Add a meter to your variac

[sluckey]

This is an idea I've had ever since I got my variac 30 years ago. A current thread over on TAG got me interested again so I bought a cheap panel meter from Amazon...

     https://www.amazon.com/gp/product/B079JNY5VD/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

I chose this PZEM-061 model because it uses a CT (current transformer) to sense current rather than a sense resistor built into the meter. The advantage of this is you can sample 100 amps but only need to use 24 AWG wire to the meter.

It's very simple to hook up to your variac. Just connect the variac output and neutral to the meter using small stranded wire. Then run the heavy variac output lead through the CT "doughnut" to the AC outlet. Finally, connect the CT leads to the meter. That's it!

The only caveat with this simple hookup is that the meter needs a minimum of 60VAC to operate. However, there is a simple one wire mod that allows the meter to measure down to zero volts. Simply cut the lead of that big yellow cap, C12 (Cut the lead closest to that 470µF/16V cap). Now solder a small wire to the free end of C12 and connect this wire to the input of your variac, which is a constant line voltage. That's all! Now your meter will measure all the way to zero volts. Here's a youtube video that explains it. The zero mod starts a 6:13.

&

The meter is surprisingly accurate, at least compared to my Fluke 87V. This is a good addition to any bread board. And I bet you can think of some neat things to do around the house too. Anyhow, it's a fun little project. Double sided tape will securely hold it in place, but I actually used a couple 4-40 screws to attach mine. Here are a couple pics of my variac...

[Tony Bones]

Do you understand the mod in detail? Obviously, the meter has a built-in regulated power supply. Is the AC input to the supply really through the big yellow cap?

I believe it works, I'd just like to understand it better. I guess I'll just have to spend $12.99 and look for myself...

[sluckey]

There's really not much to understand. It's just as simple as I described above. The problems that the guys at TAG were experiencing are non-existent if you use the PZEM-061 model.

The AC for the board power supply does in fact go through that yellow cap. The end that you cut is connected to the line terminal on the connector block. If you leave it unmodified then the input to the cap is your variable output from the variac. As you turn the variac down you will reach a point where the display starts flashing. Turn down a little more and the unit reaches a point where it just shuts down. When you cut the cap lead and connect the cap to a solid 120VAC source (like the input to the variac) the power supply maintains proper operating dc voltages regardless of the variac settings.

Here's another video that explains it much better than the first video. Just ignore the part where he tells you to put a 470K resistor across the yellow cap. It ain't needed. If you decide to get one of these meters, I highly recommend you get the PZEM-061 model with the CT. For a couple dollars more you can get one with an "open CT, which means you simply open the CT and put it around a wire to sense current. Then just snap it back together for it to work. Very handy if you want to connect this to a distribution power panel or load center. The inside diameter of the CT is about 5/8", so you can put a really big wire through it.

[PRR]

I have the V/I-only form of that power meter, on my main house feed.

In *that* application, I don't care if it craps-out below 60V.... I'm watching power sag 125V-108V.

AFAICT, the accuracy is very good. The V compares to my Fluke except the 250V meter does not display tenths-Volt, so it says "124V" when my Fluke shows 124.4V.

[Tony Bones]

Very cool. To earn a second nerd badge it would be necessary to add some way to monitor the relative quality of the power (HD?) and continuous data logging. Maybe even measure the phase of I vs V. Hmm...

[jjasilli]

Very cool indeed!  I've been meaning to add meters to my variac for for years.  Great that it displays V, I & W.  Looks like it could work for model trains too.  Thanks of this info.

[sluckey]

This is how I wired my variac...

[shooter]

I used to have a watt-o-meter, but it fried when I used it on my modified sine wave invertor.
at least there is a disclaimer with your find

If you measure power from a DC-AC inverter please make sure it's PURE SINE WAVE. Otherwise the meter can be burnt or melted.

[punkykatt]

Question for Sluckey on the above drawing in a previous post.  According to the schematic on the back of the meter it looks like the black and white wires going from the output receptacle to the meter terminals should be reversed from the way you have them? Or does it matter?

[pompeiisneaks]

Question for Sluckey on the above drawing in a previous post.  According to the schematic on the back of the meter it looks like the black and white wires going from the output receptacle to the meter terminals should be reversed from the way you have them? Or does it matter?

If your schematic and/or hookups for your meter specify Live and Neutral, you should always stick to that convention.  The picture he has doesn't seem to show that.  Some devices can handle miswired L/N others can't.  It's always 'right' to have your L/N correct on the connection of any devices to the power.  It can cause catastrophic problems if a device is two wired and can't support it 'wrong'  There are still some out there and having mains voltages of 120/240 v on the chassis of a device isn't a good place to be.

[sluckey]

Question for Sluckey on the above drawing in a previous post.  According to the schematic on the back of the meter it looks like the black and white wires going from the output receptacle to the meter terminals should be reversed from the way you have them? Or does it matter?

It doesn't matter ***UNLESS*** you want to do the zero mod. If you want your meter to read all the way down to zero volts you must wire it exactly as I show in my wiring diagram.

It's perfectly safe to swap hot and neutral, but if you swap them and do the zero mod, your meter will not work at all.

EDIT... added pdf attachment

[punkykatt]

Thank you !! Steve, Yes, I will be doing the zero mod. Glad I asked.

