[GreenKeys] Smoke and Fire

Tue Jan 9 14:41:35 EST 2018

Michael,
It has been my lifelong experience that it is the JOB of the component to blow up first to protect the fuse.  Your example seems to fit that rule perfectly.  I know for a fact that in the old TVs, the picture tube would always blow first to protect the fuse - especially if it was a color tube, and the more expensive the part, the more likely it would fail first.
--dave

      From: Michael Zahorik <m.zahorik at sbcglobal.net>
 To: drlegendre . <drlegendre at gmail.com>; Wayne Durkee <teletypeparts at comcast.net> 
Cc: "GreenKeys at mailman.qth.net" <greenkeys at mailman.qth.net>
 Sent: Tuesday, January 9, 2018 1:45 PM
 Subject: Re: [GreenKeys] Smoke and Fire

Actually the fuse is a 3 amp slow blow, which will take 3 amperes for quite some time.
 Mike Zahorik (414) 254-6768

      From: drlegendre . <drlegendre at gmail.com>
 To: Wayne Durkee <teletypeparts at comcast.net> 
Cc: "GreenKeys at mailman.qth.net" <greenkeys at mailman.qth.net>
 Sent: Tuesday, January 9, 2018 12:28 PM
 Subject: Re: [GreenKeys] Smoke and Fire

With a 3A fuse protecting it, that 120V coil can continuously source ~360W of heat before the fuse starts to think about letting go. Now take a look at the coil, and compare it - size, shape and mass - to a 360W power resistor. 
360W is a lot of heat rejection for a little coil that lacks any kind of serious heat-sinking. Heck, even half that sounds like it would be a major problem.. are we all sure that a 3A fuse is correct for the application?
On Tue, Jan 9, 2018 at 11:11 AM, Wayne Durkee <teletypeparts at comcast.net> wrote:

 Good info. Thanks Bruce.

Wayne
KB1FDW

 On January 9, 2018 at 11:57 AM Bruce Gentry <ka2ivy at verizon.net> wrote:

 I am not yet familiar with the coil and magnet you mention in a 33, but have decades of experience with coils, magnets and relays of all sorts in elevator controllers. If the coil in question was disigned for 120 volts, and designed to be energized for a reasonably long time-several minutes or more- then the failure was almost certainly an intra-winding short. These occur when a turn or so of the winding shorts out, almost always deep inside the winding where heat can not escape easily. One shorted turn creates heat, which causes more shorts, a vicious cycle begins and continues until the winding burns up and opens. Age is the main contributer to this, useage heats the  coil and over the years the insulation fails. In addition, older insulating materials were not as robust as newer ones can be. Other considerstions can overheat the coil as well. Over voltage will cause overheating, and under voltage can as well if the plunger or armature the coil is pulling does not move all the way to it's seated position. AC magnet coils draw far more current if the plunger or armature (clapper) do not move all the way and "seal".  As for not blowing the fuse, it's purpose is only to keep the device from starting a fire in the building. There is no consideration in most cases to preserve the equipment from damage as long as whatever happens inside does not start a fire outside. The safety codes are especially loose  for equipment that is not typically kept and operated in a residential setting.

        Bruce Gentry, KA2IVY

 On 1/8/18 6:18 PM, Michael Zahorik wrote:

    Well...... it seems like only a few hours ago, I was talking about how well my ASR33 unit was working. This evening I was reading a paper tape into my PDP8E and half way through the read, I noticed smoke and fire inside my ASR33. Turns out the distributor trip magnet caught fire. Looking at the circuit I find that it is very simple. This coil is 120 VAC, protected by a 3 amp slow blow fuse (which did not blow) and the only control  that I see is the tape tight/out or reader switch off contact. I have removed the coil and it appears to have burnt in mid coil, I can see red wire at the top and bottom. This coil reads open and I see no short to the core. This magnet has a number on it, 1800853. While the magnet is out, re energized the unit and there are no problems, smoke and/or fire that is. I can read 120 VAC at J4 pin 11 & 12, when the unit is in either local or line and there is paper tape in the reader. If the tape is removed or the Local/Line switch is turned off  the 120 VAC goes away. I'm betting the coil somehow shorted internally either to itself or the core (ground). What are my options on finding a replacement magnet? Thanks for the help.   Mike Zahorik (414) 254-6768  

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