irf540 stays "ON" / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

irf540 stays "ON"

1 2 next

Move Thread

LAN_403

Solidacid

Mon Sept 28 2009, 02:24PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i recenlty bought some irf540's for several projects, my current one being a negative ion generator for charging myself(like this guy), because i have been unable to buy one.

i'm going for an astable 555 pulsing a 230v-9v transformer in reverse, with a irf540.
i did get the transformer to make some tiny (<1 mm) sparks, which my 9-stage CW-multiplier turned into 4mm sparks at about 1Hz.

since the circuit was performing so poorly i decided to do some tests.
i discovered that when the irf540 had been turned on(by something as little as lightly touching it.) it stayed conductive, the only thing i had connected to it was a 9v-battery and my voltmeter. after triggering it on it simply would not stop conducting, even after completely disconnecting the battery.

however, if i touch it while the 9v-battery is disconnected, the resistance across it will drop from full conductivity to none at all over a few seconds.

is my irf540 broken?
is it supposed to behave like this? if not, can i use a irfp450?

Avi

Mon Sept 28 2009, 02:37PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Yes, it is supposed to, mosfet gates ARE capacitive (a thin sheet of silicon dioxide)
so if you touch it to +, it will remain on unti you touch it to - (or you leave it long enough for the charge to leak away).

By the way, the thread title does not match the said mosfet that you used.

Mads Barnkob

Mon Sept 28 2009, 02:42PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

EDIT "nothing to see here, keep passing."

Proud Mary

Mon Sept 28 2009, 02:59PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

With 9V pp drive from your 555 into a reversed 230V-9V transformer, the most you could hope to get out of it would be 230V - so you are doing well to get 'tiny sparks.' before putting it into your C&W, son.

Your 9 stage C&W could give you a maximum amplification of 9*230V=2070V (but in practice it will be less because of what is called 'sag').

Given that the breakdown voltage of air at STP is ~3.3kV/mm your little outfit is doing very well indeed to produce 4mm sparks - which one would normally expect at 12-13kV.

I'd say your IRF540 seemed to be working very well, regardless of your other observations.

Solidacid

Mon Sept 28 2009, 03:06PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

the most you could hope to get out of it would be 230V

wouldn't the DC pulses into the transformer give a high voltage "kick" when the magnetic field collapses, like an ignition coil?

Proud Mary

Mon Sept 28 2009, 03:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Solidacid wrote ...

the most you could hope to get out of it would be 230V

wouldn't the DC pulses into the transformer give a high voltage "kick" when the magnetic field collapses, like an ignition coil?

Yes, I s'pose that's a thought, but I wouldn't like to guess its size.

Can you not measure the voltage on the 'new' high voltage side of the transformer to see what is going on there?

Avi

Mon Sept 28 2009, 04:06PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Harry wrote ...

With 9V pp drive from your 555 into a reversed 230V-9V transformer, the most you could hope to get out of it would be 230V

This is incorrect considering the mosfet would allow the circuit to go open allowing for high back-emf voltages to be generated via the flyback effect.

To test the circuit is functioning correctly, you can use a purely resistive load (instead of the transformer) and put a scope directly across the load or across the mosfet. Both cases should result in a 0 to vcc square waveform.

klugesmith

Mon Sept 28 2009, 04:12PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

With 9V pp drive ... the most you could hope to get out of it would be 230V

[edit] I agree, under the specified condition. But...

230 V RMS is a fixed limit, based on core saturation, with sine waves at mains frequency.
With higher frequency, electronically switched input, Solidacid could get substantially higher output voltage.
(Limited, very importantly, by insulation strength within the "230" winding! )

With only 9V on primary side rail, could induce a higher voltage by interrupting the current.

Proud Mary

Mon Sept 28 2009, 04:20PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Yes, of course, you're both right, and I was thinking in 50Hz sine wave terms.

Needless to say, with a 4mm spark output from the 9-stage C&W, the input from the transformer must be of the order of 1500V, or 1600V, allowing for 'sag,' so I'd say Solidacid's fear that his N-Channel MOSFET is dud are unfounded.

Also, he doesn't tell us the size of the capacitors in the C&W, nor its working frequency, from which we could calculate its maximum loaded and unloaded voltage, wattage, and so on.

Solidacid

Mon Sept 28 2009, 05:49PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

my plans for this circuit was to use the mosfet as a switch that would repeatedly disconnect the transformer from the battery.

i just tried attaching the positive lead of a 9V-battery(8.2v actually) to the gate of the irf540.
the positive lead of my scope probe on the source of the irf540. and the negative lead of my scope to GND on the battery.
then i pulsed the 555 output into the gate of the irf540

here is a pic of the results

1 2 next

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