I'm building an electronics workbench in my basement and want to hard-wire a dim-bulb tester and mount it to the wall. The DBT has two receptacles, one has the bulb in series, the other doesn't. Both receptacles are switched (see photo).
Is it a good practice to connect the component to the variac and then to the receptacle which has the DBT when powering up an old tube amp for instance?
Is it a good practice to connect the component to the variac and then to the receptacle which has the DBT when powering up an old tube amp for instance?
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Is the question:
a] AC wall outlet >> dim bulb tester >> Variac >> Component Under Test
b] AC wall outlet >> Variac >> dim bulb tester >> Component Under Test
Good question, I don't know.
a] AC wall outlet >> dim bulb tester >> Variac >> Component Under Test
b] AC wall outlet >> Variac >> dim bulb tester >> Component Under Test
Good question, I don't know.
A bulb is designed to function on some static voltage so I would vote for option a] .
I think you are asking which test to perform first- variac or dim bulb correct?
Use the dim bulb first. You are attempting to determine if there is a hard short somewhere. A variac can damage something before you determine that is the problem, unless you are monitoring voltage and current very carefully. The dim bulb is the safest, because it will not only absorb the energy rather than the device under test if there is a short, but the behavior of the bulb will tell you which ballpark you are in better than a variac. The purpose of the variac is to allow a non-shorted chassis to "soak" in lower voltages allowing the electrolytic capacitors to reform and stabilize before you hit them with full voltage.
I made my bulb tester with an old incandescent 3-way bulb. First I switch on the lowest wattage filament (usually 25W or 50W). This will usually glow fairly bright for almost any chassis, but over time you can get a feel for "how bright" is normal. Assuming this is a tube chassis, you will typically see the 25W glow brightly at first, then as filaments start to warm up you will see the bulb dim slightly. After being there for a bit, then I can "change gears" and click into the next highest wattage filament (typically 75W or 100W) and see what it does there, then proceed. As long as the bulb is glowing to some extent, it is limiting the voltage to the chassis the same as the variac. If, however, you don't see any dimming, and the bulb stays bright, the variac will be of no use, and you need to troubleshoot what is going on before you proceed.
Over time, I just stopped using the variac. If you can find a lower wattage version of the 3-way bulbs (25, 50, and 75 with both filaments in parallel) I would practice with that. (of course, it must be an old-school incandescent bulb, not LED- sorry, have to say that if others read this and do not understand)
Use the dim bulb first. You are attempting to determine if there is a hard short somewhere. A variac can damage something before you determine that is the problem, unless you are monitoring voltage and current very carefully. The dim bulb is the safest, because it will not only absorb the energy rather than the device under test if there is a short, but the behavior of the bulb will tell you which ballpark you are in better than a variac. The purpose of the variac is to allow a non-shorted chassis to "soak" in lower voltages allowing the electrolytic capacitors to reform and stabilize before you hit them with full voltage.
I made my bulb tester with an old incandescent 3-way bulb. First I switch on the lowest wattage filament (usually 25W or 50W). This will usually glow fairly bright for almost any chassis, but over time you can get a feel for "how bright" is normal. Assuming this is a tube chassis, you will typically see the 25W glow brightly at first, then as filaments start to warm up you will see the bulb dim slightly. After being there for a bit, then I can "change gears" and click into the next highest wattage filament (typically 75W or 100W) and see what it does there, then proceed. As long as the bulb is glowing to some extent, it is limiting the voltage to the chassis the same as the variac. If, however, you don't see any dimming, and the bulb stays bright, the variac will be of no use, and you need to troubleshoot what is going on before you proceed.
Over time, I just stopped using the variac. If you can find a lower wattage version of the 3-way bulbs (25, 50, and 75 with both filaments in parallel) I would practice with that. (of course, it must be an old-school incandescent bulb, not LED- sorry, have to say that if others read this and do not understand)
AC = bulbs => Variac => Object under testing gives the advantage that the bulds limit inrush current of the variac.
Thanks for all of the input.
My thinking is that when powering up an old tube amplifier slowly with a variac, the dim-bulb-tester would let me know there is a short.
If I were to use only the variable power supply in my photo above, I suppose I could rely on the built-in AC ammeter to monitor the current.
I also have another "variac" that does not have a built-in ammeter, so it seems that without a means of monitoring current, the bim-bulb-tester would be essential.
My thinking is that when powering up an old tube amplifier slowly with a variac, the dim-bulb-tester would let me know there is a short.
