Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 28 Sep 2005 11:36:31 +0100
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From: "OSBORN, Chris" <Chris.OSBORN@astrium.eads.net>
To: "'rsgb_lf_group@blacksheep.org'" <rsgb_lf_group@blacksheep.org>
Date: Wed, 28 Sep 2005 11:36:07 +0100
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Subject: RE: LF: Advice will be gratefully received
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Many thanks Rik,

I shall put in an order for some more robust fet's.
I would have thought that the aerial system was resonant but something seems
to have changed !

73 Chris G3XIZ

-----Original Message-----
From: Rik Strobbe [mailto:rik.strobbe@fys.kuleuven.be]
Sent: 28 September 2005 11:21:AM
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Advice will be gratefully received


Chris,

if the load differs from 50 Ohm (and especially if ot has a reactive 
component) the drain voltage can easily sing to 5 times the power supply 
voltage.
I would recommend to use FET's with a higher Vds.
I have been using IRFP350's and IRFP450's in my 450 W PA running at 55V 
supply voltage and they survived all kinds of misthreatment.

73, Rik ON7YD

At 10:43 28/09/2005 +0100, you wrote:


>         The PA FET's in my new home brew transmitter keep exploding !
>
>         I'm using push-pull  IRF520's rated at 100 V Vds and my supply is
30
>volts.
>         I am careful not to exceed the V gs.
>
>         The gates are driven with a toroidal transformer and the whole
>circuit is just a scaled up version of my original TX, which works fine.
>         It generates healthy power into a 50 ohm dummy load but on
>connection to the aerial circuit - bang !
>         The aerial system is identical to that used with the old TX !
>
>         Has anyone any ideas on what's causing the problem and how to cure
>it ?
>
>         I'm running out of FET's and my nerves are somewhat frayed (shell
>shock!)
>
>         73 Chris G3XIZ
>
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This email is for the intended addressee only.
If you have received it in error then you must not use, retain, disseminate or otherwise deal with it.
Please notify the sender by return email.
The views of the author may not necessarily constitute the views of EADS Astrium Limited.
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EADS Astrium Limited, Registered in England and Wales No. 2449259
Registered Office: Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2AS, England