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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The 28V 60mA filament lamp has two 50mm long wires soldered to it. It illuminates to full brightness with only one wire connected to the breadboard. The remaining wire is left disconnected.
I thought 4HV readers might find this ammusing, as it initially appears to go against what we are taught about circuits having to be "complete" in order to work. Although I'm sure many of you will have seen it before, and know how it works...?
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
I really wish 4hv would add some sort of thumbs up so that I can say "This post is awesome" without having to come up with something to post with that.
But while i'm at it: how much charge is moved to/from that wire on the other end of the bulb? It can't have more than a pF or two of capacitance, what frequency do you use to drive it? I can't read the crystal
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The lamp is lit by the current flowing through it that charges and discharges the remaining wire that isn't connected to anything. As you said this floating wire will only have a capacitance of a couple of pico-farads at most.
The operating frequency is 25MHz. A CMOS crystal oscillator module drives a Class-C amplifier consisting of a modern equivalent to the 2N3053 NPN biploar transistor. This driver stage serves only to provide enough gate drive power for an IRF510 MOSFET PA stage. The MOSFET also operates in Class C and drives a series resonant load network.
The voltage at the mid-point of the load network rings up to a hundred volts or so with a frequency of 25MHz. This combination of high voltage and high frequency is sufficient to drive a significant current through the lamp to cyclically charge and discharge the few pf of capacitance of the wire the other side of the lamp.
RF engineers can think of the series resonant network as an L-match network between the PA transistor and the lamp load. The lamp with one lead flying presents a load that is resistive in series with a large capacitive reactance. The L-match network transforms this "difficult to drive" impedance to something much lower and real that the PA can drive power into.
-Richie,
BTW, The IRF510 is massively over-spec'd for the trick of lighting up that little bulb! You can actually dispense with the MOSFET PA altogether and get it to light almost as brightly using just the bipolar stage alone.
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Filament lamp lit with one wire
