Really basic transistor question

[Bob Engelhardt]

I want to build a device that will take my phones off line. The phones
are connected to a Vonage modem that (I'm sure) uses the POTS standard.
Which is 50v DC at "idle"/on-hook and 90v (RMS) 20 Hz superimposed on
that for ringing. 90v RMS is 250v p-p offset by 50v is -75 to 175 p-p.
Off-hook is 10v DC with low voltage audio superimposed on that. The
phones on the line are all electronic, so (I assume) that the ringing
current and the off-hook transmission current are small. ma's?

I want to put a transistor in the line to disconnect the phones. I have
either 5 or 12v to drive the transistor. I'm assuming that the "on"
resistance should be small so as not to mess with the normal use of the
line. What kind of a transistor is best suited for this? Or is there
another device which would be better? I'm tempted to use a relay, but
that's so 19th century.

Thanks in advance,
Bob

 

Just use the relay. It doesn't care about polarity or anything and you only need one for like 150 volts an under one amp. Get off the two bucks.

Get a DPDT and you get to choose whether the circuit is open or closed when not energized. A transistor can't really do that. Well I guess it can with a little bit of thought but you got a solution right there at "Relays-r-Us".

I have this hammer, it is older than I actually, came from the early 1900s. When I need a hammer it works better than any screwdriver, pliers or anything else in my toolboxes. The concept of the automatic transmission in your car is about 80 years old, same with front wheel drive and disk brakes.

Those "19th century" items still do the job and there is a reason they still sell them.

Hmmm, sitting here PBS is running on the boob tube with no sound but I see this is like "this Old House" or something and what do I see ? The guy is using pipe dope and not that garbage ass teflon tape. A hundred year old method is there ever was one. They were all agog about the teflon tape but I know it is garbage.

 

[default]

On Sun, 24 Mar 2019 10:51:41 -0700 (PDT), jurb6006@gmail.com wrote:

Just use the relay. It doesn't care about polarity or anything and you only need one for like 150 volts an under one amp. Get off the two bucks.

Get a DPDT and you get to choose whether the circuit is open or closed when not energized. A transistor can't really do that. Well I guess it can with a little bit of thought but you got a solution right there at "Relays-r-Us".

I have this hammer, it is older than I actually, came from the early 1900s. When I need a hammer it works better than any screwdriver, pliers or anything else in my toolboxes. The concept of the automatic transmission in your car is about 80 years old, same with front wheel drive and disk brakes.

Those "19th century" items still do the job and there is a reason they still sell them.

Hmmm, sitting here PBS is running on the boob tube with no sound but I see this is like "this Old House" or something and what do I see ? The guy is using pipe dope and not that garbage ass teflon tape. A hundred year old method is there ever was one. They were all agog about the teflon tape but I know it is garbage.

Just use a relay... I agree, or just a toggle switch.

Or if you want to be "State of the Art" (in other words biting your
nose to spite your face) you might try IC analog switches, power
mosfets, etc..

When in doubt or if it is really simple, choose a switch or relay. No
need to get all hyper over it - unless that's what gives you pleasure?
Engineering is about finding the low cost solution that meets all of
the objectives (cost, simplicity, time to implementation,
reparability,elegance, etc.)

Yet there's a certain satisfaction in doing things the hard way..., so
it becomes a matter of your own satisfaction, versus corporate exigent
necessity.

 

[Bob Engelhardt]

jurb & default - thanks for your replies. I will go with a relay, but I
did find an interesting transistor solution. It uses 2 serial MOSFET's
head-to-head (or "back-to-back", or "anti-serial") with a common gate
signal. When one MOSFET is on, the other is conducting through its body
diode.

This is appealing for it's elegance, but also for the good MOSFET
availability, they can be driven with logic, reliable, small, ... . But
the relay is fail-safe - NC contacts conduct even with no power to the
logic.

Thanks again