X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 13 Oct 2009 13:13:13 -0400 Message-ID: X-Original-Return-Path: Received: from nschwmtas06p.mx.bigpond.com ([61.9.189.152] verified) by logan.com (CommuniGate Pro SMTP 5.2.16) with ESMTP id 3883808 for lml@lancaironline.net; Mon, 12 Oct 2009 20:09:51 -0400 Received-SPF: pass receiver=logan.com; client-ip=61.9.189.152; envelope-from=frederickmoreno@bigpond.com Received: from nschwotgx01p.mx.bigpond.com ([121.221.212.198]) by nschwmtas06p.mx.bigpond.com with ESMTP id <20091013000916.DUJB22839.nschwmtas06p.mx.bigpond.com@nschwotgx01p.mx.bigpond.com> for ; Tue, 13 Oct 2009 00:09:16 +0000 Received: from Razzle ([121.221.212.198]) by nschwotgx01p.mx.bigpond.com with ESMTP id <20091013000914.ZRPI5339.nschwotgx01p.mx.bigpond.com@Razzle> for ; Tue, 13 Oct 2009 00:09:14 +0000 From: "Frederick Moreno" X-Original-To: "Lancair Mail" Subject: Transient suppression on relays and Snapjacks X-Original-Date: Tue, 13 Oct 2009 08:09:08 +0800 X-Original-Message-ID: <007801ca4b99$63d9d460$2b8d7d20$@com> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0079_01CA4BDC.71FD1460" X-Mailer: Microsoft Office Outlook 12.0 Thread-Index: AcpLSGZUdxek8eITTKutftuESIWsrwAT5G+A Content-Language: en-au X-RPD-ScanID: Class unknown; VirusThreatLevel unknown, RefID str=0001.0A150202.4AD3C52C.001F,ss=1,fgs=0 This is a multi-part message in MIME format. ------=_NextPart_000_0079_01CA4BDC.71FD1460 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit I use diodes around my relay coils, and was interested to learn that they are a mixed blessing, saving the toggle switch, but slowing the coil action. I wrote Eric at Perihelion Design to ask about his Snapjacks which use zener diode technology. (Zener diodes act like regular diodes except that when the voltage exceeds a certain level in the "wrong direction" they become conductive and remain so until the transient voltage condition goes away. Then they shut off. Thus they can trim transient voltage spikes going both ways when put in "back to back" in a single integrated device - Eric's Snapjacks.). Here is what I wrote and Eric's interesting response. These new devices look to be a big improvement. I am going to order a set. Fred Dear Eric: There is a lot of chatter in the Lancair world about transient suppression. I read your stuff on Snapjacks - very interesting - and I am thinking about ordering a set. However: there is always the question of failure modes and probabilities. What the failure mode of the Snapjacks? Can they fail closed in such a way that you can't operate the relay? Can they fail short and lead to smoking wires or a fire around the relay? The reason I ask is the following list from a text discussing the relative failure modes of various types of components:. ________________________________________________________________________ Fred, I am also interested in transient suppression. I build it into most of the devices I sell, because I can't depend on the customers' attention to this detail. As a result, the more of my parts you have, the more transient suppression you have as well. My comments on the methods of failure-- "The following list....: * Capacitors (shorted), especially electrolytic capacitors. The paste electrolyte tends to lose moisture with age, leading to failure. Thin dielectric layers may be punctured by overvoltage transients. One comment--I have avoided the use of electrolytics as far as possible in all my design work for the reasons stated. Often a REALLY BIG electrolytic gets used where a smaller tantalum plus a series inductor plus a small parallel ceramic and a zener should be used. This is sloppy engineering. * Diodes open (rectifying diodes) or shorted (Zener diodes). True as a Rule of Thumb...but it depends on parts and circumstances. * Inductor and transformer windings open or shorted to conductive core. Failures related to overheating (insulation breakdown) are easily detected by smell. True. Let's try to minimize transformers and relay coils. * Resistors open, almost never shorted. Usually this is due to overcurrent heating, although it is less frequently caused by overvoltage transient (arc-over) or physical damage (vibration or impact). Resistors may also change resistance value if overheated! True. Design note: The almost-obsolete carbon-comp resistors perform well in voltage-transient and overvoltage conditions. My SnapJacks are Bidirectional Zeners and as such never see current except during transient suppression. It is hard to judge their failure modes since their electrical failures are virtually impossible when used as specified. Mechanical failure from vibration would be possible, and then they would fail open. I have never seen or heard of a failure short of a lightning strike. The issue of zeners, bidirectional zeners --or even diodes in general-- failing short always brings up the idea of adding a fuse. This fuse could be far less reliable than the diode. A better solution would be to add a series resistor to limit the fault current, but it would be problematic as to whether or not this improves the function of the suppression diode. By the way, the question of lighning strike suppression needs addressing. Lightning is attracted differently on composite or metal-skinned aircraft, but in general, adding arc suppression to the "corners" and the main busses of the aircraft seems to be a cost effective thing to do. You're welcome to republish this on any list. Regards, Eric M. Jones emjones@charter.net www.PerihelionDesign.com 113 Brentwood Drive Southbridge MA 01550-2705 (508) 764-2072 ------=_NextPart_000_0079_01CA4BDC.71FD1460 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

