its scope-independent, which is really useful...

Mine was in a so-so shape (some plastic parts chipped, amplifier box clearly has been opened), but it seems to work (i get believable readings with DC current at least) I have to check the performance later.
At the moment im using a PULS ML30.106 (+-12V 36W, fairly low noise for a switcher) that still needs to be put into a case.
The probe is quite power hungry and the voltage regulators get quite hot quickly, even with very little load. I could probably power it from my Lecroy scope (which can sort of supply 1-2A on +-12V, total on all 4 channels though), but it would be too much for the TDS3014. Finding a Lecroy probe connector (ie. broken active probe) wouldnt be easy though.

I did find pins 1 and 2 on the RJ45 that seem to be connected to something, measuring 100KOhm to ground each, but cant see their purpose yet.
Update: Pulled up to +5V via ca. 20kOhm. Dont see any activity there.

I did disassemble the probe head itself as it had some plastic rattling inside, which were broken-off bits from the bosses the self-tapping screws thread into. The whole plastic case is made from some really cheap PS/ABS material, dont use solvents except alcohol to clean it or it will be damaged. There also was some oil around inside, which i hope came in from the outside and didnt leak out of some important part... The actual probe is a huge heavy metal block with the transducer plugged in (luykily the core is undamaged, just full of fingerprints that can be cleaned off), which i didnt take apart further as i dont really want to break it.


BTW, you can take the amplifier apart without soldering, there are 2 M2 screws under the back side label that attach the heatsink block to the case.

[0xdeadbeef]

I also came across this probe. Surprisingly cheap, as far as DC current probes go. And its scope-independent, which is really useful...

I guess we bought from the same guy if you won your auction recently. He sold like 3 or 4 of them with the same picture though I assume the picture was that of my probe.
At least on mine the very same plastic edge of the amplifier box was broken. Yet when I opened the thing, the one of the other side just fell off as well.
No rattling or oil in my probe though. I still opened it up but there was not much to be seen there without risk of damaging it.
Regarding the back side label: I didn't figure out how to remove it, I thought it was glued and didn't want to risk to break it.
Anyway, for a working probe, this was a bargain but with a certain risk as it was sold "untested" - which usually means broken.
I recently saw a used Lem PR50 (which I guess is exactly the same model before Fluke bought the rights and renamed it) for 1500€ without VAT. But ok, this one was calibrated and still within its case.
Used and working 50MHz DC/AC probes from Tek or Lecroy are usually in this price range as well but come with supplier specific connectors. So the Fluke i50s/Lem PR50 is kinda unique as it seems to be more or less the only probe in that category which has a normal BNC connector.

Regarding the supply, I'm currently figuring out three possible solutions

1) Cheap and dirty: two 12V supplies back to back - but I guess the ripple could be an issue (even though there are linear regulators from 12V to 8V anyway)
https://www.reichelt.de/Treiber-fuer-LEDs/MW-LPH-18-12/3/index.html?ACTION=3&GROUPID=6581&ARTICLE=152200&OFFSET=16&

2) The classical linear approach with a toroid and e.g. LM317/LM337 for the +/-12V (they are cheap on ebay. probably not worth the effort to do one myself)
http://www.reichelt.de/RKT-5015/3/index.html?&ACTION=3&LA=446&ARTICLE=15279&artnr=RKT+5015&SEARCH=trafo+2x15
http://www.ebay.de/itm/LM317-LM337-AC-to-DC-Dual-Power-Supply-Voltage-Converter-Module-/291382102173?hash=item43d7ba089d

3) A split rail approach like this to get +/-12V from a 24V supply:
http://www.transkommunikation.ch/dateien/schaltungen/diverse_schaltungen/power_supplys_circuits/Symmetrische%20Spannung%20mit%20einfachem%20Netzteil.pdf

[robert_]

Yes, same seller. Also same damage on the amplifier box and the back side label fell off as it had been ripped off before, also some damage on the probe itself from improperly opening it. So you probably got a better one... I got the probe put together with some epoxy to repair the screw bosses, and a bit of tape to hold the case together on the front side (clips broken off).

I wouldnt use the LED drivers. These usually are single-stage PFC type, ie have significant 100Hz output ripple (often 10-15%), which is okay for LEDs but not for anything else, also these generally are cheap, crappy and noisy as hell.
Cheap PSU in general are not really a good idea, often they fail with excessive output ripple and sometimes with huge overvoltage (had a 5V plug-in type that had over 16V on the output). I dont know if the +-12V are used directly, or only the +-8V from the regulators. Even if it doenst use the 12V, a 78xx has pretty bad ripple rejection, especially above a few Khz it becomes quite useless in reducing ripple and noise, so the supply should be relatively clean. Linear regulation from a mains transformer wouldne be the worst idea, even though its gonna be large and heavy.

I did use this one, works nicely and is very well made (and i had one around):
http://www.conrad.biz/ce/de/product/511263/Hutschienen-Netzteil-DIN-Rail-PULS-MiniLine-ML30106-12-VDC-14-A-36-W-1-x?ref=searchDetail
the conrad "data" table is wrong as so often, it is dual 12-15V (adjustable), 36W (total).

[robert_]

Just gave Pin1+2 a quick look: they are both connected to the microcontroller. Looks like I2C. Probably not really useful.

What im planning to do though: use the range indication LED to drive a small FET to switch in the 6.8k (iirc) resistor to enable the probe to be detected as 1x/10x depending on the range selected, so the scope display is scaled correctly, at least in V instead of A. This would work as long as the probe works normally and isnt trying to indicate some error by flashing the LEDs, but in that case the scale factor doesnt matter anymore.
Only real problem is how to get a BNC connector with readout pin that is connected to a wire instead of an built-in resistor. I think i can make one with some left over parts from broken probes but it might need some creative rearrangement of parts...

Also planning to put the power supply in a case, together with a 1 (or 2) channel Tekprobe/BNC adaptor/PSU (1103 clone) to power other probes like a P5205.

[0xdeadbeef]

That supply of yours looks like a good choice, although it seems a bit expensive though for my liking. You're probably right that LED supplies are not the perfect choice but that Meanwell supply I pointed to is not that bad. It has a ripple of 120mVpp specified which is like twice that of your supply but it's not completely unusable. Anyway, since it's finally a bit cooler here, I might be able to do some testing and tinkering this weekend. I also intend to build a +/-12V supply in a (somewhat) portable case, but with general type 4mm banana plugs so I can use it also for different stuff.

Which brings me to the topic that I'm not perfectly happy with my current RJ45 adapter. I wonder which solution you came up with? Slaughtered a network cable or fully customized?

BTW: interesting that there seems to be communication on the 1st two pins. I wonder if this was used by the original supply in some way (protection, auto sleep, whatever) or just for calibration.
You probably also noticed that there is a serial EEPROM inside the amplifier next to the 5V regulator. Which is surprising as it's quite far away from the µC and very close to the RJ45 connector. I didn't check, but maybe this EEPROM can be programmed from pin 1/2 which would somewhat explain the odd location.

[_Wim_]

With this constellation however the probe wouldn't work. No LEDs lit, only like 14mA flowing into the +12V supply and 300mA into/from the -12V supply.
I suspected an issue with a voltage regulator in the +12V input, took the control unit apart, didn't find anything, put it together, still wouldn't work, already had given up. However in a last attempt, I tried to connect only the +12V supply: blinking LEDs, 700mA drawn from the 12V supply, overcurrent warning. Connected also the -12V: only ~150mAp on both supplies, green range LED lit. Probe seemed to work, but with implausible values and the auto-zero didn't work. However it needed just one degauss and it showed plausible currents. Yippee Ki Yay

I recently also obtained a fluke i50s, and mine has exactly the same behavior at start-up: if +12V and -12V and turned on together, no leds. I first need to turn on the +12V, and afterwards the -12V. Sometimes it remains in over temperature warning, sometimes it starts up correctly. I did verify the current measurement on mine, and it is quite accurate. So once past this strange start-up behavior, it operates normally.

Does your probe still shows the same start-up behavior? I do not think this behavior is normal, as according to the manual you can use your own power supply, and this start-up sequence of first +12V is not described in the manual.

Did you find a service manual for this probe?

[kilobyte]

Some days ago I got the LEM PR50 with a broken slider. I replaced the missing silder temporarily with a piece of plasic.

It has the same strange start-up behavior if the +/-12V are turned on together.
I have tested only the Low Current range yet. Auto Zero and Degauss are also working.

A Traco TXL 035-1212D is already on the way as the PSU for the PR50. My plans for the power supply are to build a litte PCB that will do the power sequencing.

[_Wim_]

For the moment I do the power sequencing just with my lab power supply. But before building a psu with sequencing, I would like to give it a go at fixing them.

I presssume this is the reason they become relatively available cheaply, because they do not work any longer on the original power supply.

P.S: did you buy your probe from "suche_handys" also?  (I got the fluke labeled one)

[kilobyte]

Yes i got mine from the same seller.

