Help needed to change the analog volt meter on my Variac to digital.

[Ken from ontario]

I have a 20 amp variac that came with an anolog voltage display, I thought it was an easy task to replace it with a digital one so I ordered the proper size display units (two of them) one came with just two wires and according to the listing it does not need its own power supply, that sentence got me really confused because there's another display that has 3 wires which I also ordered ,the third (yellow) wire can be connected to its own power supply.

What concerns me is the two wire voltmeter(AC 30-500V) would start powering up at about 30 volts and my concern is it would dim bellow 30 volts, that's why I also ordered the 3 wire meter(AC 0-600V) I took a screenshot of how to connect both.

My question is, do I need a dedicated power supply so the display could show all the way to zero? or is it possible to get the power from the ON/OFF power switch to connect to yellow but don't I need a "load" or return (don't know how the pros say it) to connect to the black wire?
In the comment section of the video someone commented:

You can usually mod the two-wire devices to three wire and run the power to the module from the line. This means good brightness across the board and voltage measurement down to a few volts. As you have an autotransformer variac, there's no issue taking the supply to the module from the primary and the sense voltage from the secondary.

Here's another question, can any of you guys explain this in simple trems so some like me could understand, please., I'll get it . someone made a video of doing the very same upgrade but with a two wire module and you'll know what I'm talking about once you watch it:

 

[silence dogood]

An analog meter is like a motor. A current goes through a coil on a spindle that can deflect next to a magnetic field. If you look closely, you may see a spring. The more current, in this case since it set up for voltage, the more the needle will deflect. against that spring. So, it can start from zero voltage. A digital meter has semiconductors that cannot work unless they have a certain amount of turn on voltage. In this case, your meter must have 30 volts to turn on and it uses the voltage from the variac. The other meter has a separate supply that is independent of the variac. That is why it can measure down to zero volts. Hope this makes sense.

 

[Ken from ontario]

An analog meter is like a motor. A current goes through a coil on a spindle that can deflect next to a magnetic field. If you look closely, you may see a spring. The more current, in this case since it set up for voltage, the more the needle will deflect. against that spring. So, it can start from zero voltage. A digital meter has semiconductors that cannot work unless they have a certain amount of turn on voltage. In this case, your meter must have 30 volts to turn on and it uses the voltage from the variac. The other meter has a separate supply that is independent of the variac. That is why it can measure down to zero volts. Hope this makes sense.

Yes it makes perfect sense, thank you for the explanation.
If anyone can explain how I can get and independant source of power within the variac to act as the power supply for the digital meter , does it need to be AC or DC ? could an LED driver like 600mA 1x3W Led Driver, be used as the power supply?,I used that driver but it did not work, either because the output is too low or it was in DC?

 

[silence dogood]

From looking at your schematic on the bottom left on your picture. The black wire would be connected to the neutral wire which should be white. Ground is usually green, and you don't want the black connected to that. The red wire would be connected to output of the switch. The yellow wire would be connected to the output of the variac on the hot side. According to specs, your voltmeter can run on AC just fine.

 

[silence dogood]

I should had looked at 1:21 on the utube. Opps, I mislead you, ignore my last comment. The black wire will go to the neutral white wire or blue wire on the switch. The red will go the brown fuse side that goes to the variac. The yellow will go to red wire side of the plug-in. Again, looking at the first schematic on the 4. ac 0-600 volts according to the specs, it should work just fine on 110 ac.

 

[Ken from ontario]

If I understand correctly, the old analog volt meter was connected the same way, one wire to neutral, one to output hot side, the new digital has an extra yellow wire, what if I connect the black and the red wires to the same spots as the old analog one and then connect the yellow to the power switch's hot side and run a short length of wire from the load side of the power switch to the black wire which is neutral, that may only make sense to me right now lol.

 

[Ken from ontario]

I just read your last post, I must have been typing when you posted it, anyhow, let me read and digest it and see if I can make sense of it.

 

[silence dogood]

Just got your latest message. I like your thinking. Just like the analog meter, the black will go to neutral. The red is the power and should go to the fuse. I suggested this instead of the switch is because if the fuse blows, the meter will go to zero just turn off. The yellow is your sensing wire just like the other wire on the meter. So, you can connect it just like the other wire on the meter. Too bad that you don't appear to have the room. Because you could leave the analog meter in too.

 

[Ken from ontario]

I see your point on using the fuse instead of the switch, as the matter of fact I have also bought a manual reset circuit breaker to replace the fuse, if what you said works, I'll leave the fuse alone for now and just connect the volt meter for now, what can I tell you I got the bug to fiddle with this variac and upgrade it within what I think I can do,.
I sincerely appreciate all your help, I'd imagine it was hard to make it sound simple so I could understand the concept, thank you .

 

[silence dogood]

Now that I had a chance to sleep on this. I'm going by the schematic or diagram here. The black wire is connected just like the meter wire to neutral. The yellow wire (sensor wire) is connected to the output like other meter wire. The red wire (power wire) is connected to the fuse or reset circuit breaker (good idea). If you put the red wire on the switch side of the fuse, if the fuse goes the meter will still be on but go to zero. If you put it on the other side of the fuse, the meter will turn off.
I have two older models of these variacs, very useful devises. Just curious, what do you plan to use this for? For many years back in the eighties, I repaired many solid-state amplifiers and power supplies. Ran an AC ammeter, in this case a 5 amp, in series on the hot line. If the rated input current was at let's say 1 amp, I turn the voltage up till the meter reached 1 amp. It made it really easy to trouble shoot without worrying about maximum smoke.