auto-trigger? / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

auto-trigger?

1 2 next

Move Thread

LAN_403

Lem0nHead

Wed Apr 29 2009, 02:37AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
Location:
Posts: 18

hello again
sorry my ignorance on this matter, but what's an easy/cheap way to auto-discharge the capacitor when it's charged?
my idea is something like:

the 110V is my home power, after a diode bridge (I'll probably use a voltage multiplier to get at least 300V later)
so how could I do something like: when I close C1,
1) if capacitor is charged, close C2
2) else, charge the capacitor and then close C2

(I consider charged = 250V, for example)

is that what people use SCR to?

and another (probably dumb) question... do I need to open C1 before closing C2 if I don't wanna risk my home power?
I guess closing C2 will transfer almost all the capacitor energy to the coil, since its resistance is way lower than 1000ohms, so the "home power" circuit wouldn't feel a thing

thanks!

Camel

Wed Apr 29 2009, 07:32AM

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

Instead of 'C2', use an SCR. The gate of the SCR could be triggered by a voltage comparator, an LM311 would work.

Slid

Sat May 02 2009, 04:29PM

Registered Member #853 Joined: Thu Jun 21 2007, 03:08PM
Location:
Posts: 14

The easiest way to just trigger a photoflash capacitor when it is approximately the required voltage I think is to just use a transient protector device.
Some tech I found: Unactivated=10^10Ohm. Activated= 2mOhm in 1uS. Where I order electronics they have them from 75 to 350 volts.
They are made to kill a voltage spike like a static discharge or whatever, so therefore they come in different trigger voltage types, like a really big digital zener diode.
They don't last forever, but can handle many big fast discharges good, like you cap.
And, not so expensive. They are not silicone-devices, so they are very rugged.

edit: Oh, yeah, I use these instead of back emf-dides. My scr's are rated at 1200 volts, capacitor voltage 400 volts. The back emf will build a voltage, and when it reatches about 470 volts these nice devices triggers and shorts the spike out.

Lem0nHead

Sun May 03 2009, 11:33PM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
Location:
Posts: 18

I just had an idea on this matter
what if I put a big resistor (probably more than 10MOhm) on the "?" resistor and, on "something here to clos...", the gate for the SCR, which would be C2?

that seems to make sense for me
the gate needs a minimum current, which would be archived when a minimum voltage is reached for the capacitor

big5824

Mon May 04 2009, 09:17AM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

could just use a very small spark gap. Would be pretty hard to tune, but should work if you can get it right.

Lem0nHead

Sun May 10 2009, 12:50AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
Location:
Posts: 18

I tried to make it on different ways unsucessfully
so I went back to the LM311 solution, that seems the most likely to work

but although it worked on a protoboard while I was testing with a 12V PSU it's not working on the real thing
any ideas on what may be wrong? schematic:

(I tried swaping pins 2/3)

a quick explanation:
since 120V (actually 120*sqrt(2) = ~170V peak) is too much for the LM311, I used a voltage divider both on the main source and capacitor
on the main souce I used smaller resistance to give enough current to the LM311 (it needs at least 12mA)
on the capacitor I used a smaller proportion on the resistors so that it triggers when it reaches 95~98% of the main voltage

the problem is that output pin is always 0V

Camel

Sun May 10 2009, 05:10AM

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

edit* blah. I was wrong.

Lem0nHead

Sun May 10 2009, 05:30AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
Location:
Posts: 18

I can see that it makes sense, but it worked on a 12V PSU lighting up a led when one voltage was bigger than the other, exactly as in this schematic I did before (later I changed to 5V and added a LED instead the voltmeter):

is there a difference because the capacitor, or something like that?

Edit: hm, I understood this better now
shouldn't a 10k resistor between pin7 and SCR solve the issue?

Camel

Sun May 10 2009, 05:45AM

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

If pin 3 (-) is higher than pin 2 (+), the 311 will turn on. When it turns on, pin 7 shorts to earth. So as soon as you turn up the voltage on pin 3, the 311 turns on and your volt meter should read close to zero. Is that what happens?

Lem0nHead

Sun May 10 2009, 05:52AM

Registered Member #2096 Joined: Mon Apr 27 2009, 03:08AM
Location:
Posts: 18

It never goes up from 0V
what you're saying makes sense, because when it's low, pin 7 will sink current so voltmeter will show 0V
when it goes high, it will short to ground, so it should keep at 0V

a better way to test it would be to unplug pin7 from the SCR and leave it just plugged to the voltmeter, right?

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob