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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
In coilguns, there is often a diode in antiparallel across the capacitor to protect it from high negative voltages due to ringdown in the coil, right? Well I'm using an SCR for switching (like most people), and doesn't that eliminate the need for an antiparallel diode? Because the SCR is in the capacitor-coil-SCR circuit, and the SCR can only ever conduct in one direction, does that mean that negative voltages can never be applied to the capacitor from coil ringdown?
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Yanom wrote ...
In coilguns, there is often a diode in antiparallel across the capacitor to protect it from high negative voltages due to ringdown in the coil, right? Well I'm using an SCR for switching (like most people), and doesn't that eliminate the need for an antiparallel diode? Because the SCR is in the capacitor-coil-SCR circuit, and the SCR can only ever conduct in one direction, does that mean that negative voltages can never be applied to the capacitor from coil ringdown?
You still need a freewheeling diode, otherwise, the voltage across the SCR will rise until the SRC explodes.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
You still need it. In an RLC circuit, voltage and current are out of phase by 90 degrees. So as inductor current falls to zero, voltage across the capacitor reaches its peak negative value.
If an SCR and quenching (anti-parallel) diode were included in the circuit, when the capacitor voltage reaches zero, the current stops flowing through the capacitor & coil and instead flows through the diode & coil, and the capacitor voltage drops no further. The current waveform changes from the steep negative slope of the under-damped RLC response, and takes the form of the steady exponential decay of a simple RL circuit. This extends the time that current flows in the coil, and that explains why the quenching diode increases suckback.
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Does an SCR eliminate the need for a quenching antiparallel diode?
