[RFI] Doorbell Transformer RFI ??

[Roger (K8RI)]

I'll try again:

The "door bell" problem has been around since before switch mode power 
supplies existed.  I think it may even predate the disconnected TV 
preamp in the attic.
I think we can rule out the old mechanical door bells, but the early 
electrical ones were nothing more than an electrical circuit closure to 
electromagnetic hammers hitting resonant tubes.  These involved a small, 
low voltage transformer and many, many feet of wires to momentary push 
button switches. Usually wired so the back door provided a single note 
and the front door two tones in succession.  This evolved into a number 
of tones, but these were nothing more than a transformer, 
electromagnets, and a LOT of wire.

Once solid state came about, many of these old systems were simply 
disconnected and the new ones installed, leaving, in some cases, many 
hundreds of feet of wire.
  Our old farm house probably had at least 200 feet of the old, fabric 
covered, solid, door bell wire.  IIRC this wire insulation (such as it 
was) was white and red, or blue in equal amounts,  The wire was held 
with spring clips. I'm not sure of how to describe those clips, but 
those clips were not the best electrical connections.  Given hundreds of 
feet of this wire connected to the low voltage side of a 5:110 VAC 
transformer that may have the primary floating, or connected to a pair 
of wires back to the panel, with a poor connection to the secondary.  We 
"might" have over a thousand volts of low current on the primary side 
with the wire back to the panel acting as an antenna?

I don't know either Jim. I'm just speculating.

73

Roger (K8RI)

On 3/14/2016 Monday 11:55 PM, Jim Brown wrote:
> 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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