driving a tesla coil - Page 1
Forum section: electronics / beginners

author: GIGGINO94
author_rank: Contributor
timestamp: July 10, 2024, 12:35:41 pm
content: Hi i'm trying to make a tesla coil but in the process of winding the secondary i used an online calculator to calculate roughly the inductance of the secondary and it turned out to be around 0.09 uH for a secondary of 6.6 cm lenght and a 4.3 cm diameter and roughly 850 turns, so i calculated the resonance frequency of the secondary and topload to be around 160 Mhz, since i can't make a 160Mhz generator for now i was thinking of friving it with the capacitor with a spark gap that puts it in parellel to the primary so they can resonate ( hopefully at a similar frequency of the secondary) so to generate a high voltage to charge the capacitor with i designed the circuit on the image wich is simply an astable multivibrator made with bjts an then an h bridge that powers a transformer ( i don't know if driving the transormer with a square wave is the problem but i tought that since the square wave has infinite armonics it shouldn't be a problem becouse one of that armonics will also be at the resonance frequency of the secondary of t1 and the capacitor) but the h bridge doesn't work as intended and i don't get why, there seems to be close to no current in the load ( for now i'm trying without the tesla coil itself ) i also tried with two reversed leds but they don't turn on at all, but using only two mosfets of the h bridge seems to work just fine , but then of course the capacitor would get charged at a slower rate so i think that either i didn't use the mosfets right (is the first time i'm trying to design a circuit with them ), or i got all the idea of the driving circuit wrong.Also i wanted to ask if i should cut off the secondary of the first transformer when there is an arc to let the capacitor and primary of t2 resonate.Details:power source : 5v dcmosfets:IRFZ44Nbjts: 2n2222avoltage on the secondary of the first transformer(T1) should be around 150 vrb of the astable :10kΩrc of the astable : 220 Ωc of the astable: 4.7uFThanks for your attentionupdate: i tested my mosfets and they all turned out to be broken, i guess buying them on aliexpress was not a great idea...anyways i'm trying to make the circuit work with bjts, in particular making the h bridge from tip122 darlington, i made this circuit that works on simulators but it doesn't in reality, i don't knoew what is actually going on in reality because i don't have an oscilloscope to test, so i'm just asing for your help again( second image circuit first row : the two utput , second row: 100 ohm resistor that will later be the inductor, all the other rows are transistor vbe or vce that i forgot to remove before taking the screenshot an close the tab, sorry for that and the general mess in the circuit)

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 12, 2024, 11:25:41 am
content: First of all I think there must be some typo or miscalculation in your post. A secondary inductance of 0.09 uH makes no sense. For a solenoid with L=6.6cm, D=4.3cm, and N=850, I calculate about 20mH. That should bring the resonant frequency down into the <1MHz range, depending on the topload.

author: gbaddeley
author_rank: Regular Contributor
timestamp: July 12, 2024, 02:28:27 pm
content: 6.6cm is very short. I would expect at least 20cm to achieve a useful secondary discharge length.How did you fit 850 turns in just 6.6cm??

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 12, 2024, 02:56:52 pm
content: You need a lot more than 150V for a SGTC. For further reading see "Paschen's Law" and "Paschen minimum", basically you can't have a practical atmospheric spark gap below 500+ volts.

author: GIGGINO94
author_rank: Contributor
timestamp: July 14, 2024, 08:10:47 pm
content: Quote from: gbaddeley on July 12, 2024, 02:28:27 pmi used a 0.08 mm wire for just a little more than 6.6cm, as i said it should be roughly 850, anyways i got a component tester and it turned out to be around 6.5 mh and around 300 ohm, i was actually expeting a greater inductance, anyways i will add some lenght to the secondary later onModify message

author: GIGGINO94
author_rank: Contributor
timestamp: July 14, 2024, 08:13:21 pm
content: Quote from: mtwieg on July 12, 2024, 11:25:41 ami actually tested it and it's around 6.7 mH, it seems too low to me so i think maybe the coating o fthe wire is damaged somewhere shorting some secondary so i'll just try to coat the thing myself and then test again

author: GIGGINO94
author_rank: Contributor
timestamp: July 14, 2024, 08:26:10 pm
content: Quote from: BrokenYugo on July 12, 2024, 02:56:52 pmthanks for the advise, it will probably save me a lot of headache later, i'll figure out something

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 15, 2024, 12:57:58 pm
content: Quote from: GIGGINO94 on July 14, 2024, 08:13:21 pmLikely either a shorted turn or just miscounted the turns. Or maybe just measurement error. Generally solenoid inductance models are pretty accurate.Quote from: BrokenYugo on July 12, 2024, 02:56:52 pmYeah 150V is not enough for a spark gap. I recall long ago some people trying to replace the spark gap with a diac or thyristor to operate at lower voltages. Not sure how it worked out though.But your circuit for driving the first stage transformer will probably have its own issues (cross conduction in the H bridge is likely destroying it)... I would develop and test the two stages separately. Can test the tesla coil by driving its sparkgap with a traditional step-up mains transformer.