Mauriscivic's Pulser

[garrypm]

Hi all,

Looking for advice on what type of N channel mosfets to get this simple circuit running -

http://www.youtube.com/watch?v=KQtBPR5GmuI

I am referring to the second part of the video where 2 x N channel mosfets are powered via
a coil that induces a drive via the second coil.

If tinselkoala can spare a few minutes and investigate, it would be most appreciated.

I have tried with 2 x IRF840 but can not get them to start.

Many Thanks, Garry

[gyulasun]

Hi Garry,

This circuit seems to be a kind of oscillator and the winding sense of the coils is important to get positive feedback to maintain oscillations. Have you turned for instance the sensor coil (L2) 180 degree mechanically? so that its axial left hand side would be facing L1 coil instead of its right hand side, for instance?   or what is equivalent to this, keep the orientation the same as per it did not start and replace its wire endings with each other.  The IRF840s sounds good to me to use, provided they are not faulty.  It maybe also helpful to adjust the supply voltage by using a variable power supply if you happen to have one that is.   Also, the turns ratio between L1 and L2 may also count.

My understanding on this circuit is that when the switch K1 is closed, the lamp load gets the input DC voltage directly via coil L1 (this is why it is much brighter then) and both MOSFETs are 'inhibited' during this time. When you open K1, then the supply voltage can get to L1 via the Drain-Source of the right hand side MOSFET but this MOSFET can only pass current whenever BOTH MOSFETs are ON because the left hand side MOSFET is able to supply current via its Drain-Source path to the Source of the right hand side MOSFET when it is also ON. And for both MOSFETs to be ON, the sensor coil must feed their paralleled gate-source electrodes with higher than the treshold gate-source voltage which should be at least 5V peak AC coming from L2, at least a voltage this high should be induced in it by L1 to start oscillations. And the phase of this induced AC is also important to maintain oscillation.  If you happen to have MOSFETs that are so called logic level types, this means they already fully ON by a 2 or max 3V gate-source voltage level (not by 4 or 5V), this may also help when your L2 coil has a smaller number of turns.  But I do not think you would need such MOSFETs:  just try to increase coupling between L1 and L2 by inserting a common ferrite core into them if the reversing of the winding sense does not help.  The common core helps in getting a much higher induced voltage in L2 when you kick L1 via K1 with current pulses. If you do not have ferrite cores to insert, try inserting welding rods or (isolated) iron wire pieces bundled into a rod form. (Of course when the circuit does not start with the coils on a common core, you still must change the winding sense of one of the coils.)

Gyula

[Groundloop]

Hi all,

Looking for advice on what type of N channel mosfets to get this simple circuit running -

http://www.youtube.com/watch?v=KQtBPR5GmuI

I am referring to the second part of the video where 2 x N channel mosfets are powered via
a coil that induces a drive via the second coil.

If tinselkoala can spare a few minutes and investigate, it would be most appreciated.

I have tried with 2 x IRF840 but can not get them to start.

Many Thanks, Garry

Hi Garry,

If you have mounted the two mosfets onto a heat sink, then first make sure that the metal
on both mosfets is insulated from the heat sink. Then try to reverse the trigger coil connections.

Attached is my drawing of the circuit. I also think that it should be possible to get this circuit
to work with only one mosfet (the Q1) because the body diode of Q2 is always conducting
the current even if the Q2 is switched on.

GL.

[gyulasun]

Hi Garry,

If you have mounted the two mosfets onto a heat sink, then first make sure that the metal
on both mosfets is insulated from the heat sink. Then try to reverse the trigger coil connections.

Attached is my drawing of the circuit. I also think that it should be possible to get this circuit
to work with only one mosfet (the Q1) because the body diode of Q2 is always conducting
the current even if the Q2 is switched on.

GL.

Hi Alex,

Thanks for your valuable additions, I fully agree with you.

Gyula

[garrypm]

Thanks GL and Guyla,

I had tried all of your suggestions before I posted.

One thing that comes to mind is that to ensure there is enough current draw through the main coil so that it induces enough
into the trigger coil I guess.

Anyway, I will persist.

GL, thanks for the suggestion of the single mosfet - I will also give that a try.

Many thanks, Garry