Reason for using A/C power adapters for telco equipment?

[Thomas Tornblom]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

Thomas

 

[Sam Goldwasser]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

Kind of doubt it's anything that subtle.

One possibility is that it's easier to generate positive and negative voltages
using an AC input than a DC input. So, where there is analog circuitry,
it makes sense to go with an AC input. You get +/-VDC almost for free.

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[Homer J Simpson]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

Price and reliability.



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[hr(bob) [email protected]]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

Thomas

Reliability

 

[me]

Reliability

Naw, price. Both are reliable enough that you will most likely replace
the device before the power supply. So shell out 2 yen/peso/etc for
transformer times 8 million or 15-20 for same. Charge retail for
something in the middle and use the cheap one...

 

[Franc Zabkar]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

Dialup modems need to generate +/-12V for the RS232 serial interface.
This can be done with a voltage doubling rectifier and an AC supply.
The alternative is to use a DC adapter with a +5V RS232 interface IC
(eg MAX232) which generates its own RS232 supply rails.

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

Look inside these devices and I believe you will find switchmode PSUs.
If so, then you will gain no significant efficiency benefit from an
external switchmode DC adapter.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

Thomas

If the device requires an AC adapter, then it can usually tolerate a
DC adapter of either polarity without damage ... unless there is a
transformer on the input.

- Franc Zabkar

 

[rebel]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

No, the isolation comes from the transformer regardless of whether the device
delivers AC or DC.

Think price/volume, and then consider that you can't get the connector polarity
wrong with (isolated) AC.

 

[Thomas Tornblom]

Dialup modems need to generate +/-12V for the RS232 serial interface.
This can be done with a voltage doubling rectifier and an AC supply.
The alternative is to use a DC adapter with a +5V RS232 interface IC
(eg MAX232) which generates its own RS232 supply rails.

Yes, this was my thought also, but the DSL and SIP stuff have no
RS-232 type serial connection. The SIP adapter needs a ringer
generator, which is on the order or 80V AC, and with selectable
voltage and frequency, so I see no immediate gain from feeding it AC.

Look inside these devices and I believe you will find switchmode PSUs.
If so, then you will gain no significant efficiency benefit from an
external switchmode DC adapter.

Unless the transformers, as in my case, appears to be undersized and
runs fairly warm.

If the device requires an AC adapter, then it can usually tolerate a
DC adapter of either polarity without damage ... unless there is a
transformer on the input.

You are correct that many of them use switchmode PSU:s internally. I
connected a 12V DC switching adapter to the DSL modem before noticing
that the original adapter was AC, and it worked fine. I guess 12V DC
may have been on the low side though.

I was just curious why it appeared that anything telco related used
AC, while almost everything else is DC.

- Franc Zabkar

Thomas

 

[Lionel]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

There are several reasons why modems (whether POTS or xDSL) usually
use transformer-based, AC output plugpacks:

(1) Modems need multiple power rails for the digital & analog
circuitry, typically +5V for digital & +/-5V or +/- 12V for the analog
circuitry. Supplying AC & regulating onboard only requires 2 way
connectors + cable instead of a 4-6 way cable & connectors, which are
much more expensive & fragile,

(1a) Switchers are much noisier than transformer supplies, which is
bad for analog I/F circuits.

(2) Onboard regulators are much easier to shield,

(3) AC plugpacks are cheap, "off the shelf", stock items, but
multi-rail DC packs are usually custom-order units. (Eg; for some
Cisco xDSL modem+routers.)

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

It's much less relevent than the reasons I've listed, but it /is/
harder to get telco certification for anything powered by a switcher,
due to line/power isolation rules - at least in my country,
(Australia), which is 240VAC mains, & the toughest phone-line
isolation rules (2.5KV, IIRC) in the world.

 

[James Sweet]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high
voltage
to the telco switch?

I think Sam's response is probably pretty close. At some point I had an
external modem which had an AC output adapter, on DC the modem would light
up and appear to work, but it couldn't communicate with the PC. In that
case, apparently they were using the AC to get the + and - voltages for
RS-232.

 

[James Sweet]

Naw, price. Both are reliable enough that you will most likely replace
the device before the power supply. So shell out 2 yen/peso/etc for
transformer times 8 million or 15-20 for same. Charge retail for
something in the middle and use the cheap one...

I don't see how price is affected one way or another. You either put the
rectification and filtering in the adapter, or you put it in the device,
since the two are bundled it's the same either way.

 

[me]

I don't see how price is affected one way or another. You either put the
rectification and filtering in the adapter, or you put it in the device,
since the two are bundled it's the same either way.

The poster asked why they use mains transformer power supplies versus
switching power supplies.

 

[Ross Herbert]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

Thomas

I am convinced that Lionel is on the right track. Any item of telecom
electronic equipment which is powered from the mