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a few switching questions

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JoeDarkon

Sun Nov 18 2007, 08:32PM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

So far I have successfully built several basic single stage coilguns using caps starting at photoflash all the way to 8000uf 420v using a simple switch and a large scr. Currently I am attempting to make a multistage coilgun using several 330uf 450v caps and this scr

pdf warning. the problem is that I cant get the scr to trigger(im using barrys schem

Also I could never get a photogate to work and I have no idea what im doing wrong. Any help would be nice.

Shaun

Mon Nov 19 2007, 06:20AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Well, its hard to say since neither of your links work. Just a guess, but perhaps barry's circuit was designed for a different SCR. If yours is bigger, it will need a smaller value gate resistor.

Barry

Mon Nov 19 2007, 07:31AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Could it be this datasheet for the MCR265-10 rated for 550 amps surge and 800v?

The spec sheet says you'll need 20 mA gate current (typ) and 50 mA (max) to trigger it.

Use Ohm's Law R = V/I to estimate the gate current-limiting resistance. If you drive it with a 9v transistor battery, assume 1v is dropped on the gate junction. For example:

for 20mA: R = (9v - 1v)/(20mA) = 400 ohms

for 50mA: R = (9v - 1v)/(50mA) = 160 ohms

Make a little test rig to experiment with, such as with a dummy load to start with, such as an ordinary light bulb. You can even drive it with a small power supply or car battery for testing, whatever provides some glow in the bulb.

What gate resistances and load have you tried so far?

Cheers, Barry
If at first you don't succeed, call it version 1.0

JoeDarkon

Mon Nov 19 2007, 06:42PM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

sorry about the links I fixed them...thanx barry I will try that but since my ultimate goal is to use a photogate to trigger it how to I make that give the correct current?

Barry

Mon Nov 19 2007, 09:49PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Start with the simplest possible lash-up ... Get the SCR working with a simple switch - battery - resistor first, and build up from there later.

After the SCR works reliably, you'll know the working combination of gate current and applied voltage. Then the mechanical switch can be replaced with a transistor driven somehow by the photogate. The exact schematic depends on which particular photodetector you choose; there are lots of possible things that can work. You could even use use an optointerrupter assembly designed for applications like this.

Sorry but I don't have a particular photogate to recommend, since I haven't built a multiple-stage coilgun myself. (Anyone want to suggest one??)

Cheers, Barry
Life would be easier if I had the source code.

JoeDarkon

Tue Nov 20 2007, 06:09AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

how would I wire something like that up? The way I know how to hook up a SCR is using your website. Using the scr wiring schem could I just replace the cap with a multimeter and instead of a coil have a regular wire?

Myke

Tue Nov 20 2007, 07:08AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

You would want a load like a light so that the SCR stays on. Am I right?
Yeah, you would want to just not have the coil but it won't make too much of a difference if it's there or not.

JoeDarkon

Wed Nov 21 2007, 12:27AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

so I am trying to set up my scr to test and I used barrys diagram but it wont work with high voltage the charger will charge to somewhere around 15 volts then the cap will discharge... it works with 5 volts but thats too low for what Im doing... I want around 400v

EDY19

Wed Nov 21 2007, 01:23AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I saw that you mentioned optical triggering- I thought i would mention this design:

I just built it on a protoboard tonight, and it seems to work pretty well for such a simple circuit.

JoeDarkon

Wed Nov 21 2007, 02:34AM

Registered Member #485 Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36

ya I have tried that before I think the problem is that I just dont understand circuit schematics well enough... I made the attached schem (not sure if it works) is there a way you can convert the schem on the hiviz site to one like the one I made?

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