Power Meter for Variac

[ledtester]

The Peacefair units use the V98XX series of energy metering SoC chips:

https://youtu.be/G0AlH3vtT0k?t=9m12s

http://www.vangotech.com/int/uploadpic/157234475389.pdf

Update: In section 18.3 (physical page 275) of the V98XX datasheet there are some tables showing the update and settle times of the metering registers along with this note:

... In 60Hz power grid, the time for updating and settling is 1.2 times of that for 50Hz power grid. ...

[coppice]

I also find the chinese power meters have very long averaging time in firmware, it takes a few seconds to get a settled reading with a constant load. They aren't useful for fast response unless you communicate directly with the IC.

I wonder if that's because they use cheap/slow parts and a low sample rate?

Its because they use the cheap, but quite accurate, devices made for the utility energy meter market. That market cares about accuracy over a wide dynamic range, but not about the update rate.

[passengert78]

Hello guys!
I don't know if this topic is alive yet, but any help would be appreciated. So, I have an updated version (V5.0) of this PZEM module (see attached images). My goal is the same as everyone elses here: I want to get this meter to read down to 0V. As I've searched for solutions, I found this topic which solves this by cutting the connection between the mains line input terminal and resistor R13, and soldering a wire to the latter, which connects to the variac's hot output. The other solutions that I've read was running the metering side from external DC (bypassing its own AC to DC conversion), but that was for older PZEM modules (like PZEM-021 with capacitance drop solution). So my questions are:
- would the "trace cutting method" work for this module too?
- i would prefer to power this thing from external DC, since I have several wall adapters that I could use for this, but the sources that I found were quite inconclusive on where to connect the DC positive and negative wires (the ones I saw did this to the puffer capacitor's two sides), and even the voltage it requires (some say it needs 5 Volts, some others say 12 Volts)
Regards,
G

[Kleinstein]

In principle a separate, isolated DC supply works.
A wall wart would however not work, as the supply is connected to mains (though ideally the neutral side - but can't be sure). So the circuit to apply a dangerous voltage to the thin cable from the wall wart to the circuit. If the whole thing is part of variac unit one would have a separate transformer for the supply, mounted inside the same case.

One could test how much voltage the SMPS part really needs. This may be well less than the given 80 V, as the power requitement is low. One could get away with less with a lower series resistor and full wave rectifier - the ciruit seems to have just 1 diode.
Not sure if one would really need to cut a trace, one could also remove the large resistor or a diode instead.
The question still is if one would supply on the SMPS input or output.

As the SMPS does regulation, the required voltage at the SMPS output side may have a relatively small range (like 4.5 -5 V) to work. One may have to look at the ciruit in details and maybe measure the voltage of the SMPS output under normal operation.

[passengert78]

Thanks for the quick reply. I did my "homework" and took my time to analyze how this module works, and I found that driving it from DC would be more complicated than doing the trace cutting hack. As far as I understand, thet trace with the R13 resistor basically leads to the voltage measuring circuitry of the module. If I cut that trace from the AC input terminal, and solder a wire to it, that connects to the variac's live output, I make a dedicated sense line for the variac output voltage. This way I can connect the meter's L and N terminals to the mains part, so that it always gets what it deserves to stay on all the time. Just have to make sure that the trace is cut properly so that no arcing happens or any microscopic connection remains.
Well, I'll try this then. Hope this works with this version of the meter too.
Regards!
G