How to wire a vintage wattmeter

[Food Scientist]

Hi everyone,

I'm working on a library for the basement and bought some vintage panel meters that I want to be functional, including a wattmeter I purchased. I am planning to hook it up to a single 60 watt light bulb but am having a hard time finding info on how to wire it. I have a basic understanding and have an idea how to do it, but wanted to confirm.

Would my incoming positive be attached to the lower left post marked L, my load positive attached to the V1 post, and my load negative and power negative both be attached to the lower right post, S?

I already tripped a breaker test wiring something else so I figured better to ask first for this.

 

[PiperOne]

Does the meter have a light in it for the display? Might be what the extra terminals are for? Nice meter ....love that retro look.

 

[MrMoody]

Test it with an ohmmeter. If I am correct about the terminals, you should see 0 ohms between L (load?) and S (supply?), a high reading between V1 and V2, and open from either V1 or V2 to L or S.

A wattmeter works by measuring voltage across one pair of terminals and current across (through) another pair. The current-measuring pair is almost a dead short, so don't get it wrong! You may want to protect the meter with a 1A fast acting fuse.

120V AC wiring has hot & neutral, no positive. Assuming I am correct, Hot would connect to S and V1. Neutral would connect to V2 and the neutral side of the light socket. L would connect to the hot side of the light socket.

 

[Food Scientist]

Originally Posted by PiperOne
Does the meter have a light in it for the display? Might be what the extra terminals are for? Nice meter ....love that retro look.

That would be cool but I don't see one. Neither do the other meters I have, although I have thought about adding one.

 

[Food Scientist]

Originally Posted by MrMoody
Test it with an ohmmeter. If I am correct about the terminals, you should see 0 ohms between L (load?) and S (supply?), a high reading between V1 and V2, and open from either V1 or V2 to L or S.

A wattmeter works by measuring voltage across one pair of terminals and current across (through) another pair. The current-measuring pair is almost a dead short, so don't get it wrong! You may want to protect the meter with a 1A fast acting fuse.

120V AC wiring has hot & neutral, no positive. Assuming I am correct, Hot would connect to S and V1. Neutral would connect to V2 and the neutral side of the light socket. L would connect to the hot side of the light socket.


Here are the readings I got with my multimeter:
S to L = 1 ohm
V1 to V2 = 6k ohms
V2 to L = less than an ohm (so open)
V2 to S = 1 ohm (assume open)
V1 to L = 6k ohms
V1 to S = 6k ohms

I read your post and thanks for the explanation, makes a lot of sense. Only question I have is that is the hot connected to both the S and V1 terminal so it can measure the current and voltage? And would I just make a jumper wire from S to V1?

 

[MrMoody]

1 ohm is not open, digital meters will usually flash 0 or 1 same as when nothing is connected. There are apparently internal connections I didn't guess, so it seems V1 & V2 may be backward and some connections not needed ... will have to wait until tomorrow afternoon to analyze and post. Edit: connecting it wrong will not only pop a breaker but probably ruin it.

 

[Lou_Boyle]

Ummmm....approximately 1 ohm is a direct connection, not an open circuit.

 

[EdwardC]

Originally Posted by MrMoody
Test it with an ohmmeter. If I am correct about the terminals, you should see 0 ohms between L (load?) and S (supply?), a high reading between V1 and V2, and open from either V1 or V2 to L or S.

A wattmeter works by measuring voltage across one pair of terminals and current across (through) another pair. The current-measuring pair is almost a dead short, so don't get it wrong! You may want to protect the meter with a 1A fast acting fuse.

120V AC wiring has hot & neutral, no positive. Assuming I am correct, Hot would connect to S and V1. Neutral would connect to V2 and the neutral side of the light socket. L would connect to the hot side of the light socket.

Originally Posted by SVTCobra
Originally Posted by MrMoody
Test it with an ohmmeter. If I am correct about the terminals, you should see 0 ohms between L (load?) and S (supply?), a high reading between V1 and V2, and open from either V1 or V2 to L or S.

