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My First attempt at capacitive discharge launcher

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LAN_403

Induced

Wed Sept 15 2010, 03:34AM

Registered Member #3011 Joined: Sun Jul 18 2010, 11:33PM
Location:
Posts: 9

Hi everyone.

I have been playing around with a small capacitive discharge setup, using it to launch a small neo magnet, and having alot of fun while Im at it.

Im interested in developing a solidstate switching circuit that will automatically fire the caps once they reach a preset voltage level. Im hoping someone can offer me some advice.

Currently I am just mechanically closing the switch, Im looking for something that will trigger an SCR or Mosfet to fire at a set voltage level, with ease of use/build in mind. Im hoping that the switching system can fire repeatedly at fast intervals (high repetition rate) further down the track, would an SCR be suitable for this?

There is a short video here:

The power supply is similar to a SMPS, the caps are 560uF 400v x2 in parallel. I want to discharge these caps at a set voltage, so that they fill and discharge automatically. Increasing the input voltage will increase the rate at which the caps charge and hence the rate at which they discharge.

Any ideas or pointers?

Regards

Adam Munich

Wed Sept 15 2010, 03:42AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Does that power supply spin a magnet in front of coil to generate high voltage? Because that is a very clever way to make a HVPSU!

I would imagine you could use an SCR with the gate hooked up to a voltage divider. I'm not sure what voltage you want so I can't give you resistor values, but you'd want to make them high to minimize leakage.

Induced

Wed Sept 15 2010, 03:54AM

Registered Member #3011 Joined: Sun Jul 18 2010, 11:33PM
Location:
Posts: 9

Hi Grenadier.

Its something like that, though the high voltage to charge the capacitor doesnt come from induction from the magnets on the rotor. The high voltage comes from the inductive discharge which presents itself when the switches open. So yes the power supply spins the rotor (magnets) via the electromagnets (stator) and the discharges are collected into the capacitors. So its kind of like a two for one, you turn your rotor, and you charge your caps all from one impulse.

Have a look at this video, you can see what the magneto looks like inside.

At the moment Im just looking to reliably trigger my caps to fire at say 250v. You can see from the above video that when the input is raised (up to and over 130VDC) that the rotor will spin very fast (over 8000rpm). The inductive discharges at this input will charge this cap up to 250v almost instantly, so the repetition rate will need to be able to keep up.

Regards

Torchwood

Wed Sept 15 2010, 03:32PM

Registered Member #2668 Joined: Sun Jan 31 2010, 12:11PM
Location: South Australia
Posts: 18

Nice idea for the charger, I like the bit at the end where the magnet sticks to the metal plate too.

Saz43

Thu Sept 16 2010, 07:03PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

That has got to be the coolest charger I've ever seen.

To offer an idea for your switching question:

A simple comparator circuit will do. One comparator input should be the output of a voltage regulator (say 5V). The other input should be the output of a voltage divider connected to your capacitor bank which takes a set ratio of the capacitor voltage (reducing it so you can compare it to the 5V). Thus you are comparing the capacitor voltage to the reference voltage. If you do the math right, the capacitor voltage divided by whatever will rise above the 5V reference just as your capacitor bank is fully charged, causing the comparator output to go low to high and triggering your SCR.

You may need to connect the comparator output to an opamp which would then output to the gate to your SCR because the comparator may not be able to source enough current to reliably gate your SCR.

Thus you trigger once you are fully charged. Use of a pot on your voltage divider will allow you to adjust the firing voltage to your liking. To see an example, look at the lower left-hand side of this charging circuit:

It outputs to a transistor that shuts off the charging process, but it could just as easily gate an SCR or MOSFET. This also is suitable for rapid-fire if that's what you're going for. I'm not sure if the SCR will turn on and off quickly enough- this depends on which SCR you're using and how long the current pulse is in your electromagnet.

Induced

Thu Sept 16 2010, 09:55PM

Registered Member #3011 Joined: Sun Jul 18 2010, 11:33PM
Location:
Posts: 9

Thanks for your replies guys.

Saz, I have limited knowledge and experience with ICs, I havent used comparators or opamps before. However the circuit looks simple enough. Maybe I could use a simple darlington arrangement instead of the opamp to boost the triggering of the SCR? So in that diagram the 2n3904 could trigger the SCR/Mosfet directly?

Thanks for the ideas, Ive got some homework to do now.

Regards

Saz43

Thu Sept 16 2010, 11:16PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Induced wrote ...

Thanks for your replies guys.

Saz, I have limited knowledge and experience with ICs, I havent used comparators or opamps before. However the circuit looks simple enough. Maybe I could use a simple darlington arrangement instead of the opamp to boost the triggering of the SCR? So in that diagram the 2n3904 could trigger the SCR/Mosfet directly?

Thanks for the ideas, Ive got some homework to do now.

Regards

Pretty much. I didn't have any experience either until I built that circuit. You may have to modify it a little for your application but that's the idea of it.

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