Halo Style 2 stage COILGUN

[AnselmoFanZero]

Barrys Coilgun Site
RLC and Inductor simulation....will give you pulse-durations as well...

 

[millirad]

I'm not sure how much inductance a coil 21AWG, 16mm long, 8mm ID and 4 layers has. Possibly only 0.04mH, which would mean a spike over 1000A. ^^ That coil wouldn't work with this SCR if that is the case.

Now I know why the spark that came out of the barrel was so strong for some very tiny leads/contacts. xD

Edit: if I use an equal sized coil to the first stage, I'm still showing an ~800A spike. Looks like the most difficult decision is with the SCR's. There seems to be no alternative except to stack some cheaper SCR's, to meet the current spike reliably. I'll get my $16.00 back and purchase some cheap SCR's, by driving a longer distance to a better electronics retailer on Monday. I'm starting to see the difficulty with deciding on a coil too. Any low current coil in the 2nd stage will not have a short pulse and will not accelerate the projectile very well. ^^

 

[AnselmoFanZero]

1000A can be handled by a single SCR, depending on the type.
But its cheaper to stack smaller ones.

You will figure it out

 

[millirad]

I like this image of how the SCR is placed in my second stage circuit.......Credits:Barry's Coilgun Design Site.

But I'm not sure if the gate of the SCR would really be hooked to the positive terminal of my batteries?

 

[AnselmoFanZero]

Didnt work with my batteries...sounds crazy, but I connected the gate to the charged cap+...works great. Like that:

SCR gate -> 180R resistor -> contact 1 / contact 2 -> cap+

For the first stage you have to replace contact 1 and 2 with terminal 1 and 2 of the push-button switch.

Dont be afraid to ruin anything! The pulse for the gate is so damn short. As soon as the projectile touches the contacts (or you pull the trigger), the cap is discharging...so no chance to overcharge the gate. Simple idea but efficient ;-)

 

[Leodahsan]

IIRC the scr gate is maximum rated in Watts. eg, there is a maximum 'power', not current/voltage on the Gate, before it breaks the scr. I'd still recommend a small resistor depending on the battery, to not exceed around 1 or 2 watts on the gate.

 

[AnselmoFanZero]

No need to try anything...just told you how its done ^^

Very reliable method! I tried it with all kinds of batteries....nothing. Sucks.
Trust me....my way is better. Might sound cocky, but I tried alot...trust me.

 

[millirad]

I'm starting to finish the second stage with the single SCR, and it will be a risk I'm willing to take. The gate will possibly be OK, so we will see what it's peak current abilities are. ^^

 

[AnselmoFanZero]

My fingers are crossed... ^^

 

[millirad]

Well I fired it and there was a medium sized sound of the caps discharging in the second stage, like a "snap". I'm not thinking that the 2nd stage worked too well. There shouldn't have been a sound when it was fired right? It wasn't as fast moving as I expected, more like when only the first stage functions. Tomorrow, I'll try firing again to see if there is any response from the second stage. I tried sliding the .25" steel rod past the contacts and they seemed a little rough, but it passed and ohmed out successfully. I also noticed on the barrel from the last time where the contact wires melted a pattern into the barrels exterior slightly, so that's what 100 amps would do when running without an SCR. ^^

Edit: So the second test that I did today, wasn't any good. The 2nd stage just arced into the air (during the charge cycle) with a medium sized "pop" sound and prevented the firing from happening as a 2 stage. It may have something to do with using the Kodak charging circuit in a second stage with the SCR. The Kodak charger works with the projectile shorting the caps on the coil, but that's too risky and only good for a single shot. ;-)

 

[millirad]

Found an error in my circuit for the second stage. ^^ So I'm going to build a new barrel with spring style contacts and wind 2 new coils for the first and second stage. Also a new charging circuit board for the second stage. This "might" get my 2nd stage to work.

 

[AnselmoFanZero]

Maybe you could up my plan....just in case somebody wanna try make a reliable 2 stage.

 

[millirad]

Good idea! Anyone who just wants a working 2 stage will want to give this a try CREDIT: AnselmoFanZero .......................

 

[AnselmoFanZero]

The resistance might be too high/low for your SCRs. Try 420R on the first stage, if its not working, try 270 and so on.
I am using 180R and its working well.

 

[millirad]

The barrel replacement has been completed today, and has 2 equal length coils. The first stage coil is 6 layers of 20AWG and the second stage is 5 layers of 19AWG. I found another SCR that is rated at 400V and 55A, with a peak surge current of 650A. The second stage charging circuit will be built tomorrow and then the first test of a "possibly" working 2 stage.

I also installed spring contacts on the barrel for the second stage from an ordinary spring. It wraps around the barrel with 2 turns to secure it(epoxy adhesive also) and has a small loop that is under tension in the groove/hole of the barrel. Mechanically it bounces when the .25" steel rod is inserted and ohms out OK. It moves up and down when it is tested with "spring" action. ^^

 

[AnselmoFanZero]

Sounds good so far.
On my next coilgun, the spring wires will enter the tube vertically. Slightly V-shaped...slits on both sides.
They have to bounce back into place...you dont want a coilgun you have to re-adjust after a shot ^^

I would test the second stage if I were you. Just to be sure contacts are working fine.