John,
I would love a schematic and am willing to wait as long as it takes... Your
last schematic for me (a TTL sensing relay setup for an old video poker
machine, worked great and is still working!!)

thanks in advance.

bob in phx.
"John Fields" wrote in message
...
On Wed, 18 Oct 2006 15:35:57 -0700, "Bob in Phx"
wrote:

To All,
I have a few great old Boatanchor tube type radios. The number one issue
that I hear from others with these types of radios is transformer failure.
The majority of transformer failures are from bad capacitors that slowly
allow the amperage being pulled from the transformers to go up. This
causes
the transformer to overheat and the internal windings to short. The best
protection for this issue appears to be replacing all the old capacitors
with new ones. I have done that, but still worry and am constantly check
the
heating of my transformers. I do this by hand and if it feels " to hot" I
turn the radio off. So how hot is two hot, I am not sure, but I am
guessing
someplace in the 110 degree to 150 degree area. I will be posing the how
hot
is too hot question over on the antique radio newgroup... But for now I
was
wondering if an externally powered circuit could be built to shut off the
radios main power, if the outside temperature of the transformer goes
above
a certain level????? I would suspect that a termisitor, a power supply and
a
relay would lashed up together. I am not sure of how to do this... I would
also like to add that I may be barking up the wrong tree and maybe all I
need is a very accurate fuse that is matched to the "highest" amperage
encountered during normal operation of the radio. Over amperage, thermal
runaway and damage of the transformer is what I am trying stop. Any
ideas?????

---
The fuse idea isn't a bad one, but what you'd have to do would be to
match the fuse's rating to the increase in the transformer's current
draw as a function of time.

Most fuses are rated to carry their rated current forever, but as
the current through them increases, they'll fail sooner. For
example, if you have a 1 ampere fuse in a circuit which is pulling 1
ampere through the fuse, the fuse will never blow, but if your
circuit starts pulling 2 amps the fuse will blow in a certain amount
of time. 4 amps will cause it to blow more quickly, and so on.

The various fuse manufacturers' data sheets will give the "time to
blow VS current" data.

OTOH, if you'd like to disconnect the radio from the mains when the
transformer gets to a certain temperature, you could use something
like an LM35 epoxied to the transformer laminations followed up with
a comparator and a normally-closed relay which would interrupt the
mains when the transformer got hot.

The nice thing about this scheme is that it's a thermostat.
As long as the transformer laminations stay below the trigger
temperature the radio will play, but when they get hot enough
that'll disconnect the radio from the mains and allow the
transformer to cool down until it gets cool enough to let the relay
drop out and connect the mains to the primary of the transformer.

If you like I can post a schematic for you, but with my current
workload it'll have to be next week sometime.

Maybe someone else can pick up the challenge?


--
John Fields
Professional Circuit Designer