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…
.
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.