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

[Ken from ontario]

I'm just starting to take the front panel off right now, (for the second time, (I put it back together last night until I get a good understanding of the connections), as I mentioned I already have the two wire display as well but I'm planning on installing the 3 wire volt meter and if I couldn't make it show zero to 130v all the way, I know I can always tie yellow and red so I could still use the variac ( with the new volt meter showing 30 volt and up) already tested it by connecting red/yellow to hot and black to neutral side of a wall plug and it showed 115V. if yellow is not connected the volt meter only shows zero.
I bought this variac not to repair anything electrical but for soldering , my old 100W Weller takes a long time to reach to maximum temp but I want to be able to lower it half way in stand by mode, while preparing my next connection, ( making copper light fixtures, clocks, etc.), it may also help me with my shop made (LRS) Low resistance soldering unit when full amperage is too much to solder two very thin pieces of copper or brass.

I also have 3 amp variac that I use with my Foredom grinder, it's been working for 2 years without a single issue.

 

[silence dogood]

Good use. Those variacs are very useful. The most usual use that I used my variac for was to slowly turn the lights on the Christmas tree for the Nutcracker Ballet. We brought the lights up in steps to the beat of the music. The variac made it so simple.

 

[Ken from ontario]

I got side tracked today, wife wanted me to install a dimmer switch on a wood (tongue & groove) wall, took me 2 hours to cut a square in hard wood to fit the switch. I'm slow but I'm thorough LOL.
now I can tackle the variac. I hope I can rely on your help again if I get stuck.

 

[silence dogood]

I got side tracked today, wife wanted me to install a dimmer switch on a wood (tongue & groove) wall, took me 2 hours to cut a square in hard wood to fit the switch. I'm slow but I'm thorough LOL.
now I can tackle the variac. I hope I can rely on your help again if I get stuck.

That's the wonderful thing about this site. We try to help. Any time my friend. The only thing that makes me nervous is have I made it understandable and clear in what I'm trying to pass on. P.S Wife is a priority, so you don't need to apologize.

 

[Ken from ontario]

That's the wonderful thing about this site. We try to help. Any time my friend. The only thing that makes me nervous is have I made it understandable and clear in what I'm trying to pass on. P.S Wife is a priority, so you don't need to apologize.

Yes you have, and I appreciate your input.

 

[Bi11Hudson]

Without seeing the documentation for a specific meter, any ideas I have will be SWAGs, no better. For the two wire meter, I will bet for DC only. For the three wire meter, since it has only three wires, I would say 50/50, leaning toward DC. A proper AC digital voltmeter should have four wires minimum for what you are attempting.

For most any voltmeter, analog or digital, the range is simply a matter of how much resistance is in series with the meter. Within reason, the higher the range, the more resistance. All analog meters (D'Arsonval) are milliamp meters. If they have a series resistor, they are voltmeters. If they have a resistor in parallel(shunt), they are likely amp meters. Many other measurements can be made with sufficient resistive networks. Resistance readings are just current meters with a different scale. Getting into voltages above 600 volts is limited by OSHA rules. The meters are the same, just using more complex circuits. I predate OSHA. . .

In the case of the displayed meters, I would consider using a resistor and a rectifier in the power lead. And a rectifier in the sense lead. I don't have high(?) voltage versions of those meters, I am working 15 volts and less. But I have been inside one, changing the scaling. There is no room for the high resistance needed for power in the 120 volt region. Or cooling space, the resistor for that would get fairly hot. Those meters are small, with plastic cases. That would melt. . . For the 3 wire version, I would bet on a 12 volt supply with the sense lead set up to read high voltage DC, with a common return. As stated above, all of this is a Scientific Wild A$$ Guess witrhout reading the docs.

You might consider a low end multi-Meter hardwired into the circuit. Hook a battery eliminator into the line side and glue the switch to voltage. Or even more creative, jacks on the front and a pair of inverse diodes to read current as well. Or even a bridge with a shorting strap. . . There are many options there.

.