I use diodes around my relay coils, and was interested to learn that they are= a mixed blessing, saving the toggle switch, but slowing the coil action. = ; I wrote Eric at Perihelion Design to ask about his Snapjacks which use zener diode technology.  (Zener diodes act like regular diodes except that w= hen the voltage exceeds a certain level in the “wrong direction” th= ey become conductive and remain so until the transient voltage condition goes away.  Then they shut off.  Thus they can trim transient voltage spikes going both ways when put in “back to back” in a single integrated device – Eric’s Snapjacks.).

 

Here is what I wrote and Eric’s interesting response.    Th= ese new devices look to be a big improvement.  I am going to order a set.   Fred

 

Dear Eric:

 

There is a lot of chatter in the Lancair world about transient suppression. = I read your stuff on Snapjacks – very interesting – and I am thin= king about ordering a set.

 

However:  there is always the question of failure modes and probabilities.  What= the failure mode of the Snapjacks?  Can they fail closed in such a way tha= t you can't operate the relay?  Can they fail short and lead to smoking wires or a fire around the relay?

 

The reason I ask is the following list from a text discussing the relative fail= ure modes of various types of components:…

_______________________________________________________________= _________

 

Fred,

 

I am also interested in transient suppression. I build it into most of the devices I sell, because I can't depend on the customers' attention to this detail. As a result, the more of my parts you have, the more transient suppression you have as well.

 

My comments on the methods of failure--

“The following list....:

  • Capacitors (shorted), especially electroly= tic capacitors. The paste electrolyte tends to lose moisture with age, lea= ding to failure. Thin dielectric layers may be punctured by overvoltage transients.

One comment--I have avoided the use of electrolytics a= s far as possible in all my design work for the reasons stated. Often a REALLY BIG electrolytic gets used where a smaller tantalum plus a seri= es inductor plus a small parallel ceramic and a zener should be used. This is sloppy engineering.

  • Diodes open (rectifying diodes) or shorted (<= span class=3Dhilite1>Zener diodes).

True as a Rule of Thumb...but it depends on parts and circumstances.

  • Inductor and transformer windings open or sho= rted to conductive core. Failures related to overheating (insulation breakd= own) are easily detected by smell.

True. Let's try = to minimize transformers and relay coils.

  • Resistors open, almost never shorted. Usually this is due to overcurrent heating, although it is less frequently cau= sed by overvoltage transient (arc-over) or physical damage (vibration or impact). Resistors may also change resistance value if overheated!

True. Design note: The almost-obsolete carbon-comp res= istors perform well in voltage-transient and overvoltage conditions. 

My SnapJacks are Bidirectional Zeners and as such never see current except during transient suppression. It is hard to judge their failure modes since their electrical failures are virtually impossible when used as specified. Mechanical failure from vibration would be possible, and then they wou= ld fail open. I have never seen or heard of a failure short of a lightning strike.

The issue of zeners, bidirectional zeners --or even diodes in general-- failing short always bri= ngs up the idea of adding a fuse. This fuse could be far less reliable tha= n the diode. A better solution would be to add a series resistor to limit the fault current, but it would be problematic as to whether or not this improv= es the function of the suppression diode.

By the way, the quest= ion of lighning strike suppression needs addressing. Lightning is attracted differently on composite or metal-skinned aircraft, but in general, adding = arc suppression to the "corners" and the main busses of the aircraft seems to be a cost effective thing to do.

You're welcome to rep= ublish this on any list.

Regards,
Eric M. Jones
emjones@charter.net
www.PerihelionDesign.com 113 Brentwood Drive
Southbridge MA 01550-2705
(508) 764-2072

 

 

 

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