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What's wrong? Poor man's variac

mr_wetland

mr_wetland

Well-Known Member
OK, a little help here please. I just finished re-capping my old Lafayette LA-90, and I want to bring it up nice and slowly as I've been advised to do. So I built a PMV based on so many that I've seen here at AK. And it doesn't seem to do what it's supposed to do. Attached is a photo of my wiring. I have one side of the power cord going directly to one side of the outlet. The other side goes to the light assembly first, then out the other side of the light assembly to the other side of the outlet. In my mind this should be working, but clearly I'm no experienced electrician. I bought a light fixture with a pullcord, thinking I would use that as a switch to turn it on and off. Here's what I get.

I plug it in with a 60W bulb. The light does not come on regardless of the position of the switch, and I get 120v at the outlet regardless of anything. Obviously I wired it wrong. Or am I really ignorant and nothing is going to happen until I have a load at the outlet? Help please?
 

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The one thing missing from your pic is the plug. Make sure it is the Hot, not the Neutral, that is going through the bulb first. That would be the only thing that I could consider a possible issue because your device looks correct to me. Show us the plug.
 
Yeah, what KingBubba said.

I had to redo mine a couple of times before I got it to work, cuz the wiring is not what one usually does with lights and switches.

Mine is set up like this:

Hot lead to switch (I have a separate toggle switch), switch to bulb, bulb to one side of the outlet, then the neutral lead from the power supply romex is connected to the other side of the outlet.

Gotta make sure the hot lead goes through the bulb first, then to your equipment.
 
Oh, and another thing . . .

If nothing is plugged into the outlet of a PMV, the circuit is open. You won't get the bulb to light. I tested mine by cutting off an electrical plug and twisting the two leads together. I plugged that into my PMV outlet. Just don't let kids get hands on your test plug. Great for tripping breakers.:D
 
We need a neutral wire to power the device with AC, to light the lamp. We also need a power wire. A ground wire is also nice.

Pics of one way to wire it below. The neutral basically passes through the switch and lamp box and is terminated at the receptacle.
 

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The cord I am using is a basic 16 ga extension cord with the outlet end cut off. Is the hot side of the plug the side that is wider (don't know the technical terms, but it makes it so you can only plug it in one way). If that's the case, mine is wrong, because I put the meter on it and have continuity from the fat side of the plug to the outlet box and the skinny side to the light bulb. Easy enough to change, just let me know.

Once that's fixed, I can plug it in, put say a 25 watt bulb in there, and plug in my amp, then turn it on and see if something bad happens? If not, I can go to a 40, then a 60? Am I on the right track here?
 
If you unscrew the light bulb, there should no longer be any voltage at the outlet, if there is, then you somehow have the hot wire connected to the same light base terminal. So voltage is not going through the bulb only to one part of the filament.
 
Well, you guys have confused me a little bit, not that it's all that difficult to do. KingBubba and qpagoda said I want the hot wire running through the bulb, westend said I need the neutral running through the bulb? Tonight I will try it with a load, and see if it affects the voltage at the outlet. Maybe it is wired right and I just was testing it wrong. My goal I think is to put a load in one of the outlets, with a low wattage bulb in the socket, and show a reduced voltage when I stick my probes in teh other outlet. Does that sound right? Oh, and the light should come on too, even if just a little? Sorry for the newbie questions, I'm getting pretty good with caps and resistors and power supplies and using deoxit, but i am still learning.
 
Wide prong is the neutral - goes to silver screw on outlet. Narrow one should go to the center terminal of bulb, outside one goes to "hot" (brass) screw on outlet. You'll see 120V at the outlet with no load. No current: no voltage drop. Plug a table lamp into it and both bulbs should light, dim. You size your bulb in this tester according to current draw - for a tuner, a 60W one may work. For an amp, you may not get anything out until you put in a 150W (or bigger!)
 
It is preferable to have the hot wire to the bulb.

You was testing it wrong. As qpagoda said on post #4, you will have 120V at the outlet if there is no load. If you connect a 60W bulb at the outlet, the AC line voltage will be reduced to ~60V at the outlet (with another 60W bulb in the light assembly)
 
westend said:
Pics of one way to wire it below. The neutral basically passes through the switch and lamp box and is terminated at the receptacle.
Click to expand...

mr_wetland said:
Well, you guys have confused me a little bit, not that it's all that difficult to do. KingBubba and qpagoda said I want the hot wire running through the bulb, westend said I need the neutral running through the bulb? Tonight I will try it with a load, and see if it affects the voltage at the outlet. Maybe it is wired right and I just was testing it wrong. My goal I think is to put a load in one of the outlets, with a low wattage bulb in the socket, and show a reduced voltage when I stick my probes in teh other outlet. Does that sound right? Oh, and the light should come on too, even if just a little? Sorry for the newbie questions, I'm getting pretty good with caps and resistors and power supplies and using deoxit, but i am still learning.
Click to expand...
I did not state "running through the bulb", please, reread what was written.

