LF: RE: TX Cores, Gap from DK 8 ND

["Hans-Albrecht Haffa" <dk8nd@web.de>]

Dear Lowfers, 
any ferrite core should have a gap. This is necessary to reduce the remanent flux from about 180 mT to as low as 30 mT. It may be just the residuous gap of lapped cores, preferrable is to put a layer of capton tape on one outer leg of an EE-Core (EDT59 and the like). Lowering remanent flux will rise flux swing at operation. Apparently it is not a problem at 136kHz, as core saturation is unlikely at this frequency.  Saturation will occure at frequencies below 50 kHz during startup of push-pull switching circuits (symmetrical half bridge or full bridge), designed by beginners. A gap as low as 0.05 mm (one layer self adhesive film tape or a bit of varnish) will hardly  lower permeability, so from urel=2700 down to urel=1500. If You can, use this effect. A toroid core is often second choice in power electronics. Two paralleled ETD cores have greater surface than two stacked toroids. With the ETDs copper volume is doubled too. not so with the stacked toroids. So if You can buy ET
D cores , do it. They are available at Conrad.com or conrad.de.

55, Hans-Albrecht
DK 8 ND


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Von:      rsgb_lf_group@blacksheep.org
Gesendet: 01.04.05 11:11:24
An:       "Hans-Albrecht Haffa" <dk8nd@web.de>
Betreff:  Re: LF: RE: TX Cores


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Von:	rsgb_lf_group@blacksheep.org,
Gesendet:	01.04.05 11:11:24
An:	"Hans-Albrecht Haffa" <dk8nd@web.de>
Betreff:	Re: LF: RE: TX Cores

Dear Mal, LF Group,

 

No – it is possible to design these transformers with a small air gap, but there does not appear to be a great advantage to doing so because the required number of turns increases, increasing the “copper” losses and offsetting any reduction in core loss. There is always a quite large gap in the original LOPT – often this is produced by inserting plastic film spacers between the core halves. These, and any glue, should be removed so that the core halves fit together perfectly. Sometimes the gap is produced by machining the core legs to unequal lengths, in which case you can’t really use the core in this application.

 

Cheers, Jim Moritz

73 de M0BMU

 

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of hamilton mal
Sent: 31 March 2005 13:36
To: rsgb_lf_group@blacksheep.org
Subject: LF: TX Cores

 

Hi All Jim M0BMU.

Jim ur mod to the G0MRF using a TV line output transformer. Did u leave a gap between the cores or not ?

73 de Mal/G3KEV