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Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
Hey, Every coil gun schematic I've seen uses the + terminal of the battery to trigger the SCR gate. Is there any particular reason for not using the + of the capacitor bank in series with an appropriate resister?? A silly question perhaps, but I'd rather ask before I go a fiddling and accidentally destroy stuff.
Registered Member #1774
Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135
I guess you could but it would waste energy from the capacitor as heat in the resistors, it is just easier to power it with a small 9v battery or powersupply in my opinion (also I like to have as few HV terminals as possible so its safer to not do it this way)
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
The question is whether to get the SCR gate drive voltage from a separate supply (battery) or simply divide down the capacitor bank's voltage.
Either method works fine. I went with a battery because I like to test with a wide range of capacitor voltage. You can get around this variable voltage with some simple voltage regulator, but that gets more complicated than a battery+resistor.
The other reason to use a battery is to sidestep the ground reference design issue. The battery simply goes between the SCR's gate and cathode regardless of how everything else is connected. If you use the capacitor voltage instead, then you must be sure the capacitor negative terminal is connected to the SCR cathode. Otherwise your voltage reference levels don't match. This is easy enough to design around but it does provide another chance for things to go wrong.
I've been using one 9v transistor battery in my Mark 2 for like four years now. It's soldered and glued into place and still working fine.
Cheers, Barry So, I see this frisbee and ask myself "hmmm, why is it getting bigger?" and then... it hits me...
Registered Member #680
Joined: Fri May 04 2007, 03:20PM
Location:
Posts: 15
I tried to buy an SCR gate drive from the largest manufacturer of gate drives in the US. The engineer for that company said they did not make a drive for my applicaion, "just anode fire it from the capcitor bank". I discharge the caps at 200V with a 470 ohm resistor in series to the gate. It works fine. The engineer said he did not think the 9V battery would properly fire the SCR.
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
big5824 wrote ...
another problem is if you want to fire at different voltages, you will need to keep changing the value of the resistor
Hey, that's what I said, lol.
But I might add, there's another design problem with resistor dividers ... you must also ensure there is sufficient gate drive current. So the resistors can't be too big -- don't build a divide-by-ten with a 1M and 9M resistor! Or too small, for that matter, or your 1-ohm and 9-ohm network will burn out the SCR gate.
Barry hi say 2 wanted just I that out find u wen irritating very it find may u ... CONFUSED?? Now read it backwards...
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Silly SCR question
