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Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874
Ash about the leakage current or drain on the battery, they are the samething but negtively corlated, leakage current is small at first, drain is high, then leakage is high and drain is low.
Is a voltolic pile a battery ,a capactor, or a diode with the part that has the large effect on its outcome, a electrliclight capactor at a rating of 400volt can only withstand 5 volt wired wrong, but at the factory when the set the porilty they can supply 10mA to a 10000uF cap to set is oxide layer, with the rest of the current being supplyed by the reaction.
Just some thoughts ;)
If its a battery, a non-polar substance with high charge carrier density, if its a capactor a polar substance with the miniunm possable charge carriers.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Andy, with a well made capacitor leakage current s negligible. The leakage current of most electrolytics is awful in comparison, but I don't think it's the leakage current that's the problem. An uncharged cap 'looks like' a dead short to a DC supply, at first, anyway. When it starts charging the current slows exponentially, to the point where it never actually fully charges to the potential of the DC supply.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874
True that but with 4nA the 1700volt would drop to 1volt with a cap that large making the short draw 3nA, as long as the device can continue supply 4nA the size of the cap shouldnt be a problem.
Does the above make any sense?, I dont think it is a dc source as we know it, so to speak.
The easyest way to make this work is have a 70volt neon keep the cap the same, but make the pile 20-30kV, Q=CV2 and Ctot = 1/(C1+C2+C3...)
.whatever the reason, it makes the capacitor system untenable.
If the capacitor is small enough, current drain will be much less.
3.3nF is obviously far too big.
If you reduce capacitance by a factor of a thousand, say a few picofarads current drain may well be acceptable.
Maybe reducing by a factor of one hundred would be sufficient?...say a few tens of picofarads, or maybe somewhere in between these values?
Adding a 'very small' amount of capacitance, and using your string of high efficiency LED's should give a brighter flash.
I think the issue here is that 3.3nF is far too large for your pile to charge.
I'm actually working on a project that uses tens of picofarads and a spark gap myself, but mine is designed for much higher voltages.
I'm building my own caps, and hope to post some 'very early' test results soon. My initial setup will be very crude, and will still need a lot of development after I start firing the gap, I just want a 'base point' from which I can develop the idea.
If the capacitor is small enough it should work, and it should be brighter than no capacitor at all.
Edit: I've just had another idea. You can do away with the spark gap if you use enough LED's in series. I'm not sure what the exact 'forward voltage drop' of your high efficiency diodes are, but the averave LED is around 3-3.3V. If you string enough of them together in series, say 150, the forward voltage drop of the string will be around 450V (you can adjust this to the exact voltage you require). Every time the capacitor charges to whatever total Vfd you choose to set, the string of diodes will 'fire'. Spark gaps are lossy, you don't actually need one
Maybe someone else can confirm this idea, or can find a flaw in it?
EDIT: LED's can sustain much higher current for a very brief time, generally 100mA for a diode rated for 20mA. If current becomes too high at these voltages you may need to add an inductor, but at the moment I'm assuming that won't be necssary.
EDIT: I can think of a few tricks you could use to use a larger capacitor, which doesn't fully discharge, so, due to the exponential drop in current draw as it charges, current drain on the pile would be less, but it's nearly 3am here now, and I need to give it a bit more thought before posting.
Your point is well taken. I will try a few pico-farads and see what happens. I think I will hook up my picoammeter while it is charging so I can see what kind of current draw is happening as the cap charges.
My batteries have a 4 nanoamp draw while running my Device, I have never measured the short circuit draw...mainly because short circuits are so harmful to my batteries.
This time I will hook up a "factory seconds" battery and watch the draw. Once I an confident that it is OK, then I can hook up a bigger battery.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ok. As I said, if current draw is still to high I do have a couple of other ideas, but I'd like to see the results of your experiment first
EDIT: I think I may have found a flaw with my 150 LED string idea, but I'm sure there is a way around it. Using a shorter string and spark gap is one way, but there are probably other solutions.
First thing is to experiment with some smaller caps, then maybe experiment with caps that just partially discharge, but I'm jumping ahead here.
I think the 150 LED string idea only works with eg rectified AC, I did a Spice simulation of it a couple of years back. I think the diode string may prevent the cap from completely discharging, though.
Maybe I should do another simulation, but not tonight.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
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Zamboni, to help understand the current draw, teate the pile as a low Farad cap being charged by another battery, at 20kv max, it will charge upto 1700volt in a half second 0.01RC and then taper off, so the current draw max from the thing is based on max voltage(ie the voltage it reach with a dc short, removed then one second later), then resistance.
To save the expermint just look at ,capactor discharge time constant, match the voltage source to 1700v the capance 1pF, and keep changing the resance value to the amps reach 4nA in one second, that will tell you the tc of the load aswell
You have 25000000000ohm for the pile, and a max contious charge curret of 6.8e-8 A for a 1nF load cap, add a 200k resistor, for a tc off 0.024sec to reach steady current of 3.4nA for one second, with approx 300volt reached
The sim acrees with the values.
[Moderator edit by Mads Barnkob, double posts merged]
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Hi Andy,
I have other numbers for the time to charge the 0.001cap with 400V.
1 - assume that the max current to be draw from battery could be 20 nA (ZAmbone, please confirm) This 20nA is the current drained from the battery to start to charge the cap 0.001uF
2 - If the battery supply 1700V, the series resistance (battery internal resistence + outside resistor ) could be 1700/ 20x10e-9 = 85000megohm.
3 - them the time to charge charge the 0.001 cap with 400V may be: dt = Cxdv/i = 0.001x10e-6 x400/20x10e-9 = 20seconds or more .( if the current is constant)
The value of 200k resistor not was considered , it is too low in comparison with other resistors.
Do you have in your junkbox any resistor in the range of 500000 megohm ?
And
What are the values of your high resistors ?
Largest I have is 1 mega ohm.
Paul
Newton Brawn wrote ...
Hi Andy,
I have other numbers for the time to charge the 0.001cap with 400V.
1 - assume that the max current to be draw from battery could be 20 nA (ZAmbone, please confirm) This 20nA is the current drained from the battery to start to charge the cap 0.001uF
2 - If the battery supply 1700V, the series resistance (battery internal resistence + outside resistor ) could be 1700/ 20x10e-9 = 85000megohm.
3 - them the time to charge charge the 0.001 cap with 400V may be: dt = Cxdv/i = 0.001x10e-6 x400/20x10e-9 = 20seconds or more .( if the current is constant)
The value of 200k resistor not was considered , it is too low in comparison with other resistors.
I believe that you are asking about the maximum safe current that can be withdrawn?
I do not know the answer to that question. I know that the maximum I have used is 7 nano-amps. My Picoammeter is not working properly, so I don't know the "short circuit" amp draw.
You time estimate almost matches what I observed. It took the battery 59 seconds to charge the 3.3nf capacitor that I was using. I have ordered a 2 PF capacitor, it should be here late this week. I will probably test on Saturday.
Paul
[Moderator edit by Mads Barnkob, merged double post]
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I came up with this, I think it can be 'tuned' to any current we specify, to a point. I think it's self explanatory. The capacitor only discharges partially when the gap fires, then the second cap, which is a smaller value than the first slowly discharges through the resistor. When the voltage across the spark gap reaches breakdown voltage, it fires again, and the process repeats, ad infinitum.
Not sure how feasible it is to build, though. You charge the first capacitor before connecting the pile. If it only discharges partly, due to charging a smaller capacitor, the current stays in the 'pointy bit' at the top of the exponential charging curve.
The trick is to select suitable (and realistic) component values. Not sure what the frequency would be either, but that could be tuned to 'how bright you want the LED to flash', at least in theory. Components will certainly be a challenge.
EDIT: I think the value of the second capacitor is approximately the same as whatever value capacitor charged to the breakdown voltage of the spark gap, when discharged through the LED, lights it sufficiently. The first capacitor may be ten to twenty times the value of the second one (It needs to be big enough so that, once it partially discharges, it only re-charges at a rate the pile can handle). It should be possible to find a suitable resistor.
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Light LED with 4 nanoamps
