[TowerTalk] Relays for High Power

[Jim Idelson]

Here's a real-life experience that may shed some light on the capacity of these 
relays.

I have a switchable 2-element 40 meter delta loop array [described in Low Band 
DXing]. One evening back in October, after working a little 40 meter cw DX, I 
put the headphones down and walked away from the rig for a dinner break. In the 
middle of dinner, nearly an hour later, we were interrupted by loud beeping 
sounds coming from the radio room. When I arrived in the room, I found the 
transmitter was key-down and the ACOM amp was very hot and had shutdown due to 
thermal overload. The source of the keydown problem was an intermittent short 
in one of the key line cables. After letting everything cool down [including 
me], I determined that the equipment had not been damaged [thank you ACOM], but 
that the 40 meter loops were no longer switching and SWR was bad. I made a trip 
out to the tower with flashlight and screwdriver in hand to look at the switch 
box. As soon as I opened the box, I knew I was at the source of the problem. 
The smell of burned and melted parts was strong! I removed the switching relay 
from the socket [yes! a socket!], and brought it inside for inspection.

I am looking at the relay right now. The relay is an NTE DPDT unit purchased 
from Radio Shack. It is rated at 10A 240VAC/28VDC. The unit has a 12v coil. The 
damage to the unit was clearly due to HIGH CURRENT. The entire unit was melted 
to the point where all the plastic structural pieces stopped being structural. 
Even the outside clear plastic case is very deformed from the heat. Eventually, 
things inside the relay moved around so much that the relay developed a short 
across both elements of the antenna. Looking closely at the relay, I see that 
ALL the metal parts had become very hot, including the contactors, the 
interconnecting wires and the relay frame. I believe that the current through 
the relay greatly exceeded the capacity of the contacts, and that the heat 
generated at the contacts was distributed to every metal part of the relay, 
causing all the plastic parts to melt.

This is the only relay failure I have had in any homebrew antenna switching 
setup here. I replaced the relay with an identical one, and haven't had any 
problems since. Bottom line - I think the 10A contacts in this relay are WAY 
BELOW the required rating for handling continuous power at the legal limit. I 
would be quite concerned if I did a lot of RTTY operation. However, they seem 
to handle application of cw and ssb ok.  But, I'll bet that the relay is pretty 
warm after an hour of running Europe at 140/hour. In the future, I will 
consider designing in bigger relays.

73,

Jim K1IR

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm