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Mains Flyback trouble

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uzzors2k

Tue Feb 28 2006, 06:26PM

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

I'm in the process of rigging up an independent flyback setup, one with a power supply, flyback and driver in a box. Everything is ready, but earlier this week I managed to blow a limiting resistor.

I rectify the mains with a part of an old computer PSU, rated at 200W. But the transistor I'm using to drive the flyback is only rated at 125 W, and I suspect I need some sort of load or limiting when dealing with mains? I tried a 10W

load resistor, but you see how that went. It did work however, for a short time....

What should I use to limit the current? Or am I on the wrong track all together?

Marko

Tue Feb 28 2006, 06:46PM

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

If you post the schematic of driver most probably current can be limited by primary with higher number of turns, or inductive ballast with series (maybe im just babbling without schematic)

For limiting AC mains big inductive ballast (neon or mercury lamp ballast, series more if needed) o capacitive ballast.

vasil

Tue Feb 28 2006, 06:50PM

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

I used for my first 2N3055 drivers, car batteries as source (12 v at high current capability), with no problems. All your active power is burned in the resistor, thats why it is burning. Use an apropriate number of turns on the flyback primary. The transistor will take only whats is consumed on the secondary side.

If it is a merely 555 driver, for a SGTC HV source, you will have hot transistors trying to charge a capacitor. Hmm...good luck.

uzzors2k

Tue Feb 28 2006, 07:30PM

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

Not much of a schematic to it really. Just your standard astable, single transistor 555 driver. Very little extra. Reticfied mains run straight through primary and transistor to ground. Transistor is a 2SK2038

Not sure if it will make its way to a SGTC, but if it does, I be sure to fix it up a little.

So more turns then? How many aprox. 20, 30, 60 as much as I can fit?

Marko

Tue Feb 28 2006, 08:11PM

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

More than 60 turns for 220V. Also, mosfet will cause trouble connected drectly to pin 3 of NE555.
Firstly, 555 cannot supply enough current for fast turnon. Also, if output hangs for any reason on high for few moments mosfet will conduct and be ripped apart by high current from capacitor. With luck it will supply +300V to 555 itself pulverizing it and all his conponents around.

Gate drive is a must, just two transistors push-pull or driver IC, GDT is also good idea.
For your mosfet something like this may be used:

Here resistors of about 470 ohms must be added on bases, so that 555 is not overloaded.

Maybe better idea is usind bipolar transistors in darlington, but i didn't try this for mains directly.
These methods can stand for low-power flyback driving.

If you dont have some special need of flyback being mains powered i strongly recommend using low voltages, ZVS is great for achieving high powers and still considerably simple, you could drive even bigger DC tesla coils using ZVS without trouble.

uzzors2k

Tue Feb 28 2006, 10:31PM

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

Ok, thx for the advice, I'll fit it on the driver somehow.

This is my first real mains project, so mains is a must. Half the point is to make the whole setup standalone. Anyway, I already have a ZVS driver!

Marko

Tue Feb 28 2006, 10:42PM

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

I also davise, do not connect anything directly to mains even trough ballast.
Use isolation transformer of limited power (or better two small transformers with connected secondaries, I use that, prevnts blowing your fuses and may save your life also.

You should look at some drivers used in monitors/TVs, even there mains is not used directly, but trough ballast or SMPS output 80V. beware that this driver is for low voltage and it would probably not stand mains if primary isnt big enough, and you must use HV mosfet rated for lots of current.

Make sure NE555 oscillates before connecting mosfet, do not ever connect it etc. with gate hanging in the air or so, use lots of primary turns (maybe 100+ is a good idea), frequency must be high enough not to cause saturation etc.

And dont blow or shock yourself.
cheers...

Hazmatt_(The Underdog)

Wed Mar 01 2006, 07:29AM

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

I dunno about you guys, but I like a little more protection in my designs so it lasts for 20years!

There's my design, yes its adapted from the one already posted up there. I think originally there was a MOV across the flyback with the diode and I had to change that because it destroyed the MOV. Anyway, this thing works really well, AND I respect my Mosfets! Notice the 1 ohm resistor!! If you didn't take another look!
The heatsink is horrendous overkill, I'm not sure if its necessary. You may be able to get away with a tiny heatsink, but I dunno yet, no time for playing.

Anyway, there it is. Gives a nice arc for 36W.

Marko

Wed Mar 01 2006, 01:57PM

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

High power mosfets usually have their internal diode, but of course external can be added + maybe TVS or zener for high voltages.

Resistor like your 1 ohm one are frequently used for mains-powering where current is small, fusable resistor is actually used to blow instead of mosfet if shorting occurs.

For 12V whree lots of current flows it seems just to be too big heater

Circuit is actually very similar, diode across flyback priamry is not recommended actualy because it just dissipates power. I used big resistors here (for one mains SMPS) in series but after while I realized there is no need of it, TVS or zener + fast diode across transistor will offer him full overvoltage protection.

Also resistor of few hundred ohms is a good idea for pin 3 of NE555, overloading can only cause problems (maybe).

vasil

Wed Mar 01 2006, 02:18PM

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

Take a look here:

http://www.kronjaeger.com/hv-old/hv/src/fly-col/index.html

If you want much current, you need a limiter though. Jochen used a hairdrier. Another alternative would be your solder gun (with ohmic resistence instead transformer), you can find a little one for 300 W.

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