DIY vacuum-pump? (for vacuum impregnating)

Hi!
I need a better vacuum pump and a new shirt*)
Has anybody got plans for a vacuum pump? Or at least a clue what pressure I
need for vacuum impregnation coils?
My injector pump is only making -0,85 bar and I think this is not enough.
*) I'm winding small ignition coils for a new 4 stroke. One part is impregnating the coil in vacuum (8 kV!) with some special paint. The other one is casting the coil into a container. I did that with 2K silicone. When I injected the silicone from the buttom (buttom up) the cap popped off and spread the silicone all around. :-)
Nick
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Nick Mueller wrote:

Search Ebay for something like an Edwards Vacuum pump. Though generally heavy little buggers.
Wayne...
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Make sure you include "edwards" in the search or even better "edwards vacuum pump" some very strange things on eBay...
:-)
Steve
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Steve W wrote:

No disfunction here. :-))
Nick
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On the European side of the channel you're more likely to come across Haereus vac pumps although I _believe_ they may all be the same company now.
Richard
wrote:

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Have you any articles on the coil construction? I'm building a four stroke but abandonded spark ignition because I couldn't find any home brew in this space.
Steve
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Steve W wrote:

No articles. It's a bit complicated. The coil I have built (version 2) is quite small. OD 23mm, length 40mm. Sizes for just the coil. Some pictures:
The test setup with my first coil (OD 30mm): <
http://www.peters-cnc-ecke.de/forumupload/uploadFiles/16348_111971633898_sparcs.jpg
The V2 coil compared to the V1 together with the housing: <
http://www.peters-cnc-ecke.de/forumupload/uploadFiles/16348_111971633898_coil-1.jpg
And the finished coil: <
http://www.peters-cnc-ecke.de/forumupload/uploadFiles/16348_111971633898_coil-2.jpg
Neat, isn't it? :-))
Problem is, without electronics, it will not work. The coil is charged with 20 A for 0,5 ms. It can't be loaded for a longer time, because of its low resistance of the primary winding. This coil has a energy of 50mJ, in cars you find 120mJ. To make the coil even smaller, I'll have to find out what the minimum energy in the spark is. Maximum voltage is about 8kV.
Electronics is more ore less a IGBT and a suppressor diode to limit the primary induction voltage. I had 400V induction voltage but now will switch to 500V. A IGBT is fast enough, no problem. Initially I thought it won't work. Can't scope the high side of the coil. No 10kV HV probe here and to expensive. Just bought a 2.5kV probe for the primary side.
The processor is a Propeller from Parallax. Software now is ***very*** primitive. This is next to be done, beside making better coils. Anyhow, it's running on a single 87ccm 4 stroke.
Here is the data for the coil: Core is a Micrometals P2448-240. primary winding is 100 windings of 0.95mm, secondary is 1500 of 0.2mm. The later being double isolated (IIRC 3.5kV). and isolation layer (Polyester sticky tape, 0.06mm, 4.5kV) between each layer of windings. I tried TEX-E wire for the 0.2mm wire but it makes you poor, is thicker and doesn't make things better. The isolation now should be on the save side, so the coil can be used "open" (no spark plug connected). If someone is verifying my data and finds some errors. Yes Amp-windings is high! Design isn't finished, because electronics and coil have to match and its a bit of try and error. Tweak this parameter, tune there ...
I'm expecting to get about 50000 sparks per minute. But I'll see...
Nick
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On Sat, 08 Sep 2007 10:37:45 +0200, Nick Mueller

Some odd comments
AC coupled high voltage probes are easy. If you shunt the 1 Mohm scope input impedance with 10,000 pf and couple it to the HV source with a few pf you have a coupling time constant of 10mS which should be low enough for your application. The "few pf" is conveniently a few turns of wire wrapped round the high voltage leadout wire.
I assume that your high voltage winding is the first winding on the cylindrical powder core so that the primary is on the outside and near ground potential. In this case it would be useful to reduce the effective airgap of the open solenoid by adding some iron on the outside diameter. Ideally this would be a thin iron powder cylinder but in practice a few "I" laminations from an old "E" core would serve.
High voltage is more important than high current for effective spark ignition. The RFI suppression series resistance in spark plug leads is typically 10,000 ohms! This means that the resistance of the high voltage winding is unimportant - for maximum voltage in a given volume it should be wound with the thinnest practicable wire - 0.05 or 0.1mm.
Because you are layer winding the HV secondary the interturn voltage is only a few volts so standard enamel or Lewmex wire is quite OK - double insulated or TEX-E wire is not necessary. However, in order to keep down the secondary self capacitance, be generous with the interlayer insulation.
With single insulated thinner wire you've room for a lot more secondary turns. Your turns ratio is only 15:1 - the turns ratio on a 12v car coil is about 60:1.
For experimental trial winds, oil or wax are convenient impregnants that still permit dissasembly
Your're working on an interesting project so keep us in touch with the results - experiments that don't work are just as interesting as those that do!
Jim
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snipped-for-privacy@yahoo.com wrote:

