[Elecraft] QRM and filtering

[Ron D'Eau Claire]

Just a thought, but a couple of dollars (less than $20 IIRC) buys a battery
operated doorbell that uses a couple of alkaline C-cells that last for many
years. Makes it very easy to add a second bell in a basement shop or garage
that also rings, if needed. 

Been decades since I had a doorbell transformer in the house. 

Ron AC7AC

-----Original Message-----
--- On Sat, 10/23/10, Phil Kane <k2asp at kanafi.org> wrote:

>   If you mean the doorbell transformer, the insulation dries out
>   and turns start shorting, leading to excessive heating and
>   internal arcing.  They usually have an internal thermostat to
>   cut it off when hot, and after it cools down it starts all
>   over again.

If that was true then why don't ALL transformers have thermal cutouts?  Or
stated another way, why are only doorbell transformers allowed to be built
so cheaply that their insulation dries out and arcs?  The thermal switch is
there to protect against cheap doorbell pushbutton switches that
occasionally get stuck in the closed/ON position, which is only supposed to
be momentary.  If that happens the transformer will heat up rapidly and the
thermal switch opens to protect the circuit.

Unfortunately, it looks like the energizing current in the transformer
primary creates enough heat to open the switch just enough to arc and keep
the doorbell operational.  That's what's happening at my home...I really
must replace my transformer again.

73,

Ken Alexander
VE3HLS