Difference between a dim bulb tester, current limiter & a variac with current meter?

Perkinsman

"I've never met a fixer that I couldn't break"
Is there a difference between these? I was told that I need a dim bulb tester for old tube gear. I looked on youtube and saw many variations and I think some are calling them a current limiter. It seems to be basically a light bulb mounted on a receptacle box with an on/off switch, plugged directly to A/C. The unit under test is plugged into the receptacle & you switch on. If the light get very bright and stays bright, you switch off because it is indicating a short in the until under test. Can't I just use my variac with the built in current meter for the same purpose or am I missing something? While I'm asking, my variac has a 1 A, 3A switch. How do I know which to use?
 
A variac is like a giant variable resistor sorta like a heavy duty light dimmer switch. A dim bulb tester acts like a current limiter which acts somewhat like a big power resistor but doesn't blow like a fuse.

I know what they are and how they work but someone else might be able to describe them better.
 
The dim bulb tester is also known as a 'poor person's Variac' -- it gives one discrete voltages depending on the bulb's wattage (more to the point, the internal resistance of the filament).

Generally speaking, a Variac is a better choice; a short circuit across the Variac will (should) pop its fuse.

As to what range to use, it's gonna depend on the expected current draw (power consumption) of the equipment connected to it. 1 amp at 120 VAC is (depending on power factor, of course) 120 watts. If you expect your component to consume more than 100 W or so (and less than 300 or so), use the 1A switch. If it's gonna consume more than 300 W, you're gonna need a bigger Variac. The one I have is rated for 10A .

I find a Kill-a-Watt to be a handy tool to use, too.

EDIT: No a Variac is not a variable resistor (rheostat), if it were, it would dissipate a shedload of heat doin' its job. It is an autoformer; like a transformer but with the input and output electrically connected (rather than electromagnetically connected, as are the primary and secondary windings in an isolating transformer). More or less of the winding is "in circuit" depending on the Variac setting; this determines the output voltage. There's no current limiting per se (at least in any I've owned/seen) -- there is typically a fuse which protects the Variac from dead shorts (or less subtle over-current situations).

BTW Variac was a trademark of General Radio (GenRad) and is still trademarked, according to http://en.wikipedia.org/wiki/Autotransformer#Variable_autotransformers

trans28.gif


(imagine the ability to move the location of that "secondary tap" along the winding... and you have a Variac!)
 
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See! Someone else said it better than I could! :D Autotransformer.......I knew that! :yes:

Don't use a light dimmer! :no:
 
well - one can control a dim bulb tester to the extent that one has a series of incandescent light bulbs of differing wattages.

(although this will likely get more and more difficult over time as low-efficiency incandescent lamps are phased out of production/distribution)

http://www.antiqueradio.org/dimbulb.htm
 
incandescent bulbs will continue to be manufactured for a long time for "special service", nothing else can go inside an oven (for example).

The advantage of a DBT is that it's instantaneous, automatic current limiting. Some folks say "well just watch the ammeter on the Variac" BUT if you have a fault condition in your amp, further damage may be done in the few seconds it takes you to realize "hey the current is going too high, better stop this thing!". Although I have a Variac, if I have an amp in an unknown condition, I fire it up on the DBT (after inspecting for obvious internal problems, of course).
 
incandescent bulbs will continue to be manufactured for a long time for "special service", nothing else can go inside an oven (for example).

The advantage of a DBT is that it's instantaneous, automatic current limiting. Some folks say "well just watch the ammeter on the Variac" BUT if you have a fault condition in your amp, further damage may be done in the few seconds it takes you to realize "hey the current is going too high, better stop this thing!". Although I have a Variac, if I have an amp in an unknown condition, I fire it up on the DBT (after inspecting for obvious internal problems, of course).

Absolutely spot on Bill. The number of people swearing by the use of a variac and ONLY a variac, don't seem to realise what they are missing.

I also think that a variac is more useful for valve (tube) gear and a DBT more use for SS - but like everything there are exceptions to this.
 
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I use the DBT plugged into teh variac on something unknown, like a new build!!! It saves blowing variac fuses... If it comes up to 120V, and teh bulb dims, its probably good to go on teh variac,,, bring it up slowly to line V, and get readings...

Regards,
John
 
More support for the use of a DBT, some information previously posted.

