Stoneridge Engineering Home |
--Previous Message--
: --Previous Message--
: Hello again Bert,
: I have in my possession an old "a/f stage amp"
: that tops out @ 780 watts per channel ( both channeis driven!).
: The outputs are Mosfet (2sk176 + 2sj56) and are obviously in push pull.
: I've been told these mosfets will happily go all the way up to 10 Meg.
: (it also can be configured as a 1600 watt mono block!)
:
: My questions are,
: can I use it to power up a Tesla Coil by giving it Positive
: feedback to make it oscilate, (It would be easy to tune to the dominant
: Ferq. of the secondary that way.)or does the feedback coil do that for me?
:
: Can I split the value of the feedback "tank capacitor" and use
: them to tie the f/b coil to the gates of the Mosfets or do I tie the f/b
: coil straight to the amp input?
: I also imagine that the Feedback coil, would have to be centre tapped to
: tie in with Class B operation.
:
: Can I split the value of the feedback "tank capacitor" and use
: them to tie the f/b coil to the gates of the Mosfets or do I tie the f/b
: coil straight to the amp input?
:
: Also, would I need to use protection Diodes across all of the individual
: mosfets or just across the P channel and N channel? (4+ and 4- mosfets per
: ch. ie. push pull parallel )
:
: (So many questions, so little time!)lol!
:
: Cheers again,
: Mick. ;-))
:
:
: Hi Mick,
:
: Those are some excellent questions. I don't know the answers, but here are
: some thought and suggestions. Even though your amplifiers are designed to
: operate in linear mode, I'd recommend using soft switching and driving
: them hard into saturation (rail to rail) to reduce output switching losses
: and drive the system in bursts to get longest sparks. This also permits
: you to use digital control for switching, burst length, etc. - otherwise
: you'll have a CW system that make hot arc-like flaming corona but short
: sparks.
:
: Free-wheeling diodes are also essential across each group of MOSFETS. You
: will likely need to substantially beef up your power supply capacitors to
: handle high current high frequency energy that will be circulating between
: the amplifier and load. This means using both large DC electrolytics AND a
: polypropylene "DC Link" capacitor in parallel.
:
: There are a couple of ways to drive your system. One easy way is to simply
: connect the output of your amp directly into the base of your secondary,
: either with a direct connection or through a fairly large valued
: polypropylene coupling capacitor. This method is simplest, since it
: requires no tank circuit and feedback control/switching is simpler.
:
: You can also drive a series combination of primary inductor and tank cap
: off one audio channel, with the other end of the tank circuit grounded,
: forming a "half bridge" circuit. You might even be able to
: parallel amplifier outputs to double output current capability as long as
: you are switching them softly and from rail/rail.
:
: Another other option is to drive the inputs from opposite phase (but
: otherwise in sync) to form a "full bridge" output to double toe
: voltage swing "seen" by the primary tank circuit. In any event,
: you will need to look closely at the circuitry for the amplifiers to make
: sure their transient responses are identical and there are no surprises
: (like significant internal phase shifts or excessive time delay from input
: to output) that will cause you grief when when you try to drive these at
: the operating frequency of your system.
:
: There are a variety of ways to get feedback, ranging from a
: "tickler" winding, E-field sensing from the secondary, primary
: current sensing, and/or secondary base current sensing. For a variety of
: reasons, I would recommend using current sensing via one or more current
: transformers (CT. If you direct drive your secondary, use base current
: sensing (via a simple current transformer). If you are driving a tank
: circuit, use a CT to detect primary current. In either case, you want to
: drive switch the output of your amp(s) at (or very near) the current zero
: for ZCS mode of operation.
:
: Finally, you'll want to control burst width and burst-burst timing for
: various effects, and to prevent current into your primary tank circuit
: from climbing to excessive values. This is where digital control becomes
: essential, especially when using half or full drive to your tank circuit.
: You may need to add a current monitoring circuit that will automatically
: shut down the input drive signal once a predetermned amount of peak
: primary or secondary current has been reached. Some good ideas for current
: monitoring, burst control, etc. are on Steve Ward's site. BTW, IGBT's are
: actually better suited for this mode of operation than MOSFETS.
:
: http://www.stevehv.4hv.org/
:
: Finally, have a supply of replacement output transistors available -
: chances are quite good you'll need them... :^)
:
: Good luck,
:
: Bert
:
: Hmmmmm, It looks like I've bitten off more than I can chew,
I'm just a hobbyist who knows enough about electronics to be Dangerous.
I actually built the amp about15 years ago when I swindeled a job as a electronics tech in a small Mom + POP opreation servicing the Pub and Club industry here in Melbourne
The boss traded an old 250 watt unit with 1 blown channel and gave it too me, and said if I can fix it I can have it!
He also said I could use the workshop and anything in it to fix up the old clunker.
Taking that as cart blanche I gutted it and put everything from the current series of amps inside the old case!
I don't think that was what he had in mind but I cant look a gift horse in the mouth!
( and the pay was lousy so I considered it a bonus!)
The upshot of all that is, I never took the schematics when I left so I would have to rely om 15 year old memories, as well as all that I have had a Stroke about 10 years ago so I've got fat chance of rembering anyway!
Maybe I'll put it on the back burner for a bit while I learn more about our passion!
Cheers Mick. ;-))
P.S. I have started a blog about my T/C adventures!
here is a link if you are interested.
[url=http://micksteslacoilendevours.blogspot.com
cheers, M. ;-))
:
Message Thread
« Back to index | View thread »