Thursday, November 18, 2010

NIC Solid State Tesla Coil Project Update 2

12V DC brushless fans are now attached to the back of the H-Bridge heatsinks. These fans should greatly improve cooling and thus reliabillity. One major problem with previous coil designs were no cooling fans, the higher operating temperature of the mosfets caused failure after 5min runs. Fans make a huge difference in performance, cooler H-Bridges run more efficiently. More holes were made in the conduit box's to increase air to flow.

The halfwave voltage doubler circuit is now wired in. The doubler is configured as a Delon Circuit and produces 310VDC from the 120VAC mains input. Consisting of a bridge rectifier feeding into four 450V 270uF electrolytic capacitors. Each capacitor has a 470k ohm 1/4 watt resistor soldered across it to discharge them when power is removed, full discharge time takes about 10mins after power is removed.

The cat in the picture was not part of the orignal design but was an unintended side effect of irresistible bits of wire dangling about.

In addition to the wiring up the voltage doubler more circuitry has been added to the H-Bridge. DC blocking capacitors have been added to one leg of the bridge (yellow capacitors center). These are in series with the tesla coils primary coil, the purpose of the blocking capacitors is to prevent high currents from flowing in the primary at low frequencies. You could say they prevent direct current flow in the primary when the bridge stops switching as the primary is essentially a dead short to DC. Further more the red capacitors seen at the back of the bridge go across the DC bus of each leg. These capacitors act as a filter to sudden changes in voltages across the bridge legs at high frequency. The reason behind this is to stabalize the DC bus as the inductance of the wire between the bridge and the voltage doubler causes rapid changes in voltage as current flows between the doubler and the bridge. Without these the mosfets can see high noise during switching and high voltage transients which may cause failure at high power levels.

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