[R-390] NTC Thermistors, sizing, measure twice, add once

[David Wise]

I don't run the ovens.
With ovens off, the R-390A uses about 140W or 1.2A, well below the CL-90's 2A max.
The spec says that at 1A it's 3 ohms; at 1.5A, 1.75 ohms.  At 1.2A it will be in
between, and will dissipate an intermediate power, maybe 3.5W .
It's hot enough to burn fingers, but so are power resistors and 26Z5W's.
I soldered the leads and have seen no evidence of melting, but I also
kept them as long as possible without compromising safety.
It wouldn't pass a shake test, but I'm not losing sleep over it.

Dave Wise 

>-----Original Message-----
>From: r-390-bounces at mailman.qth.net 
>[mailto:r-390-bounces at mailman.qth.net] On Behalf Of Tisha Hayes
>Sent: Friday, March 30, 2012 9:32 AM
>To: r-390 at mailman.qth.net
>Subject: [R-390] NTC Thermistors, sizing, measure twice, add once
>
>If you opt to insert an NTC Thermistor in series on the incoming AC it
>should be sized to have a minimal impact when the radio is 
>running "steady
>state". If it is undersized (underrated) they will run hot and 
>dissipate
>some electrical and thermal watts.
>
>There is a complete data sheet on the NTC Thermistors at;
>http://www.thermometrics.com/assets/images/cl.pdf
>
>A CL-90 might be a little "light duty" for a R-390/A, maybe 
>you should have
>a CL-80 or CL-70 in there instead.
>