The primary purpose of the dim bulb tester is to limit the maximum current to the device under test. This is a good idea for a number of reasons.

You can control a dim bulb tester. The control of the DBT is done by the selection of the size (wattage) of the bulb(s) used in the DBT. The operation of the DBT is automatic.

It functions automatically because of the electrical characteristics of the filament in the bulb. The resistance or opposition to the flow of electricity varies depending on the temperature of the filament. The resistance of a cold filament is lower than the resistance of a hot or all light up filament.

The DBT is not necessarily a single voltage device. The voltage supplied to the device under test depends on the amount of current being drawn by the device under test and the wattage of the lamp used. The fact that the voltage being supplied to the device under test can vary depending on the amount of current drawn is what provides the current limiting ability of the DBT.

All of the power going the device under test must flow through the filament of the bulb.

The Variac allows one to vary the voltage supplied to the device under test, but it will not limit the current. If you use a Variac and bring up the voltage, the current may be OK up to a certain voltage point, possibility even full voltage, then due to certain types of circuit defects there may be excessive current draw. This can happen so fast that you are not able to reduce or turn off the voltage. This situation may cause the fuse in the Variac or the fuse in the device under test to blow, but the device under test will be subjected to the maximum current of its fuse or the Variac’s fuse. This may not be a good thing.

This may lead to damage of the device under test.

There have been many debates regarding which is better, a Variac or a dim bulb tester. I have always used both at the same time and there have be times when I have had a device under test draw the correct amount of current at full voltage and then due to circumstances cause the dim bulb tester to light to full brilliance, with the dim bulb tester limiting the maximum current, a good thing.

If you are going to spend the money for a decent quality Variac, the addition of a dim bulb tester is cheap insurance that functions automatically.

BTW, there may be times, although rather rare, when using a DBT may be a problem due the nature of the device under test. That is another can of worms, but if you run across this situation you can simply, cautiously bypass the DBT.

Speaking of fuses, even a fast blow fuse is going to carry 100 percent of its current rating without blowing. So if the device under test uses a fuse rated for 5 amps, then there will be no protection if the current flow is 5 amps or less, maybe just smoke…:D:D:D.

However in simple terms, a typical incandescent lamp will reach about 80 percent of its full brightness (or maximum opposition or resistance to the flow of current) in about 100 milliseconds. That is, it is already providing substantial current limiting beyond its cold filament current limiting in about 1 tenth of a second. In general this is a good thing.

See the pictures below.

In this example the total current supplied by the lamp in the DBT would be automatically limited to a little less than one half of an amp in about 100 milliseconds.

knockbill's procedure is a good procedure. I use the fact that most Variacs will provide an output voltage that is greater than the line voltage. I monitor the output voltage of the DBT and adjust the Variac accordingly to obtain the correct voltage to the device under test. (to counteract the voltage drop of the DBT). This way if something fails during testing or adjusting the device, I still have the safety of the current limiting of the DBT. Of course the device must pass the initial DBT turn on test.
 

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The dim bulb tester is also known as a 'poor person's Variac' -- it gives one discrete voltages depending on the bulb's wattage (more to the point, the internal resistance of the filament).

Generally speaking, a Variac is a better choice; a short circuit across the Variac will (should) pop its fuse.

As to what range to use, it's gonna depend on the expected current draw (power consumption) of the equipment connected to it. 1 amp at 120 VAC is (depending on power factor, of course) 120 watts. If you expect your component to consume more than 100 W or so (and less than 300 or so), use the 1A switch. If it's gonna consume more than 300 W, you're gonna need a bigger Variac. The one I have is rated for 10A .

I find a Kill-a-Watt to be a handy tool to use, too.

EDIT: No a Variac is not a variable resistor (rheostat), if it were, it would dissipate a shedload of heat doin' its job. It is an autoformer; like a transformer but with the input and output electrically connected (rather than electromagnetically connected, as are the primary and secondary windings in an isolating transformer). More or less of the winding is "in circuit" depending on the Variac setting; this determines the output voltage. There's no current limiting per se (at least in any I've owned/seen) -- there is typically a fuse which protects the Variac from dead shorts (or less subtle over-current situations).

BTW Variac was a trademark of General Radio (GenRad) and is still trademarked, according to http://en.wikipedia.org/wiki/Autotransformer#Variable_autotransformers

trans28.gif


(imagine the ability to move the location of that "secondary tap" along the winding... and you have a Variac!)
 
incandescent bulbs will continue to be manufactured for a long time for "special service", nothing else can go inside an oven (for example).