If I were to use only the variable power supply in my photo above, I suppose I could rely on the built-in AC ammeter to monitor the current.
I also have another "variac" that does not have a built-in ammeter, so it seems that without a means of monitoring current, the bim-bulb-tester would be essential.
Yep, that is what I have found. With your setup you could use a sequence of individual incandescent bulbs, from low to high wattage. Nice build by the way.
I would strongly advise to not hardware the test stuff into the wall. It could get you in trouble in various ways that having it detachable won't (including electrical codes). It should also be fused and I would use GFCI's for the connections for additional safety.
I am going to have a licensed electrician do the wiring.
What is it about mounting the Dim-Bulb-Tester to the wall that makes it unsafe?
What is it about mounting the Dim-Bulb-Tester to the wall that makes it unsafe?
No immedate disconnect. The NEC is down onn that kind of thing. If it plugs in its probably OK. They also don't like extension cords in the wall, a real issue with TV's mounting on the wall.
Wouldn't the switch act as a disconnect?
How does an extension cord factor into what I am proposing?
How does an extension cord factor into what I am proposing?
The switch would only disconnect the hot lead. There are a whole history of fires related to extension cords or similar in the wall. The NEC has some specific requirements on how power goes into the wall.
I would suggest using your Variac first with its cable connected to the wall and the dim bulb current limiter on the variac's output. I would wire the switch to short out the lamp and apply full power once your device is up and running. The Variac you have has pretty complete metering so you can see if there is a problem quickly. Collect light bulbs quickly since they are being discontinued. I would want an assortment from 15W to 150W to handle the various possible devices.
I would suggest using your Variac first with its cable connected to the wall and the dim bulb current limiter on the variac's output. I would wire the switch to short out the lamp and apply full power once your device is up and running. The Variac you have has pretty complete metering so you can see if there is a problem quickly. Collect light bulbs quickly since they are being discontinued. I would want an assortment from 15W to 150W to handle the various possible devices.
I see your point about switching the hot lead only. I'll check with my electrician of course but can a double pole switch be used to disconnect the neutral as well?
Regarding your comment on an "extension cord", the cord for my DBT as shown in my photo will be removed if it is hardwired.
I recently cleared out my mother's garage, she had a stash of light bulbs that the electrical service provider would hand out for free to customers. They must be from the 1980's or 90's because the paper packaging is starting to crumble.
Regarding your comment on an "extension cord", the cord for my DBT as shown in my photo will be removed if it is hardwired.
I recently cleared out my mother's garage, she had a stash of light bulbs that the electrical service provider would hand out for free to customers. They must be from the 1980's or 90's because the paper packaging is starting to crumble.
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A double pole switch would be a good solution. And a GFCI (I can't promote them enough). Are you going to run conduit or BX to the variac if you remove the cord?
well, if I proceed as planned, i.e.,...
.....device under test.....variac.....dim-bulb-tester.....
the variac would be plugged into the hard-wired (in conduit), wall mounted dim-bulb-tester
but I think you are suggesting.....
.....device under test....dim-bulb-tester.....variac.....
if that's a better (safer) method, then I would leave the dim-bulb-tester as it is now and plug it into the variac
in either scenario, I would not be removing the cord from the variac, I would only remove the cord from the dim-bulb-tester if it were hard-wired (in conduit) and wall mounted
.....device under test.....variac.....dim-bulb-tester.....
the variac would be plugged into the hard-wired (in conduit), wall mounted dim-bulb-tester
but I think you are suggesting.....
.....device under test....dim-bulb-tester.....variac.....
if that's a better (safer) method, then I would leave the dim-bulb-tester as it is now and plug it into the variac
in either scenario, I would not be removing the cord from the variac, I would only remove the cord from the dim-bulb-tester if it were hard-wired (in conduit) and wall mounted
I do think that is better and safer. Also the variac you have is intended to operate off of line voltage. Having a light bulb in series with the supply should not affect things but it may affect the metering. Your Variac has an isolated output on the front. Its a nice feature but it limits the load to 2A, not enough for a medium amp really. What will you be testing?
The issue is both the ratings of the cords, fusing and the potential of someone driving a nail into a cord. PVC also has an issue when its under pressure of cold flow which can lead to shorts. Romex and BX are more durable which is preferable for wires you can't see to inspect.
Tube audio amplifiers, the most powerful I have is a Dynaco ST70 which is about 35 watts per channel