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Mains Flyback trouble

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Marko

Wed Mar 01 2006, 02:28PM

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

For currnet limiting (mains directly) is good idea to use mercury lamp ballast, and use rectifier after that. Lightbulbs, hairdriers, heaters etc. are less proffesional but still good choices

But the best is to limit the current by high enough number of windings and maybe inductor in series with primary, this is more elegant and less bulky than 50hz or DC limiting.
Internal flyback primaries are usually not good and external ones have to be used in the end, so just enough turns will do for direct 300VDC supply.

Hazmatt_(The Underdog)

Wed Mar 01 2006, 07:24PM

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

There are only 2 things that cause a problem that I know of for my driver
1. short circuit on the drive leads
2. trying to put 24V in (555 isn't too happy about that)

Otherwise, I have abused it in testing and it works well. The diode load network across the flyback primary is a clamp. The power shorted in the diode is determened by the resistor value! These values were chosen to dampen the intensity, not to short out completely.

Also, the 1R resistor isn't 'on' long enough to be a big heater. The duty cycle is only about 9% or so, so your heating issue does not really come into play. Now if you were to do something foolish and operate at a 50% duty cycle, where you waste a LOT of power, then yes it would be a big heater.

Marko

Wed Mar 01 2006, 07:41PM

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

Just sayng that instead of clamp diode zener + schottky across transistor can solve the same problem.
For 12V i dont think it is needed at all.

You can give 7812 stabiliser to NE555 and run the circuit to 30V or just supply high voltage to mosfet itself.

For such low voltages and duty cycles there is no need for 1 ohm resistor, far better idea is to wind your own primary with high enough number of turns.

At 40 volts i acheive 2,5cm + starting spark with single transistor oscillator so i think it can do a lot better.

Hazmatt_(The Underdog)

Wed Mar 01 2006, 10:44PM

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

Consider first off that it was designed to drive a Crooke's Tube. It cannot be throwing out a lot of power, it has to be limited in construction. The construction also Dictates that all parts are replacable! That means no fabricated components. No hand wound anything. No one-offs.

Sounds like you think I'm new to this, but im not.

I wanted to ensure that no matter what condition this thing operated in, it was bulletproof, that means extremely overdesigned.

Hope that clears things up.

...

Thu Mar 02 2006, 12:20AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Well, if you are trying to use a driver designed for 24v on 240v mains a resistor is not a good idea, as you will waste 10w for every 1w of output... Why can't you just put in a little 24v/3a transformer+bridge rectifier+filter cap (and maybe a lm350 regulator)? A 60va transformer shouldn't be too big/heavy/expensive, and prevents insane currents from flowing if something goes wrong...

Also, the 200w rating on the power supply just means that it can put out 600ma @300v before it explodes, but it does not mean that it will limit power draw to 200w. Also, the 125w rating for the transistor is how much power it can dissipate, not the amount of power it can switch. It driven correctly it can handle much more (but your flyback isn't rated fro much more than 50-100w).

If you are going to wind your own primary, I would say that for 300v you are going to need a few hundred turns...
The way to figure out how many you need is to hook up the circuit to a variac and an ammeter.
Slowly turn up the power without drawing a spark and see where it starts to rise. If it is drawing more than 20ma or so at 300v then you need more turns.

Good Luck!

vasil

Thu Mar 02 2006, 02:33PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

In my PL500 flyback driver, fed from mains (312 VDC, even more with level shifter), I am using 60 turns on the primary of the big flybacks and 90 turns on the little flybacks. just to have a starting point.

http://www.geocities.com/teslina/stdf.html

...but the current is limited by the tube impedance, bigger than a FET Ron.

Marko

Thu Mar 02 2006, 02:47PM

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

With enough turns to avoid saturation and good driver flybacks can be ran on mains voltage very efficiently.

Current limiting is done best with capacitive/inductive balast on 50HZ or smaller inductance in series with flyback primary.
High resistance resistors can be used to limit secondary current when drawing arcs.
And about 100 turns should be about enough for primary if I remember correctly, 60 is some minimum, for frequency about 50kHz.

If well made 300V circuit can be far more efficient than low voltage mosfet-driven circuit, resistive dissipation and voltage drop on mosfet is much smaller.

Dr. Shark

Thu Mar 02 2006, 03:48PM

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

Why would you want to use current limiting at all? Flybacks are designed for inductive kick, so they have considerable leakage inductance. From this it should follow that flybacks are current limited by themselves, provided you have enough primary turns and drive them at a high enough frequency.

uzzors2k

Thu Mar 02 2006, 04:21PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Ok, I just misunderstood the MOSFET rating. I just though it would explode with no apparent load to limit power. But with more turns and all, I gues it should work. I'm using a 12 xfrmr for driver supply, if this is unclear. The whole setup is a lot like what vasil linked to.http://www.kronjaeger.com/hv-old/hv/src/fly- col/index.html
The only reason I'm not just running the flyback off the 12v xfrmr aswell, is that I would like some more power. I'' wind the flyback up tonight and tell you how things went....

Kolas

Thu Mar 02 2006, 05:44PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

hi there
I don't wanna poo on your parade.
But I might have to:
240v mains is horridly punishing on your flyback!
I would seroulsy advise you step that voltage way way down. Besides current limitation at that voltage is very hard.
And with my experiance, power comes from using some sort of half bridge. be it a zvs driver, or the real thing, not a single transistor.
Heh, you can actually take one of those SSTC bridges, including the gate driver, and just feed it the 555 signal, it'll work like a charm ;)

well thats my two cents
Kolas

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