Tangent alert: Sanity check on adding DMM display to a variac?

[turbofeedus]

I bought this inexpensive variac off Amazon, as well as this multimeter display many have used in conjunction.

I was following this video for installation, but had some issues. The installion put quickly; slot 1 goes to output neutral, slot 2 goes to switch neutral, slot 3 goes to variac wiper, slot 4 goes output hot. There's a fifth wire, which is a mod to the multimeter (allows the meter to detect output voltage down to 0V), which is wired into the fuse as just a constant voltage source.

First startup I seemed to have some type of short. I was using a bulb limiter between the variac and mains, and upon increasing the output voltage, the bulb began to glow brightly, and the variac started vibrating and humming.

I double-checked my wiring, everything was correct. After a few watch-throughs however, I noticed my variac had an extra wire compared to the one in the video. Specifically, a wire directly connecting the neutral terminal of the switch (where the nuetral from mains is connected) to the nuetral lug on the output plug. In other words, the neutral from mains was hard wired to the neutral on the output plug always, not switched. The variac in the video doesn't appear to have this wire.

I decided to try removing it, and both the variac and DMM seem to be operational now. The meter detects a variable output voltage from 0-140V+AC, as well as a current of 5mA unloaded, and a varying wattage. The energy meter shows a constant 0 w/hrs, at least unloaded.

I'm having a hard time wrapping my head around why the extra wire was leading to a fault condition, and if the modification has "fixed" the problem. Can anyone lend me some brain power?

variac-1.JPG

variac-2.JPG

[SoulFetish]

Dude, this is not correct wiring. You need to not use this variac until it's fixed. First thing to do, is disconnect the meter leads and reconnect your variac the way it was.
The neutral is the load and shared by the mains and variac output. Lemme spend a little more looking over the unit you are using and your wiring to double check it, but it looks like the unit you are using has a current sense resistor. I bet internally, one of the two connectors are actually shorted together and you inadvertently you connected mixed up the polarity and shorted the live and neutral. Hang tight until we get this sorted out.

I use the one with the current transformer and would recommend that one because it's easier and much better.

IMG_4315.jpg

But you're not the only one to wire it up incorrectly the first time, if you'll notice in my photo there is a current reading with no output load on the variac? I installed the current transformer around the neutral instead of the wiper (which should be connected to the Live terminal on the plug receptical).

[turbofeedus]

if you'll notice in my photo there is a current reading with no output load on the variac?

I think the phantom current reading is a known issue with these meters, every one I've seen does this regardless of version of meter or variac they're attached to.

Only two stock connections were changed in my variac;
1) The wiper of the variac now goes to pin 3 of the meter instead of the output plug hot.
2) The wire from the switch neutral to output neutral has been disconnected.

I did successfully use the variac (with a bulb limiter) last night to power an amp up to 110VAC (stopped there for other reasons).
I also confirmed the output AC voltage was reading correctly on the meter with another DMM.

it looks like the unit you are using has a current sense resistor. I bet internally, one of the two connectors are actually shorted together

I think correct? Pins 1 and 2 are connected via a resistor, and pins 3 and 4 are shorted together.

[SoulFetish]

I think the phantom current reading is a known issue with these meters, every one I've seen does this regardless of version of meter or variac they're attached to.

It isn't a phantom current reading. The reason people are getting low current readings When they connect them to variacs is that they have the meter connected so that the "sense" resistor is is series with the neutral. What the are actually reading is the magnetizing current in the transformer at idle.
When I moved the CT (current transformer) to the wire running from the wiper to the outlet, I get a zero current reading on the meter when there is no load on the variac outlet. Only the voltage difference is present, so it will still read the wiper voltage with no problem at all. I'll shoot another photo when I can to show you.

Only two stock connections were changed in my variac;
1) The wiper of the variac now goes to pin 3 of the meter instead of the output plug hot.
2) The wire from the switch neutral to output neutral has been disconnected.

I think correct? Pins 1 and 2 are connected via a resistor, and pins 3 and 4 are shorted together.

Okay, if that is the case I think you are okay. I would recommend this, however... If 3 and 4 are shorted together, I would reconnect the original wiper wire straight to the outlet. Its a much more robust connection and you can just run one wire from either of those connectors to the outlet, since it only serves as a voltage measuring point. The way you have it right now, the entire load at the variac goes through those tiny wires.

[Tony Bones]

If you want the meter to work, do this:

(Starting with the variac wired as it came from the factory) disconnect the two power wires at the output jack, but leave the earth ground in place.

