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Can't seem to get my SCR's to work

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LAN_403

Legit_bacon

Thu Jul 28 2011, 10:58PM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

For my science fair this year i decided to make a coilgun, and after many years fidling aroud with electronics (mostly directed in the direction of coilguns + many bad electrical shocks later) I finally got one working!

I'll give you a brief overview of the specks (pics to come shortly)

-cap bank: 2 250V 1500uf caps in parrallel

-charger: 555 osilator and a NPN switching transistor powering a 2k primary to 200K secondary transformer. ( this is really slow and takes a good 3 mins takes 3 mins to get to 250V). I suspect this is because I am putting a high voltage in and the lekage on the caps is countering it (correct me if im wrong) also is there a way to improve the charging time? Ill post the curcit diagram.
But my biggest problem is getting these damn SCR's to work. I'll post a curcuit diagram as to how im interfacing them. Also how are the SCR's untripped in coilguns? is it the back emf from the coil? Oh and by tyhe way im using two 500V 122amp pulse rated SCR's (is this to low? Could I have fried them?)

Sorry but im quite new to electronics and don't reallly know what im talking about so correct me if im wrong anywhere.

Forty

Fri Jul 29 2011, 02:46AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

for your circuit, i think you might want to put a full bridge rectifier in after the transformer secondary.

the scr configuration looks just fine.

try out this popular charging circuit from uzzor:

you already have most of the parts.
I think it is unlikely that your bank fried an scr that size, so check to make sure that you've got your pins identified/hooked up right, and that you're triggering it with the right voltage and current.

Legit_bacon

Fri Jul 29 2011, 05:51AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Thanks for your swift reply Forty, So i asume it is the reverse voltage of the coil that untrips the SCR?
Is there a way to interface my 555 timer with uzzor's boost converter? As I am reluctant to build a whole new curcuit if I already have half of it built.

Forty

Fri Jul 29 2011, 06:27AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

no, when the current reaches ~0 the scr shuts off, then the back emf rings down through the antiparallel diode and the coil. it would be easier to just make the second circuit than to move or replace all the components in yours. if you don't have/can't find one to salvage, texas instruments will sample you a comparator.

Legit_bacon

Fri Jul 29 2011, 07:24AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Hmmm... thats interesting, I think shomehow i've fried my SCR's, because they are constantly conducing. I asumed this was because I hadn't untriped them but obouisly not... Many thanks for your help Forty.

Legit_bacon

Sun Jul 31 2011, 05:46AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Hi ive got another question that maybee you van help me with. Today my charging curcuit has stoped working. Any idea why (it was under normal conditions). Is there a fault in the curcuit (It wasn't inteded for this apllication and is ment to be run on 9V) ANy ideas?

Forty

Sun Jul 31 2011, 06:21PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

the 555 can handle 12 volts just fine, and i imagine your transistor can too. you may have melted your transformer primary. from the diagram it appears that you have a second primary, try running that one and see if it still works.

Legit_bacon

Mon Aug 01 2011, 02:23AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

No sadly thats just in the diagram, no ct on this one :( I was thinking maybee i could convert it into a simple boost converter? Maybe something along these lines:
But I am unsure of the values tyhat are needed for these components, Any Ideas?

1312165408 4034 FT121177 Coilgun Schematic

Legit_bacon

Mon Aug 01 2011, 02:25AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

Oh I also got a LCR meter from school today for trouble shooting, so hopfully I will be able solve my problems. Sorry for the double posting.

Forty

Mon Aug 01 2011, 02:33AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

c1=10nF
don't need c2, just leave pin 5 disconnected.
r1=15k
r2=68
r3=4.7k
L=130uH

That's just grabbing the values from uzzors boost converter. it should work, but it won't turn off when the capacitors are charged, so you'll need to manually monitor it and shut it off, or build the rest of uzzors circuit. don't test the circuit without a capacitor hooked up to it.

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