Following is a convenient collection or compendium of information and posts from elsewhere on AK and therefore from many different members. Where possible, the posting member has been credited and the source thread or post has been linked. Corrections and additions (and updates to modern info) are welcome and encouraged. Just send a direct message and your contribution will be added.
What are dim bulb testers, variacs and autotransformers? When should you use each? How to I make my own? Are these the same as isolation transformers?
What are dim bulb testers, variacs, autotransformers and isolation transformers?
What is a dim bulb tester? What is a variac?
When should one use a Variac or variable autotransformer?
Can you use an AC speed control or light dimmer instead of a variac?
Are variacs the same as isolation transformers?
Warnings about using variacs:
Notes about variacs and dim bulb testers:
Threads on dim bulb testers
Threads on variacs
What are dim bulb testers, variacs and autotransformers? When should you use each? How to I make my own? Are these the same as isolation transformers?
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Dim Bulb Tester (DBT) | VARIAC | Autotransformer | Isolation Transformer |
What are dim bulb testers, variacs, autotransformers and isolation transformers?
In the most basic terms, these are devices you would use to limit AC voltage or current going into a unit (from an AC wall socket). The why/how is different between each device.
An isolation transformer does not limit current nor voltage; Isolation transformers isolate the chassis of the unit you are working on from the AC wall socket. Isolation transformers are covered elsewhere on AK https://audiokarma.org/forums/index.php?threads/dlucys-guide-to-isolation-transformers.1069427/.
What is a dim bulb tester? What is a variac?
From @jupitersspot
A Dim Bulb Tester supplies household voltage level (110v here in the US) to the test unit, but does so with the unit in series with the incandescent bulb. The DBT limits maximum current available to the test unit to (household voltage)/(wattage of incandescent bulb), but to vary the voltage I have choose my bulb wattage wisely.
Variac is a single winding transformer where the secondary is a wiper over exposed elements of the primary… which you use to vary voltage available on the secondary by sliding (or rotating, in most cases) a movable tap over the exposed contacts/windings of the primary…so, Variac precisely varies the voltage I want to apply to the test unit, but I have no easy way of controlling or knowing the current drawn by the test unit.
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From @lumoStoria
a dim-bulb tester does not act as a fuse but as a current limiter. This is possible because incandescent bulbs (to be precise: their filament) have a positive, non-linear temperature coefficient of resistance, similar to a PTC resistor. That is, if no or only a small current is flowing through the filament it is "cold" and its resistance is therefore low, for example, 30 ohms. If a high current is flowing (when the connected device draws excessive current or even has a short circuit) the filament gets "hot" (glowing) and its resistance is therefore much higher, for example 300 ohms. This higher resistance limits the current that flows through the device and, thus, hopefully prevents damage.
When should one use a Variac or variable autotransformer?
From @Catmanboo
Variac can be used to safely awaken long-dormant or unknown equipment by ramping up line voltage slowly /incrementally, allowing electrolytic caps the time to regroup, reform & awaken safely.
Dim bulb tester protects the gear under test from going up in smoke if there's excessive current consumption like a dead short in it. Bulb reacts like you plugged it into line voltage (Bright light) if the piece under test has obvious issues to identify & rectify before proceeding further.
Lots of answers to “when to use which” in this thread: https://audiokarma.org/forums/index...vs-variac-or-variable-autotransformer.786025/
Can you use an AC speed control or light dimmer instead of a variac?
No.
 AC speed controller |  AC speed controller |
From @EchoWars
No. Dimmers use a triac to vary the duty cycle of the AC line going into the gear. Not very equipment-friendly.
From Chad Hauris
a lamp dimmer or universal motor speed control operates by chopping the AC waveform, not by actually reducing the voltage...hooking these devices to a transformer power supply would likely do more harm than just plugging the amp into the wall!
From @d3imlay
Most people use a variac to slowly bring up power on a piece of equipment. The hope is to see an unusually high current before damage can occur. A motor speed control reduces the voltage by only using part of each half cycle. Do a search on phase angle control or light dimmers to understand this. Light dimmers and motor controls are identical with one exception. The light dimmers start at minimum voltage and gradually increase the voltage to the bulb. Motor controls start at full voltage to get the motor started and then reduce the voltage. If anything you use a light dimmer, not a motor control.
Are variacs the same as isolation transformers?
No.
From @gadget73
Most variacs are not actually isolation transformers. They are whats called an autotransformer. Its one winding with a movable tap. Neutral carries straight through, hot goes to the other end, output goes to the moving wiper. The output still references to earth ground through the neutral.
Warnings about using variacs:
From awillia6
There's also a linearity issue with variacs especially with modern electronic gear. Many (most?) DUTs/UUTs don't consume electrical power in a linear fashion due to any number of reasons, primarily having to do with transformer secondaries feeding voltage regulators designed for specific voltage input and output levels. What happens is basically NO current flows even as the variac voltage applied is gradually raised. Then, all of sudden, the device's voltage regulators switch fully on, supplying gobs of power to downstream circuitry which may or may not be in good order. If you're ONLY using a variac in one of these startup ventures, you'll see little current rise even as the voltage is ramped up from 0 to say 60 or 70 VAC. Then with the next little turn of the dial, suddenly the current spikes, the lights dim, and the smoke billows. IMO, the best way to go is using a gizmo like the catman—a combo variac AND dbt. Each certainly has its place individually, but together they just about bullet-proof the hapless hobbyist.
Notes about variacs and dim bulb testers:
Members report they often use a combination of tools to limit AC voltage and current going to a unit. For example:
From allengator
I use an isolation transformer... then variac… then DBT....
From @gadget73
I have mine wired up with a volt and ammeter
From triode17
I use a 5 amp Variac fed by a Kill-A-Watt. Then you can measure current, Watts, VA and really monitor what's going on. With tube amps, I always start at 60 Vin to get the tubes to light up enough to conduct. SS, start at Zero volts.
Threads on dim bulb testers
Building a dim bulb tester (DBT)
Using a dim bulb tester (DBT)
Troubleshooting a dim bulb tester (DBT)
Threads on variacs
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