[JustMike]

I decided to put one of these together. I'm using the PZEM-061 meter and I wired it as Sluckey has shown. But I noticed that the way the Variac is wired, the fuse is after the switch. Shouldn't the fuse be the first thing in line? Or does it really matter? BTW, the soldering on this variac was horrible! The solder looked "chalky-or-chunky" not liquid like at all. I had to resolder all of the connections. Anyway, I wired it up and it works just as advertised.
Another question; Does it make sense to use the variac in series with a lightbulb limiter? I assume the lightbulb limiter would plug into the variac rather than the other way?

 Thanks again Sluckey!

[PRR]

> Shouldn't the fuse be the first thing in line? Or does it really matter?

It shouldn't matter. There are several unlikely situations which can go one way or another. Basic rule is UNPLUG FROM WALL before changing a fuse.

[fenderguy81]

I know this is an old thread, but I am trying to do the same mod to my variac like the one shown in the video. I have the same red variac as shown in the video and the same version of the meter that Sluckey has (different than the one in the video). I've tried following Sluckey's wiring diagram and am not sure what I did wrong. I attached a crude illustration of how my variac and meter are currently wired up. Any help would be greatly appreciated. My meter will not turn on with the current wiring diagram. Thanks!

[sluckey]

Looks like you used a SPDT power switch? If so, move the black wire to the center lug.

[fenderguy81]

Thanks Sluckey! I moved the black wire to the center lug and finally got power to the meter. Unfortunately the meter isn’t responding to the variance dial. It shows a 0v, 0A, 0w, and constant 7wh (energy). I triple checked my connections and everything looks like the schematic/drawing that I can see. Hmm...

[sluckey]

Do you have a variable voltage on the output socket?

[fenderguy81]

Yes, I just plugged something in to test it and the varianc started working at around 65v (0 mod not working I guess). I can feel something click in on the dial at that setting that turns the variac on 65v. The meter said it was only about 35v, opposed to 65v, I kept dialing the variac up and it started sparking internally followed by smoke and a burnt electronics smell. I think I might have fried the variance somehow :/

[PRR]

There's no markings on the Variac terminals?

If you connect the two OUT terminals to the wall, it WILL smoke.

[fenderguy81]

-red wire going to the top of the fuse holder
-black wire going to neutral plug
-green wire going to power switch

[sluckey]

Show me a pic of your PZEM as you have it connected to the variac. I want to also see the zero mod.

Then I suggest you just start over. Totally remove the PZEM-061. Connect the variac wires as they were originally. Now verify that the variac works as normal.

And connect a power cord to the PZEM-061 to verify that it works. Be sure to connect the black and white wires as shown in my diagram. Don't connect the CT. If you have done the zero mod, you'll need to also connect the blue wire to the black wire. Meter should correctly read the line voltage.

[fenderguy81]

Here it is back to the stock wiring with the addition of the meter added on
-black and white from meter to plug hot and neutral
-zero mod wire to top of fuse   
-CT looped through the wire connecting the hot plug to the switch.

It actually seems to work up until around 77v (zero mod too!). At around 77v the light on the meter turns off and the meter turns off completely after around 79v or so. I had my fluke leads plugged into the variac plug and the voltage is accurate. I'm not sure why it can only handle 77v or so. I do see in the video and on another diagram that they do not have the hot plug wire connecting to the switch. I'm not sure where to put the CT loop if I remove that wire though. Glad it's not smoking and still seems to be working though.

[wsscott]

Did you ever get this resolved?  I'm thinking of adding this meter to my Variac.

[sluckey]

Mine works very well. So does dozens of others. If you use the same meter I used and follow my instructions, you will not have any problems. You can also find these instructions in my Amp Scrapbook on my website.

[wsscott]

Where is the CT placed? Inside the Variac?

[sluckey]

The CT goes inside the variac. Disconnect the hot wire from the output socket, pass that wire through the CT, and reconnect that wire to the output socket.

[wsscott]

Sluckey-is the Wattage reading on the Meter, the actual Output Wattage of the amp as it is played when plugged into the Variac, or some other wattage reading?

[sluckey]

How could that meter possibly know the output wattage of the amp?

[wsscott]

That's why I asked the question.  If it did then the Chinese would have really come up with something, and then it might be something useful to me. 

[shooter]

the Chinese would have really come up with something

naw, they'd just steal it.


try "audio power meter" for the speaker end of what the variac measures, maybe there's a correlation that can be math'd out

[HotBluePlates]

Sluckey-is the Wattage reading on the Meter, the actual Output Wattage of the amp as it is played ...?

The meter displays the "watts of power pulled from the wall outlet."

Nothing is 100% efficient, so this wattage (Volts x Current) is higher than the amp's output power.
Depending on the type of amp & the circuit-particulars, it might be "a whole lot higher" than the amp's output power.

[angelodp]

Hi, a bit late to this thread. Here is an older Powerstat 3PN 116. I am following Steve's plan. I have the before and after graphic before I execute. I think I have this modeled closely to the original idea. The Variac pin #'s are different in this model but the wiper is #3 for sure.  The only area to work out is the CT fit into the cavity. I am thinking of ditching that big switch and putting a simpler smaller switch since I am only using one side. Looks like I will have to drill a hole for  the 24g wire and figure how to attach the box to this Variac. I would appreciate a set of eyes on this before I proceed. Have I got it right.


Best Ange



The silence perhaps was a clue. I re-did this diagram to follow Steve's more precisely.

[SEL49]

I would appreciate a set of eyes on this before I proceed. Have I got it right.

yes

[angelodp]

Thanks