The advantage of a DBT is that it's instantaneous, automatic current limiting. Some folks say "well just watch the ammeter on the Variac" BUT if you have a fault condition in your amp, further damage may be done in the few seconds it takes you to realize "hey the current is going too high, better stop this thing!". Although I have a Variac, if I have an amp in an unknown condition, I fire it up on the DBT (after inspecting for obvious internal problems, of course).

Great Advice Bill! I have been using a variac thus far with no DBT and it is always a frightening experience for me when slowly powering up used vintage equipment for the first time or powering up any electronics which I have been working on such as Recapping ect and waiting to see smoke or hearing a loud pop or bang.
 
The dim bulb tester is also known as a 'poor person's Variac' -- it gives one discrete voltages depending on the bulb's wattage (more to the point, the internal resistance of the filament).

Generally speaking, a Variac is a better choice; a short circuit across the Variac will (should) pop its fuse.

As to what range to use, it's gonna depend on the expected current draw (power consumption) of the equipment connected to it. 1 amp at 120 VAC is (depending on power factor, of course) 120 watts. If you expect your component to consume more than 100 W or so (and less than 300 or so), use the 1A switch. If it's gonna consume more than 300 W, you're gonna need a bigger Variac. The one I have is rated for 10A .

I find a Kill-a-Watt to be a handy tool to use, too.

EDIT: No a Variac is not a variable resistor (rheostat), if it were, it would dissipate a shedload of heat doin' its job. It is an autoformer; like a transformer but with the input and output electrically connected (rather than electromagnetically connected, as are the primary and secondary windings in an isolating transformer). More or less of the winding is "in circuit" depending on the Variac setting; this determines the output voltage. There's no current limiting per se (at least in any I've owned/seen) -- there is typically a fuse which protects the Variac from dead shorts (or less subtle over-current situations).

BTW Variac was a trademark of General Radio (GenRad) and is still trademarked, according to http://en.wikipedia.org/wiki/Autotransformer#Variable_autotransformers

trans28.gif


(imagine the ability to move the location of that "secondary tap" along the winding... and you have a Variac!)


Great information here helps me understand how the autoformer really works!!
 
One thing about a Variac as you are trying to adjust it up against a short it will hum and you can feel the 60 cycles in the knob as you are trying to turn it! Although not all Variacs have a current meter but it is a nice feature.
 
Is there a difference between these? ..... How do I know which to use?

I see them as two very different tools - if you are lucky enough to have both tools, then you choose a tool that will do what you want to achieve.

I have a variac, and because I normally only have old amps on the bench that I don't know the history of, then by default I wouldn't contemplate using a bulb tester. I would normally bring on power in a very limited and controlled manner, and in steps that add more parts, each step confirming that certain parts are working properly, with meters to tell me if voltages weren't coming up as expected.

If I was to use a bulb tester then I would do that on an amp that I knew was previously working, but hadn't had on for some time. Similarly I guess for a service tech that needs to expeditiously use their time in determining if an amp is working ok, and if not then more time is spent isolating faults.
 
By all means, build a DBT. But, a good variac should be in your future if you are doing more than occasional work. You cannot easily reform caps without one. Look for one that is 5A or better, has both high & low current settings, and is metered for both voltage and current. If you start out on the low current setting and there is a dead short somewhere in the gear you are working on, the meter will slam up and stay there. You quickly know there is a major problem and back off. Stick with a good brand such as GenRad, Staco, Powerstat, or Sencore. It is a piece of gear you build your test bench around.

That said, you need a DBT and an isolation transformer too. But start with a DBT. They are simple, cheap, and easy to use. When you get your variac, you will still use it. People do use them in conjunction with a variac as another inline safety. If your variac is under 3A and you hit a dead short, you could torch it before you notice the problem. In this regard, the DBT is nearly a guarantee that you will save your variac should you find trouble with it.

As far as what variac; if you are going to be a bear, be a grizzly. The Staco E-series are the bomb -

cb1nTDi.jpg
 
Just a Question, I have a restored Zenith 7S529 shortwave radio from 1941 that I havent turned on for 2 years. What size variac should I look for? Thanks! Al
 
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