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4hv.org :: Forums :: High Voltage
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Plasma speaker from rectified mains.
Move Thread LAN_403
M.A.D.
Sat Nov 12 2011, 05:09PM Print
M.A.D. Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69
 		I do not know how to do the math to see if this is a bad idea, so I could use some help.

I am plannig on building a speaker based on the design here http://www.teravolt.org/sparkspeaker2.php. However it requires huge caps for the bridge and a rather large step down transformer to power it. Both of these increase the size and expense of the speaker. Since the MOSFETs are rated at 200v anyway I was thinking it would be easier just to rectify and filter 120v mains.

I am also going to add hyperfast freewheeling doides and schottkeys to protect the MOSFETs from kick back, and I am going to reduce the gate resistor and put a UF diode in parallel with it to reduce the time that the MOSFETs are linear. I am also using IRFP260s instead because they are better.

So is there any reason why I should not do this? How large would the bridge caps need to be? And how can I tell how many volts per turn a flyback is good for?
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Adam Munich
Sun Nov 13 2011, 02:04PM
Adam Munich Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
 		It will work, but I'd use 600V mosfets or IGBTs. The capacitors can be small, but they must be 400V ones.

You *must* keep the primary coil's impedance high (with many turns).

170 volts / 2 == 85V going to the primary.
85 / 6 turns == 14V/turn, way too high for a small core.
85 / 20 == 4.25V/turn. That's more like it.
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M.A.D.
Sun Nov 13 2011, 06:53PM
M.A.D. Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69
 		Thank you very much for your help.

Do the MOSFETs have to be rated higher if I use UF freewheeling diodes in parallel and schottkeys in series with the MOSFETs, to protect them from kickback? I can get 500v MOSFETs but the Rds(on) is a lot higher (0.27 instead of 0.04), so won't they run hotter?

Also how can I find how large the caps need to be? Is there an equation or something?
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Adam Munich
Sun Nov 13 2011, 06:57PM
Adam Munich Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
 		Well 1 amp at 85V is 85W.
1^2 * 0.27 == 0.27W lost.


2 amps at 85V is 170W.
2^2 * 0.27 == 1.08W lost.


3 amps at 85V is 225W.
3^2 * 0.27 == 2.48W lost.


4 amps at 85V is 340W.
3^2 * 0.27 == 4.32W lost.


RDSon will likely be one of the smallest losses in the system.
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M.A.D.
Mon Nov 14 2011, 12:31AM
M.A.D. Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69
 		All right, thank you, since there is almost no added loss, I will use the higher voltage MOSFETs.

Do you think that 470uF is too small for the bridge caps?

Once again, thank you for your help.
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bomber315
Wed Jan 28 2015, 07:01PM
bomber315 Registered Member #43257 Joined: Thu Feb 20 2014, 03:53PM
Location:
Posts: 33
 		im curious how far this project got/ how successful it was.

Adam, i am hoping to make a mains powered version of your circuit. Have you ever tried it that? Do you have any revisions to the widely found diagram?
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Newton Brawn
Sun Mar 08 2015, 02:25AM
Newton Brawn Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
 		I Adam !

Fix the math

4 amps at 85V is 340W.
3^2 * 0.27 == 4.32W lost. ??
Regards

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Adam Munich
Sun Mar 08 2015, 07:10AM
Adam Munich Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
 		Blast from the past?
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