Connect those wires to the inside terminals on your meter (2 & 3).

Connect two new wires from the outside terminals to the output jack (1 & 4).

If you do that correctly, it should work. In the US, one wire is hot and the other neutral, so pay attention to that. You said it right on your post (neutral in 2, neutral out 1, hot in 3, hot out 4.)If that's the way you really wired it up, and all the parts are good, then everything should work as expected with no phantom readings. The voltage and current on the meter should be only what's actually being delivered to the load.

[turbofeedus]

I think the phantom current reading is a known issue with these meters, every one I've seen does this regardless of version of meter or variac they're attached to.

It isn't a phantom current reading. The reason people are getting low current readings When they connect them to variacs is that they have the meter connected so that the "sense" resistor is is series with the neutral. What the are actually reading is the magnetizing current in the transformer at idle.
When I moved the CT (current transformer) to the wire running from the wiper to the outlet, I get a zero current reading on the meter when there is no load on the variac outlet. Only the voltage difference is present, so it will still read the wiper voltage with no problem at all. I'll shoot another photo when I can to show you.

Only two stock connections were changed in my variac;
1) The wiper of the variac now goes to pin 3 of the meter instead of the output plug hot.
2) The wire from the switch neutral to output neutral has been disconnected.

I think correct? Pins 1 and 2 are connected via a resistor, and pins 3 and 4 are shorted together.

Okay, if that is the case I think you are okay. I would recommend this, however... If 3 and 4 are shorted together, I would reconnect the original wiper wire straight to the outlet. Its a much more robust connection and you can just run one wire from either of those connectors to the outlet, since it only serves as a voltage measuring point. The way you have it right now, the entire load at the variac goes through those tiny wires.

Sorry I'm not following. I understand how the resistor connecting pin 1 and 2 (assuming this is the sense resistor) is in series now with the output hot, but I'm not clear on how to sort this out?
Also, I'm not sure what you mean by current transformer? It sounds like our variacs are different, and maybe mine doesn't have this feature?

If you want the meter to work, do this:

(Starting with the variac wired as it came from the factory) disconnect the two power wires at the output jack, but leave the earth ground in place.

Connect those wires to the inside terminals on your meter (2 & 3).

Connect two new wires from the outside terminals to the output jack (1 & 4).

If you do that correctly, it should work. In the US, one wire is hot and the other neutral, so pay attention to that. You said it right on your post (neutral in 2, neutral out 1, hot in 3, hot out 4.)If that's the way you really wired it up, and all the parts are good, then everything should work as expected with no phantom readings. The voltage and current on the meter should be only what's actually being delivered to the load.

So this makes sense to me, except that all the current will be going through the meter, and I'm not sure it's designed to work like that? Also how will this effect the mod that allows the 0V reading?

I'd really like to nail this down, not only for my meter, be hopefully I can make a guide/schematic for others doing the same thing.

[sluckey]

Also, I'm not sure what you mean by current transformer?

There is another unit that looks just like your unit except it has a current sense transformer (doughnut). Much better than using a sense resistor IMO. This type does not require the entire load current to pass through the meter. The current transformer picks up a sample that represents the actual load current. Here's one...

https://www.amazon.com/dp/B00YY1KOHA/re ... s9dHJ1ZQ==

[Tony Bones]

So this makes sense to me, except that all the current will be going through the meter, and I'm not sure it's designed to work like that?

All of the current is supposed to go through the meter. That's how it measures current!

Also how will this effect the mod that allows the 0V reading?

It should work correctly. It measures only what's going to the output plug. Actual voltage and current (and calculated power.) If the variac will go to zero volts, then that's what will show on the meter... assuming the meter is accurate.

EDIT: I just scanned the video to see what the 5th wire mod is all about. To get the meter to work at low voltages you need to connect that wire to a voltage source that's a steady 115V.

I'd really like to nail this down, not only for my meter, be hopefully I can make a guide/schematic for others doing the same thing.

The meter should be very simple to use in any application. Insert it in-line before any load and it measures both voltage and current. But, you need to have all of the current through both wires pass through the meter. There should be no electrical paths between the source and load that go around the meter, except for earth ground of course.

[turbofeedus]

Also, I'm not sure what you mean by current transformer?

There is another unit that looks just like your unit except it has a current sense transformer (doughnut). Much better than using a sense resistor IMO. This type does not require the entire load current to pass through the meter. The current transformer picks up a sample that represents the actual load current. Here's one...

https://www.amazon.com/dp/B00YY1KOHA/re ... s9dHJ1ZQ==

So this makes sense to me, except that all the current will be going through the meter, and I'm not sure it's designed to work like that?

