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My Capacitor Bank Schematic - anything major wrong?

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Hon1nbo

Thu Jun 25 2009, 07:32PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

hi, attached is the preliminary schematic for my Pulse Bank Project, and besides component values not being listed, can anyone see anything I am somehow being oblivious about? - I just can't help but feel something is wrong with it.

the item labeled charge meter is not for the voltage of the Bank, but for the varied voltage for charging the bank (a separate meter will monitor the actual bank); and High Voltage Trigger might just be a 555 circuit with a MOSFET driving a basic transformer; the fuse I have not yet calculated the rating for, and the two buttons in the upper center are really one button (a big red Mushroom E-Stop button with NO and NC states); and the two relays are just your average relay which will be rated for the purpose...

the main thing that I guess I wonder is if I really should rely on the ability of the Mushroom button to switch the Charging Transformer Off and safely Trigger the Capacitor Bank... I do not think it would be a problem from my logic, as the relay would shut at most a few moments after trigger if it lags but I want some feedback on the matter

schematic

Jimmy

klugesmith

Thu Jun 25 2009, 09:18PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Hi Jimmy. Good start.

1. For good reason, it's conventional for all components (relays, switches, transformers, etc.) to have reference designators (like your diodes). Makes it easier to discuss the schematic & make physical wiring diagrams etc.

2. Disconnecting the primary of your charging transformer leaves D4 exposed to damage from a voltage-reversing pulse discharge. Consider following D4 with a small HV capacitor to GND and a HV resistor in series with your main bank. If your "HV" value is only in the triple digits, the charger HV output (at very small current) might safely be disconnected by ordinary relay or mushroom switch contacts.

3. With single-diode rectifier D1, the DC current of both relay coils flows in the secondary of your 10:1 transformer. Not a generally good practice, but OK if you know what you are doing and/or the DC current is small compared to rated current. One alternative is to replace D1 with full-wave or bridge rectifier; another is to eliminate D1 and use relays with AC coil ratings. For similar reasons, consider a full-wave rectifier in place of D4 if your charging transformer has center-tapped secondary.

4. Your description of charge meter doesn't explain why it's switched by relay, and disengaged when the charging transformer is energized.

Hon1nbo

Thu Jun 25 2009, 10:05PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

Klugesmith wrote ...

Hi Jimmy. Good start.

1. For good reason, it's conventional for all components (relays, switches, transformers, etc.) to have reference designators (like your diodes). Makes it easier to discuss the schematic & make physical wiring diagrams etc.

2. Disconnecting the primary of your charging transformer leaves D4 exposed to damage from a voltage-reversing pulse discharge. Consider following D4 with a small HV capacitor to GND and a HV resistor in series with your main bank. If your "HV" value is only in the triple digits, the charger HV output (at very small current) might safely be disconnected by ordinary relay or mushroom switch contacts.

3. With single-diode rectifier D1, the DC current of both relay coils flows in the secondary of your 10:1 transformer. Not a generally good practice, but OK if you know what you are doing and/or the DC current is small compared to rated current. One alternative is to replace D1 with full-wave or bridge rectifier; another is to eliminate D1 and use relays with AC coil ratings. For similar reasons, consider a full-wave rectifier in place of D4 if your charging transformer has center-tapped secondary.

4. Your description of charge meter doesn't explain why it's switched by relay, and disengaged when the charging transformer is energized.

a couple of clarifications:

first the charging voltage is four digits, if I stay with my current bank size when I test the circuit it will be 1000V, if my bank increases it may go up quite a bit

the "Charge Meter" is a little off in the name I admit, but it was hard to find right words for it that would fit - it is not to measure the changing voltage as the Capacitors Charge, but it is to be used when adjusting the Variac to the Desired Charging Voltage before charging begins - it tells me what voltage the Variac is actually putting out on its secondary (so variacs technically have a secondary, or are they something else by nature anyways?) and the meter is calibrated by the ratio of the charging transformer to "program in" my desired maximum voltage - it is not needed once charging begins

as for the use of two relays on one transformer, I think you are right that it might not be a good idea to have both relays there but I am not sure what I would do to make the key control both the firing circuit and the charging circuit without doing two things: making it much more complicated, and having it as a part of the 120VAC connections

Then Again, maybe I will just remove that relay as now that I think of it, there is no specific reason I had to close off power to the Variac, if I already have control over the current as it flows to the Charging Transformer

I think I will take your anyways advice and switch that to a Full Wave Rectifier

Regarding D4, I think you are absolutely right and that I should put in a little protection for it

finally, sorry for not putting references in - I had the schematic drawn out on paper and was putting it into ExpressPCB and completely forgot to label the Relays and number the transformers (BTW: for the curious Diodes 2 and 3 were removed as I realized they weren't necessary where they were, which was to my HV Trigger which will already have diodes.)

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