[Elecraft] QRM and filtering

[Ken Alexander]

--- 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