 

[tq60]

Op stated we already tested AC so will assume ac unit.

We have been tinkering with the DC version of these and they are handy.

Accuracy limited and the 2 wite version ohms/volt may be off, not as high of impedance due to needing to power the display via the measured voltage.

Some have a tiny pot on the back to adjust the voltage reading, not sure if truly linear but one could adjust when the voltage is in the critical range.

The 3 wire are great as you can use different power source, ou application is a universal battery monitor that we use with charging batteries, 12, 24 and 36 volt units.

The 2 wire are only good to about 30 volts.

3 wire allows for higher voltage.

We had another issue is we needed a regulator that would work to 44 volts.

Found a switching unit that will buck or boost, it can make 12 from 6 or 12 from 44, works great.

If the 3 wire unit the OP has will operate with 120 VAC on the red supply wire then red supply to input voltage with black to neutral.

I would add a SPDP switch to the Yellow measure wire, yellow to center and the switched connections go to the output side of the input fuse, the other side to the output of the unit.

I would suggest adding an ammeter, they have 2 line units with volts and amps in same unit.

DC uses a shunt, AC uses pickup coil.

Here are a couple of ours...

First is voltmeters for motorhome batteries, toggle switch connects to battery.

One battery behind in charge.

Second group is the battery charger unit, it has the power adaptor as well as the volt and Amp meters.

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[silence dogood]

Whoa, guys. Bill and TQ60, I have read your past posts and have a tremendous respect for your knowledge. In the situation that you two are talking about is correct. What I would like to ask you guys to do is look at the picture of the inside of the voltmeter that is in the first comment on page 1. You may have to open your zoom and make it larger. On the bottom, I see 4 diodes. It must be a rectifier bridge. On the bottom left is a electrolytic capacitor that is for filtering. There is a chip that must be a counter of some sort. On the left of that chip is a trimmer pot. Either for calibration or setting the zero. There are no numbers on the chip. Can't read the values or numbers on anything in that thing.
Now my gripe. It used to be that many electronic devices came with a schematic either printed in the owner's manual or pasted inside the unit. Some even had a parts list. Even it did not have the information, you could still get it. So, guys, I respect and understand the info that you are giving. The trouble is many of these manufacturers or suppliers are not giving us the useful information that we need.

 

[Ken from ontario]

Bill Hudson, tq60, thank you both for your posts, I need time to read and understand them and right now I can not comment in a meaningful way, I'll take some pictures and show more of what's going on inside this variac , if the installation goes well, I will take some photos of the final upgrade as well.
I appreciate your help as always.

 

[tq60]

Whoa, guys. Bill and TQ60, I have read your past posts and have a tremendous respect for your knowledge. In the situation that you two are talking about is correct. What I would like to ask you guys to do is look at the picture of the inside of the voltmeter that is in the first comment on page 1. You may have to open your zoom and make it larger. On the bottom, I see 4 diodes. It must be a rectifier bridge. On the bottom left is a electrolytic capacitor that is for filtering. There is a chip that must be a counter of some sort. On the left of that chip is a trimmer pot. Either for calibration or setting the zero. There are no numbers on the chip. Can't read the values or numbers on anything in that thing.
Now my gripe. It used to be that many electronic devices came with a schematic either printed in the owner's manual or pasted inside the unit. Some even had a parts list. Even it did not have the information, you could still get it. So, guys, I respect and understand the info that you are giving. The trouble is many of these manufacturers or suppliers are not giving us the useful information that we need.

Do not hold your breath on getting any diagram.

You are lucky if you get the connection instructions.

The pot is likely for calibration, connect it to your voltage source, then measure with a real volt meter then with an INSULATED tiny screwdriver you can adjust the pot to show same voltage.

Without an isolation transformer consider everything hot so the jewelers screwdriver that fits may bite you.

Tape it up or turn off power, turn some and power on to see.

These things are very inexpensive, we bought maybe 20 for a buck a pop years ago.

The ammeter ones are maybe 15.

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