That's exactly what I'm doing right now. Yesterday, while falling asleep, I remembered having a book (1977) titled something like "Electronic Motortesters". and I found a neat circuit including a suggestion for a HV-probe. Easy to do. Some POM, some wire and a tube over it. Epsilon of POM is known. So capacitance can be calculated and will be 4.7pF. Just bought a 2.5% capacitor for the divider. I'll add a suppressor-diode for security.

No it ain't. I know the argument better cooling of the primary, but it also would increase the length (bigger circumfence(SP?)). Next version will have 1.12mm wire (ca. 30% less resistance) and I get the layers full, so no space lost. Wire already sitting here.

OK, good hint/argument!

So I'll change that too. Will give me room for more winding. But I'd like to reduce the size too in a next step. But then, I only can base a completely new design on results I get with the current one.

No, it's getting close to the 0.5..1kV where start of one layer meets end of second layer. But the double insulated (TEX-E is triple and quite thick) is good enough (with above 2kV and thus 4kV wire/wire).

OK, thanks for that hint!

Winding ratio is 15:1 but voltage ratio is 16:1. I know of the much higher ratios of other coils, but they don't get the dI/dt on the primary I have (500V inductance voltage currently; will investigate in other IGBTs that can accept higher UCE). Anyhow, I'll measure, check and experiment with that too.

No failures until now. :-)
Thanks a lot for your input! Nick
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Nick Mueller wrote:

Works and still alive! :-)) Turned probe out of POM (> 50kV break through voltage) and cast in PUR (similar break through voltage).
Nick
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On Wed, 12 Sep 2007 23:35:29 +0200, Nick Mueller

Sounds good. A few extra comments.
With very thin wire you would find it a lot easier to work with the small diameter and smooth surfaces of an inner HV winding.
The voltage stress at the alternate ends of the HV layers will be handled by the relatively thick interlayer insulation. The stress on the wire coating will still be pretty low at this point.
A few iron laminations on the outside typically increase the nH/turn by about 50%.
You're driving the core pretty hard (2000 AT). It's worth checking to see if you're losing permeability. Plot HV output against peak drive current starting with very low currents. The HV increase should be at least 80% of the ideal linear increase.
Jim
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snipped-for-privacy@yahoo.com wrote:

OK, so I'll try it the other way round in version 4 and see what I get. Copper losses should be in the range of 10W for now, at full spark frequency (about 500Hz, yet to find out).

Problem: No L-meter here. I know that I will be off, no guess how much. I think I can see that on the current drawn. There should be a "bent" as soon as the coil is fully loaded (or the core is saturated). An L-meter for the inductance I'm working with (0,25mH) costs 250.- EUR :-(( Couldn't find a DIY solution. That's why I ignored the suggestion you already made.
I can calculate the inductance with the charge time and the current and the voltages. I'll see what I made.

Yes, I'm aware of that. Is on the to-do-list! Currently have some nasty spikes on the power-supply and will have to change the R in front of the IGBT's gate. Now, it is much too fast. I can adjust both on and off timing. Some tuning required. Anyhow, an engine is running with it, so it can't be that bad. :-)
Spark duration is 0.4ms. Not so bad. But currently only measured with a spark laying on the table. I should get different result in an engine. -> Scope back to the shop, off the kitchen table.
Thanks for your helpful input!
Nick
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snipped-for-privacy@yahoo.com wrote:

Thin wire tends to be torn appart. Don't ask how I know! :-) Tried it with 0.112mm (the thinnest double isolated I got). Works great! For some strange reason, the secondary voltage increased beyond being able to measure it. 8-/ Might be a problem with my HV-probe.

Done. Looks good. And now, I'm also getting the 20A. The Al-value of the core is in fact higher, so I had to reduce the primary windings.

Couldn't do it that way, but I reduced voltage and measured the current. I'm at about 15% off of the linear increase.
Now to less secondary windings. I'm getting above 16kV with a spark plug connected (can't believe) and I'll verify it with less secondary windings.
OD of the coil is currently 22mm. Much slimmer than the first version (30mm). Not much left to reduce, but maybe I get 21mm with my improving winding-skills and better layout of the start/end of the wires.
I'll have a beer for the very kind supporters!!
Nick
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On Wed, 19 Sep 2007 20:37:23 +0200, Nick Mueller

Sounds as if you're making real progress!
Successful winding with thin wire is something that improves with practice and an insistence on dead smooth small diameter surfaces without a single wire out of place. The mechanical disturbance of thicker leadout wires can be a problem.
One method that is sometimes used is to abandon the separate leadout wire and simply form a leadout tail by double backing the winding wire back on itself 6 or 10 times. There is only a single winding wire that usefully carries current - the rest are just mechanical support.
Ideally self fluxing wire coating is used so that the end connection can be made with very hot soldering iron. However it's still possible with ordinary enamel coating. Light a VERY SMALL pot of methylated spirit (denatured alcohol) and burn off the coating in the almost invisible flame. Just as the end of the leadout reaches red heat dunk it in the alcohol and then rapidily withdraw it.
Provided you have adequate interlayer insulation (which is also useful in reducing secondary self capacitance) I still think double insulated wire is not essential. If you're pushing for smallest overall size 47SWG (0.08mm) should be entirely practicable.
Now that you're getting decent output voltages a 2 ball spark gap might be a useful cross check on your HV probe
Jim
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snipped-for-privacy@yahoo.com wrote:

Sorry, I don't understand that all all. Does that mean to wind some smaller diameter wire in the groves to fill them? I have some stiffer/thicker isolation that I intended to try with between primary and secondary.

They can be soldered at 400°C.

I have read to press the wire onto a "Spalt-Tablette" (umm, that is an headache pill, with ... forgot the chemical name, quite easy to do) with an hot soldering iron. But didn't try it.

You might be right. My intention is to have them withstand an open secondary side. Until now, it works. But I'll try a single isolated wire with the suggested 0.08mm. Seems my collection of wires is getting bigger and bigger. :-)
Did you know that the voltage on the HV side has to be negative? All the scope-pictures (of motor-testers like Sun, Bosch etc.) you see have an inverted display. The center-electrode is hotter and thus can be easier ionzed to start the spark.

The rule of thumb I know is 1kV/mm. But that depends on the shape of the electrodes. Do you have an relyable setup with two balls? 2kV/mm?
Nick
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On Thu, 20 Sep 2007 07:33:49 +0200, Nick Mueller

snip
The usual trouble is that the soldered joint between the winding wire and the thicker lead out wire forms a bump which disturbs the smooth surface. This scheme eliminates both the joint and the thicker wire
Pull out the first foot or so of the thin wire off the reel and fold it back and forth on itself several times so that the wire from the reel now has a multi stranded tail - typically 7 or more strands.
Twist most of this fairly tightly so that the single wire that continues back to the reel is mechanically supported. Do not twist the last bit just before it continues as a single wire.
Now when you start the first layer of the HV wind this last bit can be teased out flat and taped down so that it projects no higher than the remainder of the layer.

For uniform field, 30Kv/cm is the generally accepted gradient. 20mm balls are adequate for up to 30Kv, proportionately less for lower voltages.
For air close to normal temperature and pressure, breakdown voltage is proportional to pressure and inversely proportional to absolute temperature.
Needle gaps or sharp corners break down at much lower voltages.
Jim
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snipped-for-privacy@yahoo.com wrote:

Learned a new trick! Just tried that (modified) and it works great. Now I don't have to fear to break the tiny wire when putting the coil into the housing and connecting the thick and stiff HV-cable to it.
Thanks!

OK, will build that one too ...
Still takes me 1 1/2 hour for a coil. I even missed The Simpsons! Need a food-pedal for the winder motor.
Thanks again! Nick
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Hope I have read this correctly - Hemingways new IC engine booklet has some ideas and parts.
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Sandy Morton wrote:

Do you mean this stuff? <http://www.hemingwaykits.com/acatalog/Ignition.html
Childish. <G>
Nick
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Guess what !!! Mine is an Edwards high vacuum pump !!! Bob
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