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Registered Member #2269
Joined: Tue Aug 04 2009, 02:00PM
Location:
Posts: 4
I've been a Troll here at 4hv for about 2 years now, work and school suck up most of my time so I havent had much time to tinker. Last week my vacation started so I decided to get a boost converter up and running, couldn't get the bugger to work until last night; all it needed was a pulldown resistor on the MOSFET gate...... don't know why i didn't think of it sooner. I HAD contacted some EE profs at the university to ask but being i'm a MEC E they were not about to help me. I'm sure this boost converter is highly innefficient but at least it works.
Anywho it's putting out 140V from a 12V source and i can adjust it up to 213, but i do have a question. My caps are 160V 3300uF, is a 20V difference enough so i do not have to worry about overcharging them? Or am i going to have to control the charging somehow. Pic is below (excuse the messy desk)
Planning to wind the firing coil tonight sometime, I have 69 of these caps so i'm trying to figure out how large a bank i should make. Any suggestions? I am However dreading transitioning this onto perf-board... whenever i do something always gets screwed up.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
160 Volt capacitors are ok at 160 Volts .. that's what the ratings are for. (A 160v CAP WILL PROBABLY HAVE A 30 SEC SURGE RATING OF >= 200v) If you want long lived capacitors then derate the voltage but for this application it will be excess current or voltage-reversal that will kill the caps. DO NOT FORGET THE REVERSE POLARITY PROTECTION DIODE(S).
Since this is your first attempt I suggest that you start small with just one capacitor to get a 'feel' for it (smaller explosions/less cost) then go for it!
I don't know who is the current 'expert' on coilguns here, but I do remember Barry contributing massively. Visit his website
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
- Design your capacitor bank layout to have low inductance (ie: don't do a 'coil' layout, do parallel strings and then tie them all together). - From the looks of it, your inductor is wire wound on a nail, which is probably poor for your frequency and duty cycle resulting in losses. Shorten the leads going to the inductor and change the core material to suit your frequency. - If heatsinking for either the fet or diode is needed, make sure you check to see whether the tabs are electrically common (highly unlikely) and add insulating material as needed (sil-pad). - Don't forget to add a diode across the capacitor bank to catch any back-EMF from your coil.
- Also think about adding a switch (dual pole for both - and +) to disconnect the charger from the capacitor bank while firing - Use the inductor sim and the LCR sim on Barry's website to simulate your coil and the voltage/current waveform just to make sure your switching semiconductor is rated for the peak current, and that your back-EMF diode is rated for the voltage/current ringing.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
aonomus wrote ...
- Design your capacitor bank layout to have low inductance (ie: don't do a 'coil' layout, do parallel strings and then tie them all together). - From the looks of it, your inductor is wire wound on a nail, which is probably poor for your frequency and duty cycle resulting in losses. Shorten the leads going to the inductor and change the core material to suit your frequency. - If heatsinking for either the fet or diode is needed, make sure you check to see whether the tabs are electrically common (highly unlikely) and add insulating material as needed (sil-pad). - Don't forget to add a diode across the capacitor bank to catch any back-EMF from your coil.
- Also think about adding a switch (dual pole for both - and +) to disconnect the charger from the capacitor bank while firing - Use the inductor sim and the LCR sim on Barry's website to simulate your coil and the voltage/current waveform just to make sure your switching semiconductor is rated for the peak current, and that your back-EMF diode is rated for the voltage/current ringing.
How much current would you say is safe for charging a capacitor bank? I am considering a Mazzilli driver to charge my bank, but at 50W output I'm worried about hurting my caps.
Registered Member #2269
Joined: Tue Aug 04 2009, 02:00PM
Location:
Posts: 4
Thanks for the Input guys. I ended up soldering most of the permanent board together last night, i'll put up some pictures once it's finished. I added a power switch and heatsink for the MOSFET. I was planning to add these anyway but the breadboard was just to make sure it worked. I think i tried logging in one too many times at home (couldn't remember my login it's been that long since i posted); so once i can get back on at home i'll upload the pics.
Oh and the Inductor is wound around a plastic bobbin for a sewing machine, i simply put the nail in there to increase the inductance but it's not necessary to get the 150V that i'm going to run it at right now. It will put out 180 easily with the current inductance.
Registered Member #2269
Joined: Tue Aug 04 2009, 02:00PM
Location:
Posts: 4
Alright so of course as i restart my work on this project 10 more pop up. But i managed to get the boost converter into a housing and started playing with the caps. I attached a few images, the circuitry that used to occupy that housing was damaged and i thought it would be a shame to waste such a nice housing. The MOSFET is screwed directly to the housing with some thermal paste for good measure, keeps it nice and cool. I did add a ferite core to the bobbin but i did so right before putting it in the housing so there isn't a pic. Power switch will be external, the case is a bit crammed at the moment so there wasn't space.
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Boost converter.... finally
