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Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
I finally got some pictures!!! One of the pictures includes a 26 strike, limited because the ceiling is so close (it's sitting on my workbench) I redid the bridge and it works even better! OCD is working perfectly, everything works great. Unforunately, everything's limited by my 2.5A variac. So, I modded an MOT which worked, but I messed it up and I'm going to have to rewind it.
Pictures + blog at my site: It's in the Projects>DRSSTC section. Anybody know how to make a link inside a frameset?
But, I had an idea that I could just ditch the variac and include a charging circuit instead. I've designed this, but I've never designed anything high voltage + digital. The comparator reads the voltage of the power supply, and the voltage of the capacitor. Then, when the capacitor is full, the transistor turns on. R4 is a dummy for a relay that shorts R3. I'm asking, although it checks out in sim, is everything OK before I build it? Also, what would be a good place to put a "still charged" LED?
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
you can build a pre charging circuit much simpler than with a comparator.
simply add a say 10watts 200ohm resistor for charging the cap. a voltage divider out of 1 resistor and an relais would do the job, . the cap charges up trough the 200ohm resistor and the divider switches the relais on at say 200volts, then the first resistor is bridged via the relais, thats it, works fine on all my drsstcs. the disadvantage, you have to use thick resistors with 10watts or more, the power dissipation heat them up for the whole runtime, so you will waste some 10watts of power. the advantage is that its the simplest you can build.you dont need a discharge resistor anymore, the voltage divider does this when the coil is turned off
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Since its AC, you can also use a capacitor as a ballast. Something around 10-20uF should work well (look for motor run caps, they would work perfectly here).
Your charging circuit worries me a bit. You should rather just use 2 high impedance voltage dividers, both fed from diodes (so they ignore the ripple). But, one problem is that theoretically the comparator will never trip. The cap voltage will always be *slightly* less than the supply charging it (through any real resistance). So you are better off just setting the comparator threshold from the 12V supply "artificially". Once you are within 5V of the max, its pretty safe to just switch it full ON.
Or go with a simple approach and just use a manual shorting switch.
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
Thanks for the replies guys! I think I'll go with the relay thing.
And Steve, thanks for the comparator thing. That's why I asked first, I was worried about the same thing myself, but being somewhat inexperienced, I wasn't sure what would be better. Duh.
Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
You can also find relays on Ebay that are time delay ones- I used this with a ~15 ohm resistor in series with the load, and then after three seconds, the relay would short the resistor. It worked very well.
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DRSSTC pre-charging circuit question + spark photos
