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Registered Member #1895
Joined: Thu Jan 01 2009, 03:12AM
Location:
Posts: 22
When I fire my coilgun, there is always a small amount of voltage remaining in the capacitor after the shot. It is not negative voltage from reverse, its as though the capacitor does not completely discharge.
My quenching circuit is a simple diode short in anti-parallel with the capacitor. I also use an SCR for triggering.
My thought was that the inductors redirected back voltage might be momentarily balancing the capacitors voltage. What do you guys think
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
I think the voltage your seeing is simply an artifact of the point in time where the current fails to keep the SCR latched, thus opening the circuit at a non-zero voltage.
Edit: Wait I think thats probably not it.
I've seen something a bit like that with a bank of flash capacitors in the past. They mystically "charge" back to around 12V on a mechanically switched gun after the trigger switch is re-opened, even several seconds after the voltage has remained at zero with the switch closed. I think the cause of this may actually be something related to the capacitors, and not the circuit they are in.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
This phenomenon is called dielectric absorption or dielectric soak. This effect is measured as a percentage of the initial stored charge, up to 10%. Some dielectrics are worse than others, polypropylene is pretty good, while electrolytics obviously suffer quite badly from this.
Registered Member #1895
Joined: Thu Jan 01 2009, 03:12AM
Location:
Posts: 22
I don't think so Z28Fistergod. I always thought that the dielectric absorption was a constant. The base voltage of my caps (the voltage between the terminals when left to sit with no bleeder across the terminals) is around 4 volts.
When I fire at 150 Volts, the emf remaining is around 7 volts. When I fire at 300 volts however, the emf remaining is higher, around 13 volts. This suggests, to me at least, that there is something dynamic at work. I always thought that dielectric absorption was a static property.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
It seems you've answered your own question, Marauder. As Fistergod said, dielectric absorption is a percentage of the charged voltage. Yours seems to be around 5%, as you went to 7V after discharging 150V (150 * .05 = 7.5) and 13 after 300V (300 * .05 = 15).
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
This effect also takes a very long time to manifest fully. The capacitors will "rebound" if you will to half of their terminal value very quickly, but then take many times longer to get to 95% of their terminal value. So in that respect yes it is non linear.
Could it be that as the pulse goes negative, it is correctly shorted out by the diode, but this shorting will cause a current to flow through the diode, thus creating a magnetic field, then once the negative pulse is over the field will collapse, inducing a positive voltage across the coil which goes to charge the capacitors?
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Self Quenching Coilgun?