All of the current is supposed to go through the meter. That's how it measures current!

Also how will this effect the mod that allows the 0V reading?

It should work correctly. It measures only what's going to the output plug. Actual voltage and current (and calculated power.) If the variac will go to zero volts, then that's what will show on the meter... assuming the meter is accurate.

EDIT: I just scanned the video to see what the 5th wire mod is all about. To get the meter to work at low voltages you need to connect that wire to a voltage source that's a steady 115V.

I'd really like to nail this down, not only for my meter, be hopefully I can make a guide/schematic for others doing the same thing.

The meter should be very simple to use in any application. Insert it in-line before any load and it measures both voltage and current. But, you need to have all of the current through both wires pass through the meter. There should be no electrical paths between the source and load that go around the meter, except for earth ground of course.

Alright let's see if I've got this correct for at least the version of the DMM display I have:

variac dmm schematic.png

Does this look correct?

[Tony Bones]

Does this look correct?

Yes. And the 5th wire connects to the protected side of the fuse.

[turbofeedus]

Does this look correct?

Yes. And the 5th wire connects to the protected side of the fuse.

Ah I see, so SoulFetish is saying that because I have the neutral going through the sense resistor (rather than the hot/wiper), I'm getting a current reading even with no load, which is apparently the magnetizing current.
So if I switch this around to match the schematic I posted, namely the wiper going into the meter on 2 and leaving on 1, I should read no current unloaded.

[Tony Bones]

[Ah I see, so SoulFetish is saying that because I have the neutral going through the sense resistor (rather than the hot/wiper), I'm getting a current reading even with no load, which is apparently the magnetizing current.
So if I switch this around to match the schematic I posted, namely the wiper going into the meter on 2 and leaving on 1, I should read no current unloaded.

Maybe. I don't really know exactly how you had it wired when the meter showed current when unloaded. But, if you wire it as in your schematic then the magnetizing current will not go through the meter, so there's no way the meter could know anything about it.

[ramble]I found it odd that the guy in video chose to wire the meter with the the sense resistor in the neutral wire instead of hot. There might be something I don't know about the way the meter rectifies the line voltage, what it calls "ground", or how the 5th wire mod works. I don't know. In the end though it really doesn't matter. We like to think of the neutral wire as groundy, but really it's not. Back when power cords were all 2 prong without earth ground some equipment might have cared which wire was which (hence the polarity switch on the back of old amps) but nothing you plug in today should care which wire the sense resistor is in. [/ramble]

[turbofeedus]

[Ah I see, so SoulFetish is saying that because I have the neutral going through the sense resistor (rather than the hot/wiper), I'm getting a current reading even with no load, which is apparently the magnetizing current.
So if I switch this around to match the schematic I posted, namely the wiper going into the meter on 2 and leaving on 1, I should read no current unloaded.

Maybe. I don't really know exactly how you had it wired when the meter showed current when unloaded. But, if you wire it as in your schematic then the magnetizing current will not go through the meter, so there's no way the meter could know anything about it.

[ramble]I found it odd that the guy in video chose to wire the meter with the the sense resistor in the neutral wire instead of hot. There might be something I don't know about the way the meter rectifies the line voltage, what it calls "ground", or how the 5th wire mod works. I don't know. In the end though it really doesn't matter. We like to think of the neutral wire as groundy, but really it's not. Back when power cords were all 2 prong without earth ground some equipment might have cared which wire was which (hence the polarity switch on the back of old amps) but nothing you plug in today should care which wire the sense resistor is in. [/ramble]

Just wired up as indicated by the schematic, seems to be working correctly with no current detected while unloaded. Not sure where the energy consumption is coming from, although SoulFetish's variac has this as well.

variacDMMmod-1.jpg

[sluckey]

Not sure where the energy consumption is coming from

That's a cumulative reading based on power consumed over time. The number started as zero initially and has been building up ever since, just like the KWH meter on the side of your house. You can reset it to zero with the button on the right side of the display. That button is tricky. It controls several features depending on how long you press it. Read the directions or watch a demo on youtube.

[turbofeedus]

Not sure where the energy consumption is coming from

That's a cumulative reading based on power consumed over time. The number started as zero initially and has been building up ever since, just like the KWH meter on the side of your house. You can reset it to zero with the button on the right side of the display. That button is tricky. It controls several features depending on how long you press it. Read the directions or watch a demo on youtube.

Ah gotcha, yeah I'm not messing with that. Too many functions for one button.