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Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Protection diodes prevent suck back if anything.
The reason they are needed, is because electrolytic caps are polarized. When you create an RLC circuit, (resistance, inductance and capacitance) you often get what's called ringing. Ringing goes from negative to positive voltage across the capacitors, and because they're polarized, they don't enjoy that too much.
As a result, people have learned to put protection diodes across the capacitors. As soon as the voltage hits negative, the diode will start conducting preventing the negative voltage from hurting your caps, and your SCR. (an SCR is a....sort of polarized device. Negative voltage on the cathode would make it unhappy)
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
SublimeVolts wrote ...
Dont protection diodes cause suck back?
Correct!
PSCG wrote ...
No, in fact they help the coilgun by preventing the suckback effect.
Inducktion wrote ...
Protection diodes prevent suck back if anything.
Allow me to explain!
The commutation diode (aka protection diode) actualy keeps forward current circulating through the coil after the capacitor voltage drops past zero. The current then decays exponentialy according to RL dynamics, since the capacitor has been bypassed by the diode. As this commutatuion current flows through the coil and diode, it causes suckback if the projectile is past the half way point. The RL current decay can take a relatively long time, so it really is not an efficient way to commutate coil current (but it's easy, so it's often the best way to go).
Without a commutation diode, your capacitors will indeed be damaged by reverse voltage. But, the forward current through your coil will decay much faster, reducing suckback time.
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Charging circuit