The picture of the lamp box shows the neutrals (white wires) joined with a wire nut (red). The neutral wire passes by the switch and lamp from the plug and is terminated on the wide slot side of the receptacle. The neutral (white wire) is not fixed to the switch or lamp fixture. The power lead (black wire) is fixed to the switch (series circuit) and the lamp fixture (series circuit) and then to the receptacle. When you turn the swith to the "on" position, the power circuit is connected (series circuit) from the plug through the switch, through the lamp fixture, and then to the receptacle. If you plugged in the PMV, at this point (with nothing plugged into the receptacle), the bulb will not light. When you plug something into the receptacle, it completes the total power circuit (series circuit) between power and neutral and the lamp will light.

Don't worry about being new to electricity, we were all new at one time.
 
Just something to think about safety wise. If whatever you are testing with the device (even if the hot side of the wiring is going thru the light bulb in series) and even if you are using a small wattage bulb to start there is still enough potential for a bad shock.

I strongly recommend having the dim bulb tester or whatever you are working on plugged into a GFI outlet.
 
There appears to be 3 screws on the light socket? If so, I suspect you have the wires tied to two common screws? use a VOM and check the sockets continuity.
 
tshoejohn said:
Just something to think about safety wise. If whatever you are testing with the device (even if the hot side of the wiring is going thru the light bulb in series) and even if you are using a small wattage bulb to start there is still enough potential for a bad shock.

I strongly recommend having the dim bulb tester or whatever you are working on plugged into a GFI outlet.
Click to expand...
I built a GFIC into my dim bulb tester.

A link to the tester I built:

http://www.audiokarma.org/forums/showthread.php?t=100804

And since this is AC current, I don't believe it really makes much difference if the bulb is on the hot side or not from a use standpoint, but from a safety standpoint it's probably better on the hot side. (Keeping in mind the socket is hot when the bulb is out, and again for safety wire the hot to the CENTER contact...)
 
mr_wetland said:
I plug it in with a 60W bulb. The light does not come on regardless of the position of the switch, and I get 120v at the outlet regardless of anything. Obviously I wired it wrong. Or am I really ignorant and nothing is going to happen until I have a load at the outlet? Help please?
Click to expand...

You will need a load on the outlet to complete the circuit for the bulb to light. try plugging something into the outlet like a lamp, or toaster.
 
outlawmws said:
I built a GFIC into my dim bulb tester.

A link to the tester I built:

http://www.audiokarma.org/forums/showthread.php?t=100804

And since this is AC current, I don't believe it really makes much difference if the bulb is on the hot side or not from a use standpoint, but from a safety standpoint it's probably better on the hot side. (Keeping in mind the socket is hot when the bulb is out, and again for safety wire the hot to the CENTER contact...)
Click to expand...

Does the GFI still function correctly in that position?

You could check by putting a small bulb like a 15 watt in the socket, and something with a huge wattage like a 1500 watt heater in the outlet then push the "Test" button on the GFI and see if it trips. I would imagine the voltage at the outlet would be pretty low. (Could check voltage out of curiosity)

If it doesn't trip, try decreasing loads on the outlet, like a 300 wat bulb, 150 watt bulb, 60 wattbulb, etc down to a bulb the same wattage as the "Ballast" bulb which should bring the voltge to aprox 60V.

At least you would know if you are actually protected.

I would be curious what the results would be?
 
Mine has a switch in the same line (hot) that the bulb is in.

You could get a shock from the bulb socket (with the bulb out) AND if you were stupid enough to poke your fingers in it with the switch on !
 
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tshoejohn said:
Does the GFI still function correctly in that position?

You could check by putting a small bulb like a 15 watt in the socket, and something with a huge wattage like a 1500 watt heater in the outlet then push the "Test" button on the GFI and see if it trips. I would imagine the voltage at the outlet would be pretty low. (Could check voltage out of curiosity)

If it doesn't trip, try decreasing loads on the outlet, like a 300 wat bulb, 150 watt bulb, 60 wattbulb, etc down to a bulb the same wattage as the "Ballast" bulb which should bring the voltge to aprox 60V.

At least you would know if you are actually protected.

I would be curious what the results would be?
Click to expand...

GFIC's have two functional protection modes. The outlets are obviously protected, but you have two options for downstream connections. Protected, by the GFIC, and unprotected. I wired into the protect side for the downstream dim bulb circuit.
 
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