A simple project to enhance a variac with a voltage/current/power/energy meter

[intabits]

I've made this addition of a multifunction panel meter to my recently acquired variac. It's proved to be a very worthwhile addition.
This particular variac had no input cord or output socket, just binding posts. So those items, along with a safety switch and an output voltage indicator LED were also added in this project.





I've made a couple of videos about it:-



and



I've already found it immensely useful while investigating the behavior of a ferro-resonant transformer.

[jcw0752]

Nice looking project. I also like to use a variac in my experiments and service work. For better control of mains hot however I have taken an old PowerVar line conditioner and turned it into an isolation transformer. This in turn provides the power to the variac. This way I do not have an Earth ground referenced hot coming off the variac. One still must be careful but not quite a dangerous.

John

[intabits]

Thanks.
Isolation wasn't what I was going for, so it's not built in (would be too heavy anyway).
I just wanted to make a user-friendly variac.
Something had to be done, as it only had binding post terminals, no cord or output socket.

The variac is rated at 3KVA, so isolation at that power would be challenging.
If I need or want isolation, I do have three 240-120V transformers (weighing 22kg each). Two of those brutes wired back to back will make a nice isolation setup, and handle over 2KVA.

[floobydust]

I considered using that particular power meter pzem-061 but as I recall, it does not include a power factor spec, so I wasn't sure if it has true power.
Peacefair says:
"There are three types of power: active power (P represents / unit W), reactive power (Q represents / unit Var), apparent power (S represents / unit VA).
And the power of our product is active power.
Active power is calculated as: P = U * I * cos?, cos? represents the power factor, for purely resistive loads (such as incandescent, heater, etc.) the power factor is generally close to 1, for inductive load and capacitive load, the power factor is between 0-1; so when you test the purely resistive load, the power(P) is substantially equal to or close to U * I; but when you test inductive or capacitive loads (such as refrigerators, washing machines, televisions, computers, etc.), P <U * I . As for the specific number is related to the power factor, for each appliance has different power factor, so please believe the value of our product. "

DL69-2048 I use (LED) displays power-factor and it shows true power.

[intabits]

It's actually the PZEM-021. I think the difference is that the -021 uses an internal current shunt, and -061 uses external (but I'm not 100% on that).
I already had these meters lying about for a couple of years, so that is what I used.
But yes, I should have tried to ascertain the specs for power factor, and true RMS.
I'll have a look at the DL69-2048 unit that you mentioned.

[bitseeker]

Nice meter upgrade. I hadn't heard about that supply voltage hack before. Makes the meter much more useful.

[eurofox]

My version became a key part of my lab with the following features:

Build in 19 inch 4U enclosure

Back panel:
15A fuse, AC wall socket (power my lab AC rail), 2 green ground binding post (EMC protection connections)

Front panel:
Wall socket 1 is connected to a variac, the variac is connected to an isolation transformer, large volt and ampere meter, the ground of the wall socket can be switched on/off
Wall socket 2 is connected to an energy meter providing volt, ampere, W, energy and PF.
Wall socket 3 is used to power any equipment, will be changed with a mixed connection including 2 USB 2 ampere connectors.
12VDC powered with 10A power supply “car” connector
2 binding post connected to ground for ESD protection connections.
15A switch that power the unit but as well the wall socket on the back panel, this allow me to power the complete AC lab rail.

The picture in attachment is not the final version.

[bitseeker]

Wow, nice job there, eurofox. The knob looks like the style used by Sencore.