a "what if" for local battery operation WE307

[markosjal]

What if I alrady have good stable 7VDC  illuminating my WE princess and trimline dials delivered on 2nd pair in entire home (yellow/black) . This being DC not AC.

Now I want to run a 307 that uses local battery , un-modified.

What if I use a 3V buck converter (or voltage regulator) on the voltage on 2nd pair (yellow/black) and connect this to the phone instead of local battery?

I read somewhere that switching power supplies should not be used but not sure of why.

Mark

[Jim Stettler]

I read somewhere that switching power supplies should not be used but not sure of why.

Mark

I have been told talk battery is battery powered to prevent (hum) interference .
If a power supply is well filtered it shouldn't  cause (hum) interference.
My guess is that was what they were referencing. The talk path power on a KSU power supply should be good for talk battery.

This is all guesses and opinions,
Jim

[dsk]

Some ifs ...
If it is the only one where the DC is connected to the voice circuit, and if it is not in connection with earth/ground yes it could be done.

The best way to do it would be a choke in each wire  + and -, but probably not needed.  An electrolytic capacitor rated 20V (ish) and at least 100 uF across the battery terminals and a suitable resistor in series on supply side will probably be OK. If you put in a zener-diode instead of the resistor the zener should have a voltage drop of 3V . I believe a 3.3V is standard value. If you get hum, a choke in series with the diode may help.  (Larger capacitor may also help)
dsk

[RB]

Like has been stated...
If the supply is solid, your all set.
If after you try all this and it still hums, or sings, try a different power supply.
Switching power supplies are basically made 2 ways,
1- pretty well filtered to eliminate flicker, in lite circuits. And reduce hum/ringing in audio circuits.
2- poorly filtered because flicker and ring are not an issue.
"it is up to you to find out which one you have".
I have a LB socket in each room, with a 5volt power supply supporting all of them.
I have had no issues with doing this.
Beer

[Jack Ryan]

I have a LB socket in each room, with a 5volt power supply supporting all of them.
I have had no issues with doing this.

Perhaps you were lucky - or careful. Many LB telephones connect one side of the battery to the line. If you get the polarity of the telephones reversed the power supply shorts.

There are a heap of patents and at least one BSP that address this issue.

Can be done of course, but cautiously.

Jack

[RB]

Well, so far, anyway, it has not been an issue for me.
I have heard/read some things about this very thing.
There are 3 Leich on my LB board, 3 brit phones on my pony board
all wired the same to support a LB phone, or CB phone. I need options...
I believe all are wired the same, but I will look into that even more.
Thank you Jack. for bringing that up.
Keep us infoed, Mark.

[dsk]

The problem Jack mentions is of course there, in addition a common "battery" unfiltered and with wires with a resistance may cause that if 2 and 2 phones are connected, you may have a "overhearing"  between the 2 conversations.   The risk is there, but probably not significant.

To isolate the problem of polarity and risk of shorting Capacitors in al line wires will eliminate that, but that mix will again increase the risk of "overhearing".

No reason to repair something that is working, just a good idea to know why, when tings do not turn out as expected. 

dsk