The older type doorbell transformer has a safety feature built in. A
thermister monitors the heat in the windings. If it detects overheating it
opens a relay to disconnect power so it doesn't combust.
But even with a normal situation this relay can become pitted and chatter
away causing RFI. An easy fix if it's yours but neighbors are often very
skeptical of changing anything especially if it's not bothering them.
Forced air attic ventilators thermostats can cause RFI as they age
especially if they are near the fan as it vibrates the surrounding area.
Randy
WX5L
-----Original Message-----
From: RFI [mailto:rfi-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Monday, March 14, 2016 10:55 PM
To: Rfi List
Subject: [RFI] Doorbell Transformer RFI ??
I've heard for years that doorbell transformers could be noisy, yet the
mechanism was not clear and I've never found one that was noisy.
But it just hit me -- electrical supply houses sell no-name, unlabeled
switch-mode power supplies to provide low voltage DC for low voltage
lighting circuits, and they CALL them "electronic transformers." Is it
possible that the noisy doorbell transformers I've heard about are really an
SMPS? Or is it a real transformer whose windings have deteriorated?
73, Jim K9YC
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