Class E Coil Woes / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Class E Coil Woes

1 2 next

Move Thread

LAN_403

The Lightning Stalker

Thu Dec 22 2011, 11:10PM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

I'm trying to get a simple Armstrong oscillator going and keep blowing MOSFETS. It just doesn't make any sense. There are 18V Zener diodes on the gate and the power supply is limited to 5A. At $6 apiece I can't afford to buy that many at a time and have to wait over a week for them to arrive. I can't get SPICE simulations to oscillate. It's based on WaveRider's design. That man is a genius.

This schematic is old. Scroll down for an update:

1324595456 4230 FT0 Armstrong Tesla Coil

Dr. Dark Current

Thu Dec 22 2011, 11:50PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

This post has been removed, I just realised the coil in question is class E not C...
Anyway, the drain cap is way too small.

The Lightning Stalker

Fri Dec 23 2011, 02:03AM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

Thank you for the reply. How do I determine what size drain cap to start out with? The class E calculators say to use around 1300pF at this frequency. The output capacitance of the MOSFET alone is already 1250p. Also the component arrangement is a little different than the Sokal circuit leading to some confusion on my part.

teravolt

Fri Dec 23 2011, 04:10AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Do you have a transformer or variac to test it at lower voltages? another thing you could add some resistance in series with the 100uH inductor say 5 ohms till you can find out whats going on. Once you get it resonant there will be alot of voltage developed over your 100pf cap and accross your fet. Is you FB turn phased right?

The Lightning Stalker

Fri Dec 23 2011, 04:38AM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

I was running it on an isolated variac with a 300W light bulb in series to limit the current and trying the FB winding in both directions since I'm not sure which way is right. So far I haven't had the chance to hook up the interrupter.

Dr. Dark Current

Fri Dec 23 2011, 05:23PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The interrupter transistor is missing a diode in its collector. Without the diode, the base-emitter junction will break down during the operation, and the base drive signal will get huge rf ripple.

Marko

Fri Dec 23 2011, 10:53PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The Lightning Stalker wrote ...

I'm trying to get a simple Armstrong oscillator going and keep blowing MOSFETS. It just doesn't make any sense. There are 18V Zener diodes on the gate and the power supply is limited to 5A. At $6 apiece I can't afford to buy that many at a time and have to wait over a week for them to arrive. I can't get SPICE simulations to oscillate. It's based on WaveRider's design. That man is a genius.

Circuit as it is now:

Such circuits with mosfets are extremely fiddly to bias. You might get it biased right at low voltage, only for it to become unhappy on higher voltages, or have it self destruct after the gate threshold voltage drops too much due to elevated temperature and the mosfet latches permanently on.

You'r likely need to adjust your feedback winding in steps as you progress with voltage increments. A variable loading cap would probably be a good idea too.

marko

Marko

The Lightning Stalker

Sun Dec 25 2011, 01:22AM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

Deleted double post

The Lightning Stalker

Sun Dec 25 2011, 01:35AM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

I couldn't find a decent diode, so I just took the interrupter transistor out of the circuit. After a lot of fiddling it finally has a nice arc at 30V. The transistor is just getting a little warm and the waveform is beautiful. The secondary actually gets hotter than the transistor does and it's on a smallish heat sink (out of a small ATX power supply but it's thick, like 8mm). The transistor is temporarily a Fairchild FQA13N50C out of an old ATX PSU since I ran out of the "good" Infineons.

I ended up putting a ferrite in series with the feedback winding to take care of some higher frequency ringing which got really bad as I increased voltage. It also delayed the turn on time so it wasn't turning on too soon.

It's running at about 3MHz which is way above the target of 444KHz, so more tweaking needs to be done. It's not quite running at true Class E yet, but it's pretty close.

I'll post some pictures when my mom gets done watching TV since it's interfering with it. I'll also update the schematic.

The Lightning Stalker

Sun Dec 25 2011, 01:42AM

Registered Member #4230 Joined: Sat Nov 26 2011, 05:50AM
Location: Socketville
Posts: 53

Updated schematic:

1324777359 4230 FT130774 Armstrong Tesla Coil

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob