Electronics > Projects, Designs, and Technical Stuff
Light bulb is series with unit under test
Doctorandus_P:
No, you do not want to put lamps in series.
MVS was right about the PTC thing, I'll elaborate:
A cold incandesent light has a relatively low resistance, which means it can pass a lot of current.
If you put it in series with an AUT (Appliance Under Test). and this appliance limits the current to a "normal" value then the lamp will not light. It dissipates only a small amount of power because the voltage drop over the lamp will also be small because of the small current and resistance.
At some point the current through the lightbulb reaches a point that it starts generating some heat. This heat increases the resistance of the filament, and with more resistance (and the same current) it dissipates more power. This is a chain reaction and your lamp will light up.
When using light bulbs as a PTC fuse you generally want to stay in the region where the filament stays cold enough to pass the current without a significant voltage drop.
Some repair shops used to have a construction with a few parallel light fittings screwed to some frame, and they could easily adjust the "fuse" current by adding / removing / exchanging some of the incandesent lamps. This is not an exact science. If the lamps light up, simply turn it off, ad an extra lamp and try again. If you need too many lamps for a small appliance and they still light up, then you know there is a short or similar.
This video from PhotonInduction is a good demonstration of this effect.
He starts with pre-heating a 20kW incandescent light with 12V and it draws about 40A and the current reduces while the filament is heating up.
Zero999:
--- Quote from: 001 on February 05, 2019, 08:18:11 am ---is it good idea to add two lamps in series for neutral and hot wires?
--- End quote ---
No, just one in series with phase is what you need.
--- Quote from: 001 on February 04, 2019, 08:10:52 pm ---Why no standard industrial test units like variacs with build-in bulbs?
--- End quote ---
With a variac, the voltage can be increased slowing, whilst monitoring the current and the power cut, if it rises too high.
tautech:
001, it's a 'dim bulb tester', Google will fill you in on them.
From an old post:
--- Quote from: tautech on June 30, 2014, 01:15:58 am ---My Dim Bulb Tester
Just a incandescent bulb wired in series with the DUT, and good to have a small range of bulbs.
This one is switched SPST, but in countries without a polarized main plug I would recommend a double pole switch
As is evident it was knocked together at no cost in a dead PC ATX case, using the existing IEC mains socket and a few "bits and bobs".
On power-on the bulb should momentarily flash bright, then dim after inrush currents subside.(if all is well).
If the bulb remains bright and the bulb wattage is appropriate for the DUT then "Houston, we have/still have a problem. :'(
If you have a DUT that has emitted the magic smoke or continually blows fuses this can be indispensable.
--- End quote ---
001:
Nice idea
bulb in place of fuse http://www.antiqueradios.com/forums/viewtopic.php?f=1&t=306219
bsudbrink:
Here's a picture of my dim bulb tester, almost always used with the variac also in the picture. Note the "handy" switch. It lets me take the bulb in and out of the circuit without yanking on the cords.
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