Message-ID: <5901FD30.5010902@posteo.de>
Date: Thu, 27 Apr 2017 16:16:16 +0200
From: DK7FC <selberdenken@posteo.de>
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References: <1106627522.20170427150315@gmail.com>
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Subject: Re: LF: Varistor spike management
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Hi Chris,

Which type of FETs are you using?
The IRFP460A for example has a built in Z-diode which seems to handle 
the spikes. Anyway i put a 150 Ohm / 220 pF (for LF) series RC paralell 
to the Drain-Source...

73, Stefan

Am 27.04.2017 16:03, schrieb Chris Wilson:
> Hello LF'ers,
>
>    In the hope it may help or be of interest my experiments with
>    containing the level of spikes seen at the end of a TX on the drains
>    of push pull FET's has shown that the conventional, wire lead
>    varistor of sufficient capacity to quell these spikes adds
>    significant capacitance and ringing on the drains is greatly
>    increased. I thought I'd get some surface mount ones and the
>    difference is quite remarkable. They add NO additional ringing as
>    they are physically a LOT smaller for a similar power handling
>    ability, and thus have much less intrinsic capacitance themselves. I
>    used these and spikes on the end of a TX where the back emf was
>    showing up to 400V + are kept at or below 200V, within the
>    capability of most FETs likely to be used in a medium or large power
>    amp. They also have that wonderful quality that we all love of being
>    very cheap!
>
>    http://uk.farnell.com/panasonic-electronic-components/erzvf2m121/varistor-smd-120v/dp/1845508
>
>    I am not sure if many have looked at their drain wave form when a TX
>    ceases, I have to assume everyone has some spiking, just what
>    decides how great it is I am not sure.
>
>