Electronics > Projects, Designs, and Technical Stuff
Light bulb is series with unit under test
joseph nicholas:
Keep in mind the average household has only 20 amp wires. All this comes down to a reasonably safe level when you are an able bodied person not wearing a pacemaker or a midget. This is true if you dead short 120 volts to neutral through a screwdriver. A nasty spark will explode in front of you but it will just startle you. You will not die.
Ian.M:
Sokoloff's explanation neglects the highly non-linear resistance of a tungsten filament. The cold resistance is typically only a tenth of the resistance at normal operating temperature (calculated from its rated power or measured current and voltage).
Therefore initially your 60W 12V bulb may pass significantly over 500mA without excessive voltage drop as long as the startup surge is short. If there's a problem with the SMPSU control chip and its over-driving the chopper transistor, the light bulb will save the bridge rectifier and the fuse, and if you worked up to the right wattage for the circuit, may even save the replacement chopper transistor. Of course you *should* have checked the gate or base drive to the transistor while it was out of circuit, and if it looked suspicious (or the if the current sense resistor had blown) changed the control chip as well.
Why not integrated with a variac? Well its often better to put the bulb in series with the positive DC bus feed to the SMPsU transformer, so it doesn't heat up on the inrush current of the reservoir cap, so a simple non-metallic bulbholder you can swap bulbs in equipped with a shortish length of flex you can solder the ends of into the circuit is more versatile.
N.B. with many SMPSUs a bulb that will protect it against a primary side fault is too small to allow the whole device to run. You may have to disconnect most secondary side loads to reduce the consumption enough for the SMPSU to start up without tripping its UvLO.
sokoloff:
--- Quote from: Ian.M on February 04, 2019, 10:07:40 pm ---Sokoloff's explanation neglects the highly non-linear resistance of a tungsten filament. The cold resistance is typically only a tenth of the resistance at normal operating temperature (calculated from its rated power or measured current and voltage).
--- End quote ---
I did mean to add "in steady state" to my first sentence, but in fairness, I did directly express the non-linearity in the post (120V drop at 500mA and [guessed] 30V [rather than 60V] drop at 250mA).
That non-linearity is indeed key to why the device works pretty well for most low power devices. (The lamp has a negligible voltage drop for small currents, meaning a device that works properly and is low power will see very nearly the full line voltage and work "normally".)
001:
is it good idea to add two lamps in series for neutral and hot wires?
Ian.M:
Nope. |O
I considered asking what your chain of thought was behind that question, but decided I *really* *DONT* want to know! :scared:
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