Rectifying Flyback Transformer / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Rectifying Flyback Transformer

Move Thread

LAN_403

nerdywarrior

Mon May 25 2009, 09:05PM

Registered Member #2139 Joined: Mon May 25 2009, 07:54AM
Location:
Posts: 8

So I just ripped a Flyback transformer from a TV and was just wondering about any tips (instructions) on rectifying the output.

Im trying to use it to charge a bank of (7) 12,000uf 400V Caps

Any Advice will help
Thanks!

Myke

Mon May 25 2009, 09:15PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Unless the TV were old and the flyback had a disk shaped secondary, it's a DC flyback. I would think that using a flyback from a TV wouldn't be the best transformer to use for charging such low voltage caps (the flyback can output ~10-15kV). You could use a line powered voltage multiplier with a lamp in series with it to limit the current though the diodes.

EDIT: The line powered charger is not that safe because it's not isolated from the mains. You could build some inverter that runs off 12V and steps it up to ~400V. I think Uzzors designed a charger that runs off 12V can charges cap banks.

Weston

Mon May 25 2009, 09:19PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

If the output from the flyback transformer is connected directly to the TV picture tube it has an internal rectifier.

uzzors2k

Mon May 25 2009, 10:08PM

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

If it's the modern type as seen here

then it's already rectified. It will work for charging low voltage capacitor banks as the capacitors will hold the voltage down while charging, but it will indeed be hard on the flyback to provide much current. A line powered charger as Mike suggested is easiest, quickest, and surprisingly just as (un)safe.

nerdywarrior

Tue May 26 2009, 12:56AM

Registered Member #2139 Joined: Mon May 25 2009, 07:54AM
Location:
Posts: 8

thank yall!

Backyard Skunkworks

Tue May 26 2009, 04:48AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Yet another option for charging the caps is to simply make a 120v to 360v step up transformer. It is far safer than a multipler running right off mains because it wont supply as high a short circuit current. I generally use rectified step up transformers to charge caps, they can provide a lot of current, however they are inherantly current limited and will not put a ton of current into a short. The recitified peak output of any sine wave will be the square root of two times the RMS voltage.

Sulaiman

Tue May 26 2009, 07:34PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I would say exactly the opposite !

A step-up transformer can supply massive currents (for long enough to kill you)
The current is limited only by the winding resistances and transformer ratio.
(NST, OBIT and to a degree-MOT all have magnetic shunts that limit output current)

Assuming that the first capacitor in a multiplier is insulation Class Y1 or Y2
(designed for use between humans and up to 250 Vac (from memory))
the maximum current that can flow is Vac x 2 x PI x 50(or 60 Hz ) x C
so you can design it to be 'safe' (but low-powered)

Of course, the same capacitor in series with a step-up transformer would also be 'safe'

Safety is relative ... how safe is a charged capacitor bank ?

P.S. A tv flyback transformer will be very inefficient charging large capacitors to 400V.
Replace the flyback with a common transformer, eg 12 V primary, 120 V secondary
and use a UF4006 (cheap..or similar) diode ... much better.
If you tire of the singing transformer you'll have to move up to higher frequencies and ferrite.
Most smpsu have a ferrite transformer with about the right urns ratio to reverse for a charger.

twist2b

Wed May 27 2009, 09:42PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

Sulaiman - WHA? If you step up the voltage.... using P=IE OBVIOUSLY if the voltage goes up the amp has to go down o_O

flybacks, the secondary coils will have LOW amperage... DEFINITELY lower then defibrillators so your heart should be safe. The real hazard is the high voltage can burn you. I actually saw someone have burns on there fingers (picks) for touching it... pretty cool.

The output IS rectified though, there is a diode in there. If you want I can dig up a pick that Harry gave me... its very VERY descriptive of the inside of flybacks.

Sulaiman

Thu May 28 2009, 05:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

A normal 50/60 Hz iron-core step up transformer is limited as you say,
to maintain temperature rise within cartain limits.

The short circuit current at the output is

[Vline x (Ns/Np)] / [Rs + Rp x (Ns/Np)^2]
Typically 10x normal for small transformers, 20x normal for larger transformers.

A human body (you) may seem to be high resistance at low voltage
but once the body starts to conduct it looks like a short circuit to a hv supply.

In the case of a flyback, the peak output current will not be more than
Ipri(pk) x (Np/Ns) = usually safe.

All academic .. I wouldn't want to be shocked by any of the above !
I've been present when colleagues have been shocked by 150V, 300V and 600V dc busses with 1,000's uF bus capacitance.. so far no fatalities but you may not be so lucky.

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