I think also it is a good idea to find the problem that causes this issue.

[kilobyte]

Yesterday I took some more detailed pictures of the Probe.
There are some parts (MAX4113, ET2008, MAT04) that are discontinued now.

And today I have made some pictures with my Flir of the PCBs during the "latchup" state.
There are some components that get quite warm. Maybe one of these components is causing the latchup effect.

I also noticed that sometimes the probe work directly if I plug the RJ45 cable in.

[_Wim_]

So played around a little more, it seems that the +12V has to come up first (that we knew already), but the -12V has to come pretty soon afterwards. If the -12V is a little later, then the probe indicates "overheating". If both -12V and +12V come up exactly at the same time, no indication what so ever. If the -12V comes up first, you get exactly the same result as with both +-12V together.


More in details together with current consumption:
1) first +12V, very soon after -12V => probe start up correctly with automatic degauss
+12V: 151mA  // -12V: 145mA   (remark: during automatic degauss: both +-12V approx. 600mA, after degauss, for a few seconds +12V:164mA // -12V: 158mA until relay clicks in probe, then to 151mA&145mA)

2) first +12V, one second later -12V => immediately indicates "overheating", then probes performs automatic degauss, ,and after that does not react on button presses.
+12V: 164mA // -12V: 158mA (remark: during automatic degauss: both +-12V approx. 600mA).  When +12V only => 800mA! (could this trigger the overheating?) No relay click in probe after degauss. Remains in same current as successful start-up before relay click in probe. Probably does not switch to its final state due to overheating error.

3) +12V and -12V exactly at the same time => probe does not start up (no automatic degauss, no reaction on button presses)
+12V: only 17mA // -12V: 310mA

4) first -12V and then +12V => probe does not start up (no automatic degauss, no reaction on button presses)
+12V: only 17mA // -12V: 310mA. Exactly the same behaviour as both +-12V at the same time

So I do not think it is some kind of thermal latch up, as I can repeat the above as much as I want, it behaves always the same.

So for me it seems when the internal microcontroller gets power (powered from the +12V rail), it tries to start the degauss. When the -12V is not present at that time, the degauss function consumes 800mA from the +12V, and the overheating flag is set. When the -12V comes on, the degauss function continues normally, but because the overheating flag is already set, the probe start-up sequence does not finish successfully. This behaviour seems still normal to me.

What was strange however, if the -12V is present at the moment the microcontroller tries to start up (when the +12V arrives), there is no start-up sequence what so ever. So I think we need to look at how the -12V could inhibit the microcontroller (MC705P6ACDWE) from starting up.

The reset pin (pin1) would be the obvious suspect. It is tied to the 5V of the 78L05A (next to the keyboard connector), and when the -12V comes on first, no 5V to thin pin is supplied.  So that is a problem for sure!

It seems that the 7808 regulator is completely overloaded when the -12V comes on first (output voltage is at -0.8V, so somehow the -12V (or -8V) is tied to the +8V output of this regulator when the negative supplies come on first.

I will look further into this a bit later, but for now, I have to do some other job.

[_Wim_]

Just a quick remark that crossed my mind: the 7808 cannot be overloaded, as it is powered from the +12V rail (I measure 11.3V on the input of the 7808 which is normal because of a series diode), and this rail only consumes 17mA which is way to less to be overloaded.

Could it be the 7808 will not "start" when it sees a negative voltage at is output? It is for sure not normal that its output is at -0.8V. When the +12V comes up first however, the 7808 works perfectly.

[_Wim_]

Done some further testing:
Attached are some scope power-on pictures. Channel 1 and 2: input and output of the 7808, channel 3 and 4: input and output of the 7908. Pictures show +12V 5ms ahead, +12V 5ms delay, +12V only and -12V only.
It seems the 7808 regulator will not start when it sees a negative voltage at it output. This negative voltage comes via from the LM7171AIM opamps. When they only see a single power rail, some of that power rail voltage leaks through to the other rail. I have tested this with another opamp on a breadboard, and this seems indeed to be the case. For some reason the 7808 will not start then. I have tested a similar scenario with an 7805 on a breadboard, and the 7805 happily starts even with a negative voltage at its output. So I am going to replace the 7808, but unfortunately I have to order these.

Remark:
I have discovered that when I say to my rigol DP832 “turn all outputs on”, there is a turn-on delay of approx. 5ms between channel 1 and channel 2. As channel 1 was originally configured for -12V, the current probe would not start. Swapping channels (so a delay of 5ms for -12V) is sufficient to make the probe start.