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LAN_403

Firefox

Fri Apr 24 2009, 12:02AM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

On question #2: You do need clamps for your puk SCRs. If you look in the datasheet, it will tell you how much clamp force they need. If I recall correctly, this is to bring the semiconductor wafer into contact with the anode/cathode of the SCR. If you don't have the manufacturer's clamps, aonomus has recently built some clamps for some puks of his own and would likely be able to give you some advice. I can tell you the clamps for my SCRs are two giant steel or aluminum bars with a pair of half inch steel bolts to apply clamping force. Once bar of the clamp is isolated from the SCR with high density plastic to prevent a short circuit with the clamp.

Hope that helps.

big5824

Fri Apr 24 2009, 09:08PM

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

Thanks firefox, I started working on the clamps today, the layout will be:

---thick steel sheet---
plastic disk
aluminium sheet
puck
aluminium sheet
plastic disk
aluminium sheet
puck
etc

My only concern is that the oxide layer on the aluminium will increase the resistance of the connection, does anybody know how much it will affect it?

rp181

Sat Apr 25 2009, 03:15AM

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

Right before you put the aluminium in, sand it. It should work fine then, if not, use copper. There are also electrical corrosion inhibitors, I have used kopr-shield on my bank with success.

For slotting, you best bet is a mill, and dremel as a second choice.

big5824

Mon May 11 2009, 08:36PM

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

Iv run into a new problem with the V switch schematic.

Basically, when I try to trigger the second SCR the input voltage on the gate is less than the voltage on the cathode, so both capacitor banks just discharge through the second SCRs gate into the 6v supply....what am I doing wrong?

(R1 = coil, SW1 = charge button, SW4 = first SCR trigger, SW2 = second SCR trigger)

Also, the SCR clamp is built, will get a pic up shortly.

/edit Think iv sorted the problem, do you guys think this

would work? (works in simulator).

Basically the problem was that on the real v switch, an inductive sensor is placed between the cathode of the SCRs and the gate, so the induced voltage was added to the voltage of the cap bank, meaning that the SCR sees 6v between the gate and cathode. To fix this, iv placed a transformer in the sensors place, and will use a battery on the primary to create the required pulse. What do you guys think?

Cesiumsponge

Tue May 12 2009, 01:58AM

Registered Member #397 Joined: Wed Apr 19 2006, 12:56AM
Location: Western Washington
Posts: 125

big5824 wrote ...

My only concern is that the oxide layer on the aluminium will increase the resistance of the connection, does anybody know how much it will affect it?

rp181 wrote ...

Right before you put the aluminium in, sand it. It should work fine then, if not, use copper. There are also electrical corrosion inhibitors, I have used kopr-shield on my bank with success.

I concur with the suggestion of using the oxidation inhibitors, the stuff works great. I was getting sparks when using aluminum at mechanical junctions during pulse discharges even when using machine screws to clamp down the connections. My guess is that the aluminum surfaces have enough oxide that it'll insulate the connection enough until the voltage is sufficient enough to punch through the oxide layer, resulting in "microarcing" which gives a small shower of sparks. I sanded them, globbed some Burndy Penetrox (designed for Al/Al, Al/Cu, Cu/Cu connections) and no more sparking.

big5824

Tue May 12 2009, 04:20PM

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

Wouldnt the oxide inhibitor increase the resistance of the connection? (I need to keep the total resistance of all the connections to less than 3mOhm ideally)

rp181

Tue May 12 2009, 07:16PM

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

There are inhibitors designed to improve conductivity. Mine is mostly copper, with a petroleum base. I have found that it does improve contact resistance. You do not need to put it thickly on. Also, make sure you sand it with fine grit paper, a bit pointless if the oxide layer is still there.

big5824

Tue Jun 02 2009, 05:30PM

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

Does anybody know of any similar products that I can find in the UK? Iv searched all over the internet for companies in the UK or that deliver to the UK with no luck, and most of the shops near me dont even know what im talking about. (Iv mainly been looking for penetrox and kopr-shield so far, I dont know any other common brands)

rp181

Tue Jun 02 2009, 06:07PM

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

When I asked this question earlier, someone suggested ox-guard from home depot. See if a local hardware store has that.

big5824

Sun Jun 07 2009, 09:51PM

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

More pics:
Assembled break beam sensors:
Dscf0321

Assembled SCR clamp, still needs oxide inhibitor on the aluminium plates:
Dscf0323z

Carbon fibre tube + steel rod for making projectiles:
Dscf0328n

Coil holders (not glued down yet, still waiting to slot barrel):
Dscf0327

Capacitor bank is now also fully housed with a built in discharge switch (load is 2x 60w lightbulbs), forgot to take a picture today though.

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