[Roger A. Modjeski]

This is well done. Note that in the first demonstration with the large coil of wire and magnet that the neon lamp lights alternate rods as the magnet goes in and out. Neon lamps have two electrodes when used on AC in a common orange nightlight the rods glow alternately at 60 Hz rate. It's always the negative rod that glows so this can be used as a polarity tester. Neon lamps strike at about 80 volts and then sustain 5-10 volts lower. They are useful as voltage regulators in low current applications and I am using them in my ESL amplifiers.

This video goes on to demonstrate transformer action, the effect of coupling by air and by iron, polarity, rectification. A good lesson, well presented.

http://www.youtube.com/watch?v=sdauoOVBFWA

[Ericus Rex]

Very nice video.

It does bring a question to mind though.  If we constructed a series circuit, similar to what the dude in the video had, consisting of a DC power source, a switch, a series capacitor and the little neon light would the light also flash briefly at switch on/switch off?  Or is this purely an inductive phenomenon?

[Roger A. Modjeski]

Very nice video.

It does bring a question to mind though.  If we constructed a series circuit, similar to what the dude in the video had, consisting of a DC power source, a switch, a series capacitor and the little neon light would the light also flash briefly at switch on/switch off?  Or is this purely an inductive phenomenon?

If I understand correctly you want to put a capacitor in series with the double knife switch which he is turning on and off. If you put a capacitor in series with one line you would get one flash the first time as the capacitor charged. After that nothing till you discharged the capacitor again or reversed the polarity. Polarity reversal (which discharges it and re-charges it in the other direction) is how current gets through a capacitor.

If you want to make a neon flasher... which entertained me greatly as a kid. Take a NE-2 lamp, put it in parallel with a 0.1 uf or larger cap, not larger than 1 uf or you will burn the lamp. Then put a resistor from 47K up to several megohms in series with a 100 volt or higher power supply. The flash rate will be determined by all three. A bigger capacitor is slower and brighter, a bigger resistor is slower and a higher voltage is faster. Only the capacitor determines the brightness. The way it works is that the power supply charges the capacitor through the resistor till the voltage hits about 80V and then the neon lamp lights and discharges the cap till it extinguishes around 60 volts. Then the lamp is an open circuit again and the cap will charge up. A scope would show you a sawtooth waveform going between those voltages. Adding another resistor in series with the neon lamp will extend the on time discharging the cap more slowly but reduce the brightness. Since the series resistor will reduce the current in the lamp when it lights you can use very large capacitors. At some combination of resistors the lamp will just stay on.

[Ericus Rex]

Thanks Roger!  I'm just trying to better understand caps. 

So, is the flash upon closing the switch solely due to charging current?  If that's the case, would it be true that if you moved the bulb to the other side of the cap (but still keeping everything in series) you'd get no flash at all?

Is there ever a case where DC can flow through a cap, even for only a split second, if the cap is in good working condition and the applied voltage is within spec?

[Roger A. Modjeski]

Thanks Roger!  I'm just trying to better understand caps. 

So, is the flash upon closing the switch solely due to charging current?  If that's the case, would it be true that if you moved the bulb to the other side of the cap (but still keeping everything in series) you'd get no flash at all?

Is there ever a case where DC can flow through a cap, even for only a split second, if the cap is in good working condition and the applied voltage is within spec?

Yes, the flash is due to the charging current. Im not sure what you mean about moving the bulb. Is it the one across the coil?

DC never flows through an ideal cap, only current to charge it which stops once charged. Electrolytics do have leakage current which is constant and dependent on voltage, temperature and condition of the cap but this should be small yet not small enough to be used for coupling in a tube circuit. However electrolytics are routinely used for coupling in transistor circuits and certainly not a good sounding choice. In modern home theater receivers I am amazed to see that the signal might flow through several dozen cheap electrolytics. They are everywhere. In a simple amp like the EM7 there is only one high quality film cap for coupling.

The split second you may be thinking of would be the charging current only. The time it takes to charge is dependent on the voltage, capacitance and impedance of the charging source. This can be a fraction of a second to several seconds.