How to Discharge a Capacitor Into a Coil Using SCR Without Latching From Charging Path?

Hi all. This is really a continuation of "Which way is up?" The scr pinout is sorted out, of course I dived straight into another pothole
I want to discharge a large cap into a small coil, using an scr for the switching.Here is the circuit I'm hoping to use.If R1 is 6.8K, it works but the cap is very slow to charge (supply is 14v).If I lower R1 to 1K to speed things up, it unfortunately passes enough current to hold the scr on

What I really want is for the supply to feed only the capacitor, and the capacitor to feed only the scr&coil - which on the surface looks impossible without using a switch!
I'm hoping this is a very basic problem, solved years ago :)As I'll need about ten of these units, I'd like to keep the complexity at rock botttom.Thank-you *very* much for any help,Graham

Hi Graham-What will be supplying the trigger signal to the SCR?  We're not talking about a lot of charging current here, so it may be possible to use a small pass transistor to kill the charging supply while the SCR is triggered.If you happen to have any low voltage AC available, use it to charge the capacitor but put a single diode in series so you'll have pulsing DC to charge the cap.  The SCR will have plenty of time to turn off.You may also be able to de-senstitize the SCR by reducing the value of the resistor between the gate and cathode. This may increase the holding current, making it easier to turn off. -Rick

Eventually, I want the trigger to come from a light beam being interrupted, and ideally I'd like it powered by dc only.
Yes, I'll have a think about putting a transistor in there somewhere to isolate the scr while the cap is charging. Tho it seems a bit inelegant, if you don't mind me saying so 
Thanks...!

Graham - one easy answer (but probably not the one you are looking for) is to use a bigger SCR.  Bigger SCRs have higher holding currents.Another possibility is to use a transistor or a MOSFET instead of the SCR.  SCRs are very good at what you are trying to do but they're not the only way of doing it.  Your trigger pulse amplitude, length and waveshape may all then have a bearing on how well the circuit works (which wouldn't be the case with a SCR).There is also a technique to switch SCRs off if they are holding.  If you have another SCR with a highish resistance in the anode, which triggers when you want to switch the first one off, and couple the anodes via a small capacitor (a couple of uF usually is enough) then when the second SCR triggers, it drags the anode of the first down to the point where it can't hold.  Again, this is probably more complication than you are looking for.  You'd need two trigger pulses separated by a short interval.  If your ten circuits are all together you could use the next SCR to do this...but you probably won't be doing this?  If it's a possibility let me know and I'll do a diagram.You could also try an inductance in your charging circuit.  That might just put a risetime in your charging current sufficient to allow the SCR to turn off once the cap is discharged.Good luck - nothing's easy, is it?

@davi_d: "Another possibility is to use a transistor or a MOSFET instead of the SCR."After thinking about it, I agree. I think a MOSFET might be the best solution.-Rick

Hi.  Thanks for your interest and help.But before I go off on that track,  or one of them ,  here's the latest incarnation, in case anything can be done with it.Without the 22K resistor in place, the situation is as vefore - the cap charges happily, but after discharge it won't re-charge, because there's enough current through the scr to stop it turning off.
The idea was that the 22K resistor would ground the base when the scr is on, isolating the cap  & scr.Unfortunately, the cap doesn't charge at all - because, I now see,  the base and emitter of the TRpair are virtually shorted through the coil.Before I move on, is there an obvious way to make this work? (Although it's already getting a bit complex)
Thank-you!
Graham

Hi Graham,OK, you could go that route, but (a) change the 2 Transistors to PNPs(b) swap the positions of the right hand transistor and the 1K(c) take the 22k to the collector of an NPN transistor, emitter to ground(d) connect the anode of the SCR to the base of the NPN transistor via a 10K resistor.Theory - the SCR will usually have a voltage on it when not ON, this voltage will switch the NPN on, which will switch the PNPs on, to charge the cap.  When the SCR discharges it will switch the NPN and the PNPs off.This might work too well and the SCR might not have enough voltage on it to switch the NPN on.  If this happens, put a resistor between the +rail and cap anode, high enough not to hold the SCR on.But you've now gone from a nice simple circuit to one with three transistors AND the SCR.  Rather look at using a MOSFET.BTW, when you post pics, they will appear at the cursor, so put your cursor where you want the pic before you do so.

Ok then, fine. MOSFET it is.   R.M.Marston's "110 Thyristor Projects...", due for delivery tomorrow, will be useful some other time,  I suppose!I still have nightmares about the time years ago when I blew up half a dozen FETs in a row, trying to solder in just one. Used a socket in the end, and have since avoided anything with "fet" in its name like the plague.These days we don't have to say 20 Hail Marys, make a substantial donation to charity, and buy 10 times as many as we're going to need, to get anything to work, do we?
Yes, I realised what you have to do to place a picture as soon as I saw my last post!
Cheers,

Graham...  R.M.Marston's "110 Thyristor Projects..." - great book, I bought it many years ago and still refer to it sometimes.  You won't regret buying it.Today's FETs are pretty rugged and usually have some protection built in, but take care of grounding (yourself, your iron and your workpiece) and you should be fine.As I remarked above, with a FET you want a trigger pulse that is high enough and long enough to switch on good and hard - otherwise you may have problems with the FET going outside its comfortable operating area. You show your cap as 10 000 - if this is uF then this will be important (that's a big cap with a lot of energy).  If that's nF - ie 10 uF - not so much.You can (for a limited time) edit your posts.  Click the 3 dots at top right.  You have Edit and Delete options.  You would be able to delete and reposition the pic I think, and make other corrections if needed (I use it all the time....)Have fun and let us know how you go.  Cheers // David

It's uF, I'm afraid... :(It seems projects are straightforward only when you've just thought of them - "The Devil..." etc.Anyway, thanks for all the advice.