Dim light bulb tester and the transient state

[bodzio_stawski]

Hello

I have a question about the dim light bulb tester. I know that in "normal" value of current flowing through the secured device the bulb is cold and has very low resistance. When the device is shorted, a bulb became a load with a huge resistance and for now the current is limited in the circuit.

But what between these two states, in "transient state"? Short circuit can appear immediately, but how is the bulb able to rise up its resistance so fast after device's damage? I try to imagine that it depends on the wattage of the bulb - one bulb can receive more current and the other one can be too bad to act as an "emergency circuit load"

Bodzio

[Whales]

Indeed it takes some time for the bulb filament to heat up.  This period is short enough to protect some parts, but long enough to still let others blow up. 

Dim light bulbs are a simple, cheap and accessible protection method that works better than nothing, but is far from perfect. 

[bob91343]

Chances are the dim bulb's usefulness is for when the unit is already shorted.  Expecting it to react at the time of failure isn't its purpose.  If the current draw increases as the unit warms up, it's of help.  So no, its transient capabilities aren't its strong points, although there is some merit there.

[bdunham7]

I suppose there's a difference between a dim bulb tester and dark bulb tester! 

If the bulb is sized so that it is already fairly warm--dim--with the appropriate load, it will react fairly quickly and to fairly small overloads.  A larger bulb will be dark, cold and in addition to being slower to respond, will allow larger fault currents.  You do have to select the bulb carefully for each situation.  Most of the things you would use a dim bulb tester on are not likely to blow up so quickly, but in any case I imagine a properly-sized bulb is on the order of a slo-blo fuse in terms of protection.

[Berni]

Well its pretty visual, any light that the light bulb produces is energy that got eaten up before it got to your tested device. That's actually a nice feature since you can see that something went wrong.

The reason lightbulbs work well for this is that they have a fairly low resistance when cold but once they get hot enough there resistance rises and so the light bulb sucks up more of the energy, making it hotter, making it burn even more energy. This takes some time but even when cold the lamp still has a fair bit of resistance that limits the current, so it is far from a direct short.

That cold resistance is the reason why a lamp is more protective than a fuse. Especially on mains where the wall outlet will give you >100A if you ask for it. The fuse will happily let the 100s of Amps flow trough your device for a milisecond or two before the wire inside the fuse explodes and interrupts the flow. By that time something in the device might have gone kaboom. If you use a lamp then even a dead short can only draw as much current as the cold resistance of the light bulb allows for. This resistance is typically in the 10s of Ohms so even for the brief period the lamp takes to heat up and turn on there can only be at most 5 to 10 Amps flowing.

EDIT:
A nice proof of this can be comparing a switch for a lightbulb or large heater to a dead short. If you ever stuck the live and neutral wire together you know how violent it is. There is a loud bang, flash, sparks flying, smoke, black deposit on wires etc... Yet a light switch or thermostat on a heater just has the contacts come together with no spark at all. (You do get a spark when opening the switch but that is a different phenomenon). So you rather have a short happen quietly and calmly inside the faulty device, rather than the violent bang.

[tautech]

I suppose there's a difference between a dim bulb tester and dark bulb tester! 

If the bulb is sized so that it is already fairly warm--dim--with the appropriate load, it will react fairly quickly and to fairly small overloads.  A larger bulb will be dark, cold and in addition to being slower to respond, will allow larger fault currents.  You do have to select the bulb carefully for each situation.  Most of the things you would use a dim bulb tester on are not likely to blow up so quickly, but in any case I imagine a properly-sized bulb is on the order of a slo-blo fuse in terms of protection.

This ^^
Typically you want a bulb rating nearly twice the wattage draw of the DUT.
This will indicate quite clearly as dim however any fault conditions where a greater current draw happens is quite plain to see.
To get a handle on using a DBT properly you need a range of bulbs and a few properly working subjects to test BDT behavior at power ON. Armed with this experience you better judge the behavior if a real DUT is working properly or has a serious fault.