From: Chris Wilson <dead.fets@gmail.com>
Date: Thu, 27 Apr 2017 15:03:15 +0100
Message-ID: <1106627522.20170427150315@gmail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: LF: Varistor spike management
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Status: RO

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.

-- 
Best regards,
 Chris                          mailto:dead_fets@gmail.com