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Homemade Induction Coil

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LAN_403

Omicron

Fri Mar 17 2006, 02:52AM

Registered Member #131 Joined: Fri Feb 10 2006, 09:25PM
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Posts: 185

For fun I am building a homemade induction coil. The secondary will be 26-gauge magnet wire. The primary will be 16 gauge bare copper wire. The core will be 3/8 mild steel, will that work? The caps will be Plexiglas. One more question can the secondary be in direct contact with the core? When I am finished the coil will be under epoxy or oil for safety.

dan

Fri Mar 17 2006, 03:11AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

Depending on frequency I would be worried about eddy currents in the solid steal core. also 3/8ths for a core seems really small. And 28 gauge wire sounds really thick for the secondary.. You will need 1000's of turns to get any decent output and with such a large size wire the secindary will also be vary large.

One more question can the secondary be in direct contact with the core?

Yes ignition coils do this. The secondary is connected to the core and is used for one of the HV terminals.

Btw do you mean ignition coil?

Omicron

Fri Mar 17 2006, 03:24AM

Registered Member #131 Joined: Fri Feb 10 2006, 09:25PM
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Posts: 185

dan wrote ...

Depending on frequency I would be worried about eddy currents in the solid steal core. also 3/8ths for a core seems really small. And 28 gauge wire sounds really thick for the secondary.. You will need 1000's of turns to get any decent output and with such a large size wire the secindary will also be vary large.

One more question can the secondary be in direct contact with the core?

Yes ignition coils do this. The secondary is connected to the core and is used for one of the HV terminals.

Btw do you mean ignition coil?

I chose 3/ 8 because that would give me around 1 inch around. So 1 turn gives 1 inch of wire. The wire is 26 gauge, there will be thousands of turns. Around 14,000 turns will be used. What do you think the very smallest core should be? No I mean induction coil. An ignition coil is for autos, BTW they might be called a primary secondary coil.

I will try to find 30 gauge wire for this.

Hazmatt_(The Underdog)

Fri Mar 17 2006, 03:53AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You're going to need to go with lots of smaller rods of iron or steel rather then one big core. The issue is eddy currents in the core causing large losses. Lots of smaller rods that are varnished or surface oxidized like a real transformer will improve the performance to a large extent. This could also be an experiment if you wish to prove it out.

I was going to build one of these as well, but when I figured out the turns count it was not managable. I had a 12" long form of about 6" diameter, and 29 AWG available in 10 lbs, but my target voltage was 50,000 volts and the number of layers was simply rediculous!

If you want to do it I would suggest getting Fish Paper or Cambric paper which has 2KV/mil insulation and put that between layers. You may want to wax or varnish, depending on what period or how authentic you want to go, the whole thing so that it doesn't consume itself in operation.

I think there might be a large plate capacitor in the base of these things. A lot of induction coils of the period had a condenser, like the violet ray thread too. So that may be important to look into. You might find that you don't get much spark until you connect a large plate capacitor, or modern capacitor across your primary coil.

Sulaiman

Fri Mar 17 2006, 02:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Long ago and far away I made a quick induction coil using a 1" diameter 12" lond steel rod;
(approx.) ..... it was RUBBISH !!!
This eddy-current stuff really is true, and very significant.

Recently I made a quick induction coil using 1" x 1" x 4" ferrite - excellent.

I have an old steel laminated transformer core - that works well too
I've tried iron-filings in pvc tube - ok but not great.

If you google you can find many examples e.g.

Unless you want to spend a lot of money to recreate a vintage coil
I suggest you stick to automotive ignition coils and CRT flybacks
There are so many ways to make an Induction Coil less than perfect.

I do advise making a quick induction coil so that you can experience all the things that influence the design of high voltage induction coils etc.
For this I suggest;
Dis-assemble the largest steel laminated transformer that you are willing to sacrifice;
Use about 1/3 of the original bobbin to wind the primary with insulation
Use the rest for the secondary with LOTS of insulation.
Use all the E laminations together as the core.
Later experiment with thin spacers between the E core and the reamining I core.

Omicron

Fri Mar 17 2006, 10:01PM

Registered Member #131 Joined: Fri Feb 10 2006, 09:25PM
Location:
Posts: 185

This is getting very complicated. I donâ€™t know if this is really worth it. 10 lbs. of 30 gauge wire costs $120!
Is there crude relation ship between # of turns and voltage?

Hazmatt_(The Underdog)

Fri Mar 17 2006, 10:24PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Yes! N1/N2 = V1/V2

Omicron

Fri Mar 17 2006, 10:53PM

Registered Member #131 Joined: Fri Feb 10 2006, 09:25PM
Location:
Posts: 185

N1= Number of turns in the primary
N2= Number of turns in the secondary
V1= voltage in the primary
V2=vooltage in the secondary
So if I have 50 turns in the primary and 1000 turns in the secondary, and I put 50 volts in the primary, the output will be 1000 volts?

Sulaiman

Fri Mar 17 2006, 11:03PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The above equation will tell you what voltage will appear across the primary
for a target secondary voltage.

Ignoring losses for now, it's basically conservation of energy
Energy in magnetic field = 1/2 x Lpri x Ipri x Ipri
Ideally most of this energy will go into the capacitance of the secondary
(for highest voltage)
This includes the effective winding/parasitic capacitance of the
Induction Coil secondary and any external (stray) capacitances.
So 1/2 x Lpri x Ipri x Ipri = 1/2 x Ceff x Vsec x Vsec

Vsec = Ipri x SQRT(Lpri/Ceff)

No mention of turns!

P.S. Before you start this project give some thought to the power supply,
I suggest a 12 Volt Lead-Acid battery.
The peak current in the primary will be 2 to 3 times
(depending uopn design) the average power supply current.
What will you use as a switch/interruptor....

Dr. Shark

Sat Mar 18 2006, 03:58PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Not so sure about the formula with the turns ratio. It is an inductor after all, so a high voltage peak (couple of 100V) is induced in the primary and then further amplified by the turns ratio. So in total the voltage gain will be much more than that from pure transformer action.
Anyway, there are some nice pictures (and German description) of exactly what you want (or probably not want, since it is a lot of hassle) here:

I guess what you ought to know is that is is 110,000 windings and 20 miles (!) of wire.

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