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Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
dexter wrote ...
nop... the toroids inside a PC psu are iron powder toroids therefore not good for GDT
Most of the toroids are iron powder but those multi-winding toroids are heavier, for a given size, than the iron powder ones. And I have also stripped off the coating on some of them and they appear metalic, not grey. I was told these cores are suitable for use with LM2576 which run at around 50kHz.
I am trying to do a gate drive transformer for car ignition coil driver based on those half bridge designs by Steve Ward. The optimum frequency for these is generally 10-15kHz from memory.
I have made a GDT based on a TV flyback transformer core and this seems to work well enough at 30kHz, which my working plasma globe/AC flyback transformer runs at.
But it just does not seem to work at around 2kHz which my ignition coil driver is currently running at.
So I am now wondering if perhaps at these lower frequencies, the iron core from a plug pack would work as a GDT?
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
You can use one of the smaller E core transformers from a PC PSU to make your own GDT's In fact the one in the middle it is a GDT
to open the transformer you might need to heat it up or else you risk breaking the cores to calculate how many windings you need there is a spreadsheet on this page
i used the one in the middle to drive the gates of of a IRKT91/12P thyristor module in my solid state variac at about 4.5kHz and 15-20% duty cycle i calculated the GDT to be usable at lower frequencies 1.5-2kHz and it worked just fine but ended up using it at 4.5kHz with the same windings because of higher resolution and because the thyristor was still able to turn on
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
dexter wrote ...
You can use one of the smaller E core transformers from a PC PSU to make your own GDT's In fact the one in the middle it is a GDT
to open the transformer you might need to heat it up or else you risk breaking the cores to calculate how many windings you need there is a spreadsheet on this page
i used the one in the middle to drive the gates of of a IRKT91/12P thyristor module in my solid state variac at about 4.5kHz and 15-20% duty cycle i calculated the GDT to be usable at lower frequencies 1.5-2kHz and it worked just fine but ended up using it at 4.5kHz with the same windings because of higher resolution and because the thyristor was still able to turn on
Are the large of the e core transformers any good? They would be at least more robust when taking them apart.
I tried heating a couple of the smaller ones in water in the microwave but they still broke. I got one of them apart intact though.
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
usually there is a direct correlation between transformer size and how much power can handle at a fixed frequency i don't know if by using a big transformer in a low power application (GDT) has any drawbacks beside overuse of space...
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
dexter wrote ...
usually there is a direct correlation between transformer size and how much power can handle at a fixed frequency i don't know if by using a big transformer in a low power application (GDT) has any drawbacks beside overuse of space...
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Be careful, it is possible that the cores are gapped (ie, the center leg is shorter than the outer legs), which will make them useless as a GDT. The smaller ones usually are ungapped, but sometimes the larger one is gapped depending on the converter topology used in the supply. You can also use the ferrite core from the line filter (if it has one, some supplies are made so cheaply that it is missing).
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
... wrote ...
Be careful, it is possible that the cores are gapped (ie, the center leg is shorter than the outer legs), which will make them useless as a GDT. The smaller ones usually are ungapped, but sometimes the larger one is gapped depending on the converter topology used in the supply. You can also use the ferrite core from the line filter (if it has one, some supplies are made so cheaply that it is missing).
The larger ones are E-cores and there is no gap.
This is what I am seeing on my scope and it doesn't look right although I am unsure that I have the ground connector in the right place:
Square wave straight out of the the 555 through a darlington totem pole - this looks correct to me.
Reproduction of square wave in one of the secondaries of my GDT using one of the e-cores with 15 primary turns and 30 secondary turns. Something just aint right here but I don't really understand what is happening here. It does not turn on my FETs at all as far as I can tell.
Words of wisdom and experience anyone?
I tried setting my 555 frequency to the same as my working plasma globe (about 30kHz) and the square wave coming out of the E-core and flyback core GDTs became more defined.
Although there seems to be a lot of ringing that fills in the down time and creates a sort of ramp up to the next high pulse (no photo). So the square wave looks rather trapezoidal.
As soon as I set the frequency lower to around 10kHz, the square wave output seems to totally break down such that the high pulses are crammed together as in the photo above.
So it appears that 30kHz is pretty much the lower limit of GDTs created with tv flyback cores and E-cores from PSU and that I can't avoid purchasing a proper N30 or T35 toroidal core.
The problem I can't seem to find any of these on ebay other than some ones from the UK that are $25 plus a piece.
RS Electronics have some N30 cores. The large ones are equally expensive and unnecessary. These ones are reasonable and of a manageable size:
Can anyone point me in the direction of a better mail order source of N30 and T35 cores?
[Moderator edit by Mads Barnkob: double posts merged]
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Gate drive transformer
