Voltage hubs not working


We have a few of IotaWatts whose voltage inputs are not working (i.e. we get no voltage readings). We have checked the issue with multiple eliminators, and the issue still remains. The connections and pieces look in perfect shape. In two Iotawatts we get no voltage readings, while in another one 2 out of 3 ports (13 and 14) have readings, while input 0 is mute.

We do not know if there was a voltage discharge which may have affected the operational amplifier (LM358), which we plan to change to see if that solves the issue.

Any additional help on what the root cause may have been and the solution will be greatly appreciated!

If you are having these problems with multiple units, the problem is almost certainly something you are doing that is damaging the units.

Are you using only the TDC VTs that came with the units or have you used other VTs as well?

Are you using splitters to connect one VT to multiple IoTaWatt units? If so, there is potential for damage if strict polarity is not observed, especially if you are using a common USB supply like a USB hub for those same units.

I have seen the op-amps damaged from doing the above (I’ve done it myself while being careless with banks of test units). The op-amp is a dual LM-358. One side is used exclusively for the Channel 0 VT, and the other side does everything else, so with the units that are only 0 is bad, you might try seeing if any of the channels 1-7 are working with the channel 13 or 14 voltage reference. If not, then the ADC is probably toast.

While on the subject of multiple VTs, have you tried derived reference three-phase (one VT)? I would estimate more than 90% of all installed three-phase IoTaWatt are using derived with satisfactory results.

Many thanks for the response. The TDC VTs are the ones that came with the units, but other than that, all the rest was confimed: wrong polarity, multiple iotas connected to the same USB hub, derived reference 3-phase. We will try to reconfigure the LM-358 and see if we can save the iotas. We’ll see if the derived reference 3-phase still brings problems, but likely the USB hub and wrong polarity account for most of the problems.

Sometimes the TVS diodes cook. There are the four small ICs next to the ADC s. If necessary, you can just remove them. They are only functional when there is a transient. The ADCs are rated 4kV, the diodes are only to achieve the 6kW standard for CE certification.

As long as the ESP8266 is OK, I can fix them if you can get them to me.

Hi Bob,

We are looking for the available dual LM-358 operational amplifiers from TI and found the following models:


We see the amplifiers in the IotaWatts list ASG7G4 (not matching with the available options above). Would any of the above mentioned be useful and accurate without software reconfigurations?

For your rework project it’s pretty hard to go wrong with any LM358 with an SOIC 8 package. The basic IC is the same, virtually all of the options refer to packaging, solder, reflow temps, ROHS etc.

Specifically the LM358DRG3 appears to be a close match to the LM358DR that is the specified part.

Hi Bob,

Related question. We have been working with the polarities of the Jameco VTs (some seem to be configured differently). Is the VT input expecting the phase in its center pin (vs. the outside)? Just want to make sure it is the case so we configure the VTs appropriately.

P.S. We are also replacing the common USB hub with individual supplies.

The phase is whatever you have it plugged into. The polarity is something different. AC doesn’t have an absolute polarity, as I’m sure you know. When I mentioned observing polarity with the IoTaWatt, what I mean is that a VT shared between two IoTaWatt should have the same plug orientation. That is to say that the center pins should be the same transformer output. This is easily and accurately accomplished by using standard 5.5mm x 2 mm splitters commonly available as DC splitters in the CCTV industry.

The USB hub couples the ground of multiple units, and so even if only doing single phase, a shared AC reference must use the polarity as described above. With three-phase, even without the coupled ground, reversing two shared VTs would cause essentially the same problem.

Sharing VTs in a direct-reference three-phase environment is ultimately practical and economical, but I would strongly recommend you use the commercially available splitters. They are inexpensive and come as 2x, 3x, 4x, 5x etc.