Step 4: The capacitor--updated

This is the most finicky step in the project. Your choice of capacitor will determine how well (if at all) your coil works.

Use a film capacitor rated for at least 100 volts and 1uF. 100v may seem low, but the cap I used with greatest success is rated at 100v, while higher voltage rated caps only made the coil buzz.

DO NOT USE ELECTROLYTIC CAPACITORS! These are the ones that look like little metal cans. Even high voltage, high capacitance types failed on me *every* time. They got very hot, they bulged, and one even exploded. No joke--leave them alone! Stick with film types. There may be others that will work as well, but my best result came with a cap salvaged from a computer monitor circuit board.

It is marked:

I measured the capacitance of this cap with a multimeter and got a reading of 2.178 microfarads. With this cap I get good long sparks, cool operation, and no overheating of the coil, the switch, or the cap, although I have not run the coil for long periods. The longest I have let it run was for 60 seconds.

I mount the cap in the jaws of the clothespin. The lower lead goes to the output cord alligator clip, and the upper lead to the dimmer switch.

Update as of 3/23/09: I obtained some metalized polyester capacitors rated at 2.2 uF at 250V from an eBay seller . They work very well, and give a greater range of output on the dimmer (by this I mean I get more sparks at higher settings than I did with the 100v cap). The new caps are marked:


Tolerance is rated at 10%.

Update as of 4/22/09 While tearing down a surplus microwave oven, I came across an array of capacitors I thought I would try in the coil driver. (These are not the usual metal can capacitors associated with microwaves; they are high voltage film-type caps). A couple of 3KV caps did not work, but a 4.5 uF, 250 volt cap gives much greater sparks than anything I have used before. Discharges are longer, thicker, and louder. They are no longer blue-white, as with the previous caps, but a kind of vivid orange. Does this indicate a lower temperature, I wonder?

The cap is marked:


Update as of 4/18/10:
Even better results have been obtained using a 10 uF, 330 volt VIOT motor start-run capacitor. This is a large, oval metal-bodied cap I found on eBay. With the motor start capacitor the coil produces much more powerful arcs. I have used this coil and cap combination to power my Tabletop Tesla Coil with success. If you can get a motor start cap like this, by all means use it.

It's customary to include a schematic (wiring) diagram that shows the electrical connections in a very clear form. One easy way is to just draw on paper, then cellphone-photograph it and post it. Another is to use Windows Paint program (mspaint.exe). <br> <br>Very good instructable. The high voltage will give you a poke if you're careless, and can make flammable materials burn, but shouldn't be enough to cause any real harm, in my opinion (unless it makes you jerk your hand away and jam your elbow into your buddy's face -- or a wall). The 120-volt side is lethal.
<p>It's accually lethal because unlike most transformers ignition coils are autotransformers which makes the secondary connected to mains making it lethal.</p><p>If you want it to be a little safer use an isolation transformer before the dimmer.</p>
could i use a 12v battery to power it also can i use another coil and a <br>GE 72F6211 CAPACITOR 10 uF 330 V thanks! <br> <br> <br>
If you use a 12 v battery, you have to have some method of pulsing the current to the coil. In the old days cars used &quot;points&quot; in a distributor, a set of mechanical contacts opened and closed by a cam. In order to power a coil, you'll need a relay or some kind of circuitry (like a 555 timer) to pulse the input. You can't just hook up the ignition coil to a battery; it won't work.<br><br>10 uF at 330 V will work well unless the capacitor is an electrolytic type (in that case the cap may overheat and explode). Is this a film capacitor?<br><br>Paul
Is this a good capacitor http://item.mobileweb.ebay.com/viewitem?itemId=251135299373&amp;index=4&amp;nav=WATCHING&amp;nid=54540176014 also ill use ac 120v
Looks like a motor-run capacitor. It should work fine. 10 uF will give you a very hot, loud spark, which might melt your electrodes if you run it very long. 3 to 5 uF is cooler. <br> <br>Beware of any capacitor! This simple circuit does not discharge the capacitor after you shut off the juice. It will hold a nasty, dangerous charge, so don't fool around with the capacitor even with the power off and unplugged! <br> <br>PBT <br>
How long do you mean?
Basically the charge can last for days or weeks. You can short it out with an insulated screwdriver, but for goodness sake be careful.<br><br>PBT
what exactly do you expect to happen when its shorted with a INSULATED screwdriver?
You discharge the cap without shocking yourself.<br><br>PBT
insulated screw drivers are insulated right up to just before the tip or including the tip to prevent the screw driver from shorting things out when working on live electrical systems.
That's one version, I guess. I simply mean a screwdriver with a metal shaft but has a handle that insulates the holder against the charge in the capacitor.<br><br>PBT
And would this one work? http://item.mobileweb.ebay.com/viewitem?itemId=320883429839&amp;index=0&amp;nav=SEARCH&amp;nid=58888822986
That would be just about ideal.<br><br>PBT
will any ignition coil work
Short answer, no. The modern HEI and electronicly driven coils are not a good choice for this system. Try to find one of the old fashioned &quot;can&quot; coils, or an epoxy encased coil like the one shown in the project.<br><br>PBT
How do I measure the output voltage of an ignition coil
Lacking an expensive high voltage meter (which I don't have) you have to estimate voltage based on spark length:<br><br>see http://www.kronjaeger.com/hv/hv/msr/spk/index.html<br><br>Paul
I got a 1 3/8 inch spark length how many volts do you think is coming out of it?
An inch of spark is ~20 KV, so 1.375 inches would be around 28 KV or so. Just as a rough estimate.<br><br>PBT
Hi,what cap.to use with smallmotorcycle to ignition plug(parallel conection)), <br>I used cap. 30KV 1000pf, to ignition coil's car it work good,but not small motorcycle'scoil <br>Regards <br>Sevi <br>you can email me;miracle_sevi@hotmail.com
1000 pF (which the same as 1 nF) isn't enough, nor do you need a 30KV rating. What you should use is a capacitor in the low microfarad range (1 to 3 uF) with a voltage rating of around 400.<br><br>Paul
Hi,Paul thanks for your kind email,I will try 3 uf and 400volt as you suggess(this is pararail connecting,not series),please tell me how much voltage of secondry of small motorcycle ignitiom coil(about 125 cc yamaha),for car's coil is about 30.000 volt <br>regards <br>Sevi
No, the capacitor is wired in series. See the article and diagram here: <br> <br>http://wiki.4hv.org/index.php/Ignition_Coil <br> <br>It's hard to say what the output of your motorcycle coil will be. It depends on the input voltage and the ratio of turns inside the coil, etc. I would guess it to be around 15,000 to 20,000 volts, but that's just a guess. <br> <br>Paul
how did you wire the film capacitor to ac power did you put it across the mains ?
See the article and diagram here:<br><br>http://wiki.4hv.org/index.php/Ignition_coil<br><br>Paul
I built this, and I finally have a working hv source. I am worried about my safety though, because even at the lowest level on my switch with about a total of 10 seconds runtime and that wasn't continuous, it is very hot. I didn't use a capacitor and it worked fine, although it isn't powerful enough for a jacob's ladder.
You need a capacitor to buffer the coil and cut down on that overheating.<br><br>Paul
buffer? To me it doesn't look like it does anything at all
I have run my ignition coil (with capacitor) for minutes and it never gets hot.<br><br>Paul
ok, so it is on the 120v end right?
See the diagram here:<br><br>http://wiki.4hv.org/index.php/Ignition_Coil<br><br><br>Paul
Now it isn't working, I tried going through the capacitor both directions to no avail, it is 100v and a 2.2 joules. Any ideas, I am going to try some more really fast, but this is the best capacitor for the job it seems. <br>Does there need to be a way to bypass the capacitor, I didn't pick up on it if there is. So maybe like a wire going from the dimmer to the + on the coil and a wire going through the cap to the +
100 volts is not enough. Try to get 200+ volts. I assume you mean 2.2 microfarads, not joules (joules don't figure in this). <br><br>Always use an appropriate capacitor or you face overheating, shorting, and failure.<br><br>PBT
Well you said in the instructable that you used a 100 volt one and you had to measure the capacitance because all it gave you was joules. I had almost the exact same capacitor and it didn't work.
Commercial capacitors are rated not only for maximum voltage and capacitance, but also for the tolerance of their internal components. These ranges are coded into the numbers and letter you see on the cap. If you have a cap rated at 100 volts, but the tolerance is plus or minus 20%, then obviously the cap is good at 110 to 120 volts without overstressing. Other 100 volt caps might only have 5% or 10% tolerance, and therefore will not be as resilient. My original film cap that was rated at 100 volts worked, but not as well as the motor run cap I use now, which is rated at 250 volts.<br><br>I can't answer every question you have because I don't what equipment you are using or for what purpose. If you use salvaged or 2nd hand parts, they may have weaknesses that cause them to fail more quickly than new components.
Are you using a DC or AC capacitor because, I have tried both and none work except for the electrolytic type which are DC for sure. I think that may be why they don't work, because they aren't rated for the AC, which would explain problems at voltages well below the top of their threshold.
He, he, I got it to work, it was really cool. I decided to give it a go with a can-type capacitor. It was 400v don't remember how many joules. Gave some really cool sparks for about 5 seconds. I was glad I finally had it working so I pulled apart my two rods and it would spark once and then stop, we would shut it off and then on and it would spark again, the third time it went tsssss..... and now I have to pull all the smoke alarm batteries. :( so how do I get it to work now, do I need bigger film caps.
Don't use electrolytic (metal can) type caps! They will overheat, explode, and even catch fire. No joke, don't use them. You can use film type, or motor-start capacitors.<br><br>Paul
Well, I know this, I believed you. I just wasn't getting anything else to work so I gave it a shot and it worked temporarily. I have a big box for it and i'm outside with it so its all good.
Thanks, I just pulled about 5 film caps from a CRT board so I should be set
i have a 12 V ignition coil , can it run off of 120V AC
If you follow the instructions in this Instructable and use a dimmer and capacitor, yes, you can run a coil off 120 V AC.<br><br>
does your ignition coil run from 12V
It could, but in this case it is being powered by 120 V AC wall current.
I have a few caps. Which one do you suggest I use? They are labeled as follows<br><br>474J DH<br>200SA U<br><br>823J X<br>400PP1<br><br>395JX<br>100MEF2<br><br>Thanks!
The last cap appears to be rated at 100 volts, so I would not bother with it. The capacitance is 39 x 100,000 pF, or 3.9 uF, which is OK, but the voltage is too low. You might try it, but it probably won't last long.<br><br>The middle one, 400 volts at 82 x 1000 pF or 8.2 nF. This is not sufficient to power the coil--you need something in the low microfarad (uF) range, like 2-10 uF and at 250 to 400 volts.<br><br>The first cap, 200 volts at 47 x 10,000 pF or 47 nF, is also not sufficient.<br><br>Check out this page to interpret capacitor markings:<br><br>http://www.wjoe.com/capacitorinfo2.htm<br><br><br>Paul
Thank you for the info. I also have a MKP &quot;box-looking&quot; type cap that is rated at 1 uf and 250 volts. Would it work?
It should work, but you'll get better sparks with higher capacitance.<br><br>Paul
Thanks! I just found a 250 v 22 uf i will probably use.

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