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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
I bet you aren't getting the filament hot enough for significant emission. It's tungsten like in a light bulb (think white hot), not coated with stuff for good emission at red-hot temperature. But go slow -- don't blame me if you burn it out!
Mind your x-ray exposure. A geiger counter should give you a lively response when you get a few microamps at 30 kV. Then think about what level will give a deadly (well, hurtful) response.
[edit] Why not use a low voltage battery for filament PS? For example, some salvaged NiCd or NiMH or Li-ion cells. Use low-ohm power resistors, or rheostat, or a 3-terminal regulator to control the current. Whether filament supply is battery or wallplug-based, an anode current meter should be inserted between filament PS and HV PS. Would be clear if you draw a schematic. If you tried to read anode current with meter between filament and filament PS, you should reconsider playing with HV and x-rays.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Good picture.
Now with apologies to Proud Mary and other regulars who worry, appropriately, about experimenters too casual with ionizing radiation:
1) You have a line connecting the negative terminals of the 12V PS (which generates HV) and the 5V PS. Suppose you replace that with, say, a 1000 ohm resistor, and have no other connection between the filament circuit and ground. Then any anode current will flow through that resistor, and generate a measurable voltage drop. Your voltmeter negative probe could remain where you have drawn it. The positive probe could be switched between where you drew it (to read filament voltage) and the far side of the new current-sensing resistor (to read anode current as a negative voltage).
2) The tube filament doesn't care whether its heating current is AC or DC. Perhaps you have a variac ( or suitable light dimmer) and a low-voltage transformer, even one made for 12V lighting. Ever hear the term "filament transformer"?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Now I should bow out for a while (like days). Parting notes: 1. V = IR. Across a 1000 ohm resistor, 1 volt indicates 1 mA, and it's linear above or below that. 2. In your filament circuit instrumentation, the voltmeter should go "after" the ammeter. In this application, current drawn by voltmeter (even analog) is negligible. On the other hand, voltage drop across the ammeter is probably around 200 mV at full-scale current, and (as you drew it) adds to the actual filament voltage. For best accuracy in measuring the filament voltage, use sense points as close as possible (resistance-wise) to the tube leads.
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