| 摘要 |
<p>To the common point C of two transistors of a magnetron, switched converter power circuit is connected a coupling capacitor C11 which provides input to a starter circuit (24). A transistor switch (25) is in series with the capacitor C11 and a diode D1. When the switch is off no current flows in D11. When the switch is made, D11 conducts during alternate halves of cycles present at C. A second diode D12 also conducts and allows current to pass through discharge capacitor C12. This progressively charges until the voltage across it reaches the breakdown voltage of a gas discharge tube GTD. Whereupon the capacitor discharges through the primary winding of transformer TR2. The secondary winding has many more turns and a starter voltage is induced in the starter electrode (11). This is isolated from the Faraday cage (4) and terminates adjacent the crucible, close to the void (2). Every time the discharge capacitor discharges, the void of a microwave plasma light source is pulsed. The magnetron is being driven - the starter being able to operate only as a result of the converter operating. Once a plasma in the void establishes, this is detected by a photodiode (12) adjacent the starter electrode (11). Presence of plasma is signalled to the microprocessor which opens the transistor switch (25).</p> |