A wattmeter works by measuring voltage across one pair of terminals and current across (through) another pair. The current-measuring pair is almost a dead short, so don't get it wrong! You may want to protect the meter with a 1A fast acting fuse.

120V AC wiring has hot & neutral, no positive. Assuming I am correct, Hot would connect to S and V1. Neutral would connect to V2 and the neutral side of the light socket. L would connect to the hot side of the light socket.


Here are the readings I got with my multimeter:
S to L = 1 ohm
V1 to V2 = 6k ohms
V2 to L = less than an ohm (so open)
V2 to S = 1 ohm (assume open)
V1 to L = 6k ohms
V1 to S = 6k ohms

I read your post and thanks for the explanation, makes a lot of sense. Only question I have is that is the hot connected to both the S and V1 terminal so it can measure the current and voltage? And would I just make a jumper wire from S to V1?


You would not want a jumper between S and V1 because that would short out V1 and V2, and you would have no voltage measurement.


Based on your measurements, the current path is from LS. The voltage measurement is between V1 and V2. V2 is tied to L with that jumper since you want to measure the voltage of the same line that you want to measure the current. (Hypothetically, you could disconnect the jumper and put whatever voltage across V1 and V2, and the display will be the product of the current flowing through LS and the voltage you apply between V1 and V2, which could be fun.) Re-reading Mr Moody's post, he's got it right based on your measurements.

Line from the wall (typically black) connects to 'L'
Neutral from the wall connects to V1
S connects to the bulb on the Line side
Neutral also connects to the bulb on the Neutral side (directly from the wall or you could connect from V2 to the bulb, same thing)

Super cool old meters, by the way. Would be a cool light to put a dimmer on to watch the meter move!

 

[MrMoody]

I noticed one of my meters at work shows a 1 by itself (not flashing) for open; if there are more digits it's a reading. More later.

 

[MrMoody]

I can't tell enough to figure it out from your described readings (or interpretations of them). Most likely EdwardC is correct. I'd have to see pictures or video of the readings to be sure myself.

Another thing you could do is connect the neutral side of the lamp to neutral power, a test wire to the hot side of the lamp, and another test wire to the hot side of the power, so that if you connect the test wires together and turn on the power, the lamp lights. Then connect each combination of terminals to your test wires, turn on the power and let us know which pairs light the lamp. No danger of shorting out this way. If EdwardC has it right, you should get light with L-S, L-V2, S-V2 but no other combinations. This would show that L (or possibly S) is connected internally to V2.

I'm trusting you know how to safely handle dangerous voltages. Never handle bare live wires! Turn off/unplug power, make changes, then turn on again. If you don't know what you're doing you shouldn't be messing with this at all.

 

[EdwardC]

Obviously, I could be very wrong, but here's what I think the wiring would look like based on the OP's measurements and my assumptions:

Quote
S to L = 1 ohm (short)
V1 to V2 = 6k ohms
V2 to L = less than an ohm (so open - I'm assuming that's a short)
V2 to S = 1 ohm (assume open - I'm also assuming that's pretty much a short)
V1 to L = 6k ohms
V1 to S = 6k ohms


So, to dumb it down, and pretending that jumper wire isn't installed, I'm see a 6K resistor between V1 and V2 for voltage sensing, and a short between L and S (either a hall effect sensor or a current sense resistor).

Also, like MrMoody said, please be safe!

 

[Food Scientist]

Apologies for the assumptions I made earlier with less than 1 ohm being open, that was a duh moment once reminded here on the website. MrMoody I appreciate the insight on how to test the meter without damaging it (hopefully it works to begin with) and although I have sounded like an amateur (no professional either), I am one step above complete novice and won't doing anything stupid.

EdwardC I appreciate you chiming in as well, your description of how to connect the wires makes sense.

As requested here are the photos of the multi meter readings connected to the watt meter. An additional piece of info is when the multi meter is set to continuity it beeps between L-S, V2-L, and V2-S. Anything connected to V1 does not beep. Correct me if I am wrong but that would confirm that there is a circuit, albeit a low resistance between those connections.

Since there a few comments of how cool they are I also included pictures of the other two panel meters I purchased.

 

[Food Scientist]

Originally Posted by EdwardC
Obviously, I could be very wrong, but here's what I think the wiring would look like based on the OP's measurements and my assumptions:

Quote
S to L = 1 ohm (short)
V1 to V2 = 6k ohms
V2 to L = less than an ohm (so open - I'm assuming that's a short)
V2 to S = 1 ohm (assume open - I'm also assuming that's pretty much a short)
V1 to L = 6k ohms
V1 to S = 6k ohms


So, to dumb it down, and pretending that jumper wire isn't installed, I'm see a 6K resistor between V1 and V2 for voltage sensing, and a short between L and S (either a hall effect sensor or a current sense resistor).

Also, like MrMoody said, please be safe!


That is too cool! I can see you put a lot of effort into solving this problem. Just to make sure I understand the inline connection for the hot wire from L-S measures current and the inline connection from V1-V2 measures voltage via large resistor. If I was to try to apply voltage to across the V1-V2 terminals the resistor would overheat and ruin the watt meter due to a large amperage load, correct?

So here is how I plant to wire it:

Hot wire from wall to L (bottom left post)
Hot wire from bulb to S (bottom right post)
Neutral wire from wall to V1 (top right post)
Neutral wire from bulb to V2 (top left post)

 

[MrMoody]

Thanks for the photos (good job by the way), looks like EdwardC is correct.

I hadn't noticed that there is an external jumper from V2 to L even though Edward was referring to it.

In the original photo it looked like cut wires to me ...

Originally Posted by SVTCobra
If I was to try to apply voltage to across the V1-V2 terminals the resistor would overheat and ruin the watt meter due to a large amperage load, correct?

No, 120V across 6KOhm is only .02A, and in fact is the way the voltage measurement is intended to work.
Quote

Hot wire from wall to L (bottom left post)
Hot wire from bulb to S (bottom right post)
Neutral wire from wall to V1 (top right post)
Neutral wire from bulb to V2 (top left post)

Close but not quite. The way EdwardC described it will work. V1-V2 need to have the full voltage across them to get a watt reading, and the bulb will not light because both sides are actually hot.

I would do it like this, which should also work:
Hot from wall to S
Hot from L to bulb
Neutral from wall to V1
Neutral from V1 to bulb
V2 no other connection is needed (leave connected to L).

Thus V1-V2 is measuring the voltage across the bulb after any drop in the L-S current sense circuit, and should give a slightly more accurate reading.

 

[MrMoody]

The other meters are cool, too. They'd be right at home firing up Frankenstein's monster.

You know the big screw on the front usually adjusts for zero, right?

 

[EdwardC]

The resistor across V1 and V2 is pretty high resistance, so the wattage is relatively low. For example 120V across 6,000 ohms would be 0.020A, which calculates to 2.4W. I hope it works out ok!

 

[Food Scientist]

I thank both of you for breaking this down for me. I understand more stuff related to electrical in other areas, but I always enjoy learning something new.

As for the adjustment screw, yes I knew that and tried adjusting the ammeter and no change. The volt meter needs to be level and actually reads pretty accurate.

I will let y'all know what happens when I get this put together, hopefully by the weekend. Also I plan to plug this into a GFCI outlet in case something is messed up and trips before any damage occurs, including me.

 

[CR94]

Is that wattmeter capable of correcting for power factor, or does it simply multiply voltage times current? I used to work with more elaborate analog wattmeters (Simpson brand, if I recall correctly), but never saw a wattmeter that looks so simple.

 

[MrMoody]

Originally Posted by CR94
Is that wattmeter capable of correcting for power factor, or does it simply multiply voltage times current? I used to work with more elaborate analog wattmeters (Simpson brand, if I recall correctly), but never saw a wattmeter that looks so simple.

I'm guessing it works on assumptions such as 60 Hz AC and non-inductive load.

 

[MrMoody]

Left hanging in suspense. I'm guessing it didn't work.