Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 28 Sep 2005 12:01:58 +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 12:01:37 +0100
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Subject: LF: RE: Re: Advice will be gratefully received
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Thanks for the advice Alan,

It's looking like a higher voltage device may be the answer.
My existing TX uses the same FET's but only runs at 24 volts.
They may well be running right on their limit and perhaps I've
just been lucky so far !

I must admit that I'm not using a negative voltage clamp  :-(
I monitor the gate waveform on a good 'scope and wind up the 
drive very slowly.
It never approaches anything like the published maximum.
It will be the work of an instant to place some diodes in 
circuit - consider it done.

73 Chris G3XIZ


-----Original Message-----
From: Alan Melia [mailto:Alan.Melia@btinternet.com]
Sent: 28 September 2005 11:41:AM
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Advice will be gratefully received


Chris
Ah according to my notes the peak voltage if you verge on Class E can
approach 4 times the supply.....I think that might be your problem. This is
necessary to allow a bit of headroom when tuning as "odd" reactances at the
drain will cause the peak drain voltage to soar. I use 200V devices up to
about 40 -45 volts supply and 500V above that. If you are transformer
driving I assume you have a negative voltage clamp (diode)
Alan G3NYK

----- Original Message -----
From: "OSBORN, Chris" <Chris.OSBORN@astrium.eads.net>
To: "LF Group (E-mail)" <rsgb_lf_group@blacksheep.org>
Sent: 28 September 2005 10:43
Subject: LF: Advice will be gratefully received


>
>
>
> 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.
Nothing in this email shall bind EADS Astrium Limited in any contract or obligation.

EADS Astrium Limited, Registered in England and Wales No. 2449259
Registered Office: Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2AS, England