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# Re: [femm] Total current in a high energy disharge.

- To: <femm@yahoogroups.com>
- Subject: Re: [femm] Total current in a high energy disharge.
- From: "Graham Gunderson" <infinitenergy@hotmail.com>
- Date: Mon, 16 Apr 2001 01:08:12 -0700
- References: <9bdh26+9qrj@eGroups.com>

Hi Fucian (?), I've found that calculations, in your case,almost never agree with actual measurements. Hidden (i.e., real-world) variables like turns spacing, capacitor ESR and inductance, wiring inductance (conductor spacingand current loop area) and switch characteristics each modulate the peak current. Most of the hidden variables, though, are inside your capacitor. I have personally found that physically measuring the experiment is always more reliable (and sometimes faster) than using math. Even a ballpark calculation can be a liability with high current discharges, under some conditions. To measure your setup, begin by using no SCR. Terminate the connectors that would otherwise go to an SCR with 2 heavy wires or sections of copper tubing that you can bang together, resulting in a spark that discharges the capacitor through your coil. In series with the coil, splice in an 0.1-ohm, 10-watt wirewound resistor. This will let you measure the current. (You can use a noninductive resistor if you can find one, but herea normal, inductive resistor will cause only a slight and negligible error.) Connect a 1-amp, standard silicon diode and a 220-470 uF, 35V capacitor (which can be from the junkbox) in series with each other, then place this "chain" in parallel across the 0.1-ohm resistor. Polarize the diode so it will conduct, charging the capacitor, when there is current (a voltage drop) across the resistor. For every 10 amps the coil carries, there will be 1 volt of drop across the 0.1-ohm resistor. If your coil carries 100 amps peak, for instance the capacitor will charge to about 9.4 volts. It misses 0.6 volts since the diode "drops"that much potential during conduction. To test, just bang the open wires together HARD (you can smash them together with a mallet; the faster they join the more accurate your test will be) and quickly measure the voltage on the little cap with a digital voltmeter. (It needs to have high input resistance, so it doesn't drain the capacitor while trying to measure it.) Take the voltage reading, and add 1 volt toit (really all you need to add is 0.6 volt, but adding 1.0 volt gives you a reasonable error margin). (It is wise, though to measure the resistance of the small capacitor with an ohm-meter before you use it. It should end up in the meg-ohms or off the scale on the high end. Anything lower means leakage, and resulting error in what you measure - like measuring a really leaky balloon) This voltage reading + 1 volt is your approximate peak current, divided by ten. So, take your measured value, add one, then multiply by ten and you have the peak current. The SCR you use should be able to handle atLEAST 75% of this current value. Because you are using fast current pulses, you can use an SCR that is rated for somewhat less current than the peak value, if you have to. SCRs fuse, in the overcurrent condition, from hotspots in the silicon. Fast discharges, like yours here, are done before full heating can occur - so you can "cheat" with lesser values than what would seem like enough. Having said this, I don't recommend it. Avoid downtime and frustration - spend the extra $3 on that SCR with the full rating :) I write this assuming you have a lab that has some parts, and a DVM. If you don't, just use a 100A, 600V SCR. I can about guarantee it will not blow out - if you do one thing that most people overlook. Of course, when you fire an SCR through an inductive coil (like yours here), current flows through it, causing heat and a magnetic field around the coil. However, by the time the capacitor is discharged (at zero volts), there is often still a magnetic field around the coil, still in the process of collapsing. This fading magnetic field, because it is changing, causes current to continue flowing through the coil. The SCRwill not and cannot conduct this current anywhere - the polarity is wrong and the SCR's off, anyway. Because the current is thus trapped, it will often build up to a high voltage (like the ignition coil in a car, releasing its spark) and blow out the SCR. Very large, expensive SCRs can be easily destroyed with relatively modest coils and capacitors, if you don't add a diode across the coil to short out this trapped current that still flows after the cap is discharged. Just solder in a diode across the coil, somewhere in parallel with it. Polarize the diode so that it will not conduct when the capacitor/SCR are "on". It will appear to be in "backwards" in the circuit, as if it would seemingly never have a chance to conduct. Use a diode with about 1/8 the current rating, but the same voltage rating, as the SCR. More current-rating is always better, but SCR/8 (or thereabouts) is almost always enough to do the trick. If you use a 15-amp, 600 volt diode across your coil and in "backwards" you can't go wrong. This little piece of advice here can save you a lot of frustration and money. Too many people forget this "back-diode" and learn the hard, expensive way. (Yes, I am one of them!) By the way, this FEMM list is primarily here for discussions on the FEMM software and related mathematics. FEMM, however, can not model your question (it's more of a SPICE simulator thing). You might find that you have more luck asking questions like this on other email groups that deal with practical experiments. For starters, try jlnlabs@yahoogroups.com - other members there also deal with the kind of circuits that you are using! (If you want to try the list, go to http://groups.yahoo.com/group/jlnlabs to subscribe. Or, you can check out the list owner's website at http://members.aol.com/jnaudin509/index.htm, but beware the hideous MIDI music that plays when you go there ;) Regards, Graham G. PS - A good surplus retailer that carries large,high current SCRs has a URL at: www.candhsales.com ----- Original Message ----- From: fucian@aol.com To: femm@yahoogroups.com Sent: Sunday, April 15, 2001 6:14 PM Subject: [femm] Total current in ahigh energy disharge. `Hi, Im new to this stuff and cant download right now.Can anyone tell me how to find amperage when inductance and capacitance and volts are know?Let me tell you what Ive got.I have a flat solenoid of7uH and a 7000uF 450 volt cap.I need to know how much current the solenoid will pull and how fast the current rise is so I can get a proper SCR for it.Can anyone help me?Oh yea, this is a non-repetitive application.Just a charge up...Fire type. Thank you very much.` `Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.`

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  - Total current in a high energy disharge.
    - From: fucian@aol.com

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