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LAN_403

Camel

Tue Nov 25 2008, 09:28AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Hi,

I've got 4 SCRs in parallel, rated 90amps for 10ms. Using the applets on barrys site, I generously estimated a maximum of 300 amps for 4ms through my coil. So in theory my SCRs should be more than capable of handling the pulse. On the gate, I've got a 500 ohm resister in series with a SPST switch from the + terminal of my 12V battery.
In theory, my SCRs should be running well within limits, and yet after every 4 - 5 shots my SCRs fail short.
Does anyone else have any experience with this? Am I doing it wrong??

OZZY

Tue Nov 25 2008, 02:01PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Hi Camel

If you connect SCRs in parallel you should de-rate them by 20% or so. This is because they will not share the current exactly. 4 x 90 = 360A. 20% of that is 72 so that leaves you with 288A as the pulse rating of your switch.

Reducing the pulse time to 4ms will not help a great deal. If you want to calculate the rating for a shorter pulse, just keep I^2 x t constant so if you multiply the time by 0.4 you can multiply the current by 1.58. This gives you a rating of 288 x 1.58 = 455A this is above your 300A estimate.

If you are pushing SCRs this hard you need a good solid gate drive to make sure they turn on completely and quickly. The voltage across the gate is about 2V so you have 10V across the 500 Ohm resistor, thats 20mA total split four ways is only 5mA to each SCR. This may not be enough, check the data sheet to see what is recommended by the manufacturer.

The SCR gates will not share out the current equally if they are all connected together. It is better to use a separate resistor for each SCR. Try 100 Ohm resistors, this should give you 100mA to each gate. But if the gate current is too high you will damage the SCRs, again check the data sheet.

As you can see there is nothing wrong with using SCRs in parallel but it is much easier to buy a bigger SCR.

Have fun. Ozzy

buzzer

Sun Dec 07 2008, 03:32PM

Registered Member #680 Joined: Fri May 04 2007, 03:20PM
Location:
Posts: 15

What is the pulse time of a typical coilgun?

Backyard Skunkworks

Sun Dec 07 2008, 08:05PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

buzzer wrote ...

What is the pulse time of a typical coilgun?

A well built CG might have a pulse time of anywhere from 4 to 15ms depending on a bunch of factors. A poorly designed gun with too much resistance and not enough voltage may have pulse time thats much longer, causing horrible suckback.

rp181

Sun Dec 07 2008, 10:30PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

4-15ms is really long. More like 1-10ms.

Killa-X

Mon Dec 08 2008, 01:00AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Sorry to just butt into a thread, But I rather not make a thread to ask such a stupid question. RP181, My coilgun has a pen inside the coil. It sticks less than 0.5cm out of the coill. Would extending the barrel to like 5inches out of the end, allow it to shoot beter/straighter? What will change when i make a long barrel VS my no-barrel?

Thanks! Btw my SCR is rated 10 i think? Doesn't really say...

guitardude012

Mon Dec 08 2008, 01:03AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

The longer barrel will most likely only increase friction and drag losses.

Killa-X

Mon Dec 08 2008, 01:18AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

so its best to keep it as a like 1cm barrel...thanks! lol i got to get a new material anyways from a store when i eventually multistage it...maybe that skinny PVC pipes that are 1cm and less diameter..While im here, does adding a analog meter drag how fast it charges and holds charge? I rather a analog volt meter built into it than digital. Once the batterys get bad mine can miss read the high 300s as high as 100V over what it is :(

rp181

Mon Dec 08 2008, 01:26AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

@ Killa-X
A analog meter should be fine, though ive never used one myself.
EDIT: your SCR is rated for 10ms pulsed 500Amps, should be good depending on your power source and coil.

@Camel
When you say failshort, do you mean it occasionaly fails, or are they permanantly dead?

If its occasionaly, then you might be exceeding the dv/dt of the SCR, causing it to automatically trigger. Whats your charging? dv/dt dosnt harm the SCR's much, and some applications even rely on this attribute to work. Try slowing down your charging and see.

Killa-X

Mon Dec 08 2008, 02:11AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Well RP181, I've seen one on youtube that was modded to look like a speedometer, and it looked cool. I just rather that over digital cuz then you replace the battery a lot.

Also, I was shooting 9ft distance today shattering old CD cases with my 3 caps. But I still got many changes to do over time. One i get my chargers going, id have to LM311 a stop-charging circuit. Thanks for your help as always ;)

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