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SCR's above 2,200 volts?

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Patrick

Sat Sept 01 2012, 12:31AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Well arnt coil guns current driven devices?

are you trying to increase the current over what would be provided by a 300 volt system by dumping high V (3kV) and joules in less time into the coil?

Yanom

Sat Sept 01 2012, 12:38AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

Patrick wrote ...

Well arnt coil guns current driven devices?

they are amp-turn driven devices. The magnetic power is equal to the amperage times the number of turns. My coil has an obscene number of turns - something like 1400 or more (it's slightly smaller than a fist and made of 28 awg), but because of all that wire there's high resistance. So I have to put a ton of voltage through to overcome Ohm's law, and then get a good amount of current (100 Amps) through all that wire.

Sorry for all the noob questions. I hadn't done any HV stuff before this. It's been fun learning though.

So, do you think I can make the series SCR thing work? Or should I just go buy a thyratron/triggered spark gap?

Patrick

Sat Sept 01 2012, 02:37AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i think an SCR is what i would stick with, but others will have to advise you price and availability of anything else.

Yanom

Sat Sept 01 2012, 03:37AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

Patrick wrote ...

i think an SCR is what i would stick with, but others will have to advise you price and availability of anything else.

What am I going to do about the slow turn-on problem?

Pinky's Brain

Sat Sept 01 2012, 04:18AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Ugh, you're not the only one not awake Patrick ... I completely misremembered the rise time capabilities of SCRs ... thought it was on the order of 10-100s of amp per ms, it's actually per us :/

Patrick

Sat Sept 01 2012, 04:44AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

yeah my brain was limping from that hangover, but 100 amps in 0-4 mS doesnt seem outrageous.

id stick with the SCR Yanom... a saturable reactor will delay the full current conduction, even 50uS would be enough for survivable kA shots into your coil gun (from even weak phase control scrs). i think the SCR is the cheapest, fastest way to get a predictable coil gun working.

2Spoons

Sat Sept 01 2012, 06:29AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

That di/dt should be fine. The fence energiser I designed went from 0 to 2000A in 50us. Nothing died, and it was just an 85A rms stud thyristor.

Patrick

Sat Sept 01 2012, 07:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

That di/dt should be fine. The fence energiser I designed went from 0 to 2000A in 50us. Nothing died, and it was just an 85A rms stud thyristor.

excellent! then he just needs a good gate drive circuit with fast rise time and enough current to trigger.

Sulaiman

Sat Sept 01 2012, 08:42AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

In the circuit diagram the output of the generator is taken from the wrong ends of the end capacitors,
The switches that are drawn in the open position need to have a voltage rating equal to the series voltages of the capacitor bank,

The switches that are drawn as closed could be individual thyristors which would remove the need for a secondary switch
(a mechanical or electromechanical isolating switch would still be required)

A high voltage relay such as found in some microwave ovens may be useful.

To trigger many thyristors simultaneously,
pass a large current pulse through a well insulated wire
(e.g. discharge a capacitor using a small thyristor)
that passes through one ferrite toroid core per gate circuit.
One FT50 size core per gate is adequate, an FT100 size core makes insulation easier.

Yanom

Sat Sept 01 2012, 02:09PM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

wrote ...

In the circuit diagram the output of the generator is taken from the wrong ends of the end capacitors,

yes, I accidentally put the switches in backwards

wrote ...

The switches that are drawn in the open position need to have a voltage rating equal to the series voltages of the capacitor bank,

oh yes... I forgot about that.

wrote ...

The switches that are drawn as closed could be individual thyristors which would remove the need for a secondary switch
(a mechanical or electromechanical isolating switch would still be required)

So the circuit should look like this pic (charging/triggering circuitry not shown)? That last switch is still having to switch some high voltages.

wrote ...

To trigger many thyristors simultaneously,
pass a large current pulse through a well insulated wire
(e.g. discharge a capacitor using a small thyristor)
that passes through one ferrite toroid core per gate circuit.
One FT50 size core per gate is adequate, an FT100 size core makes insulation easier.

That's essentially a DIY pulse transformer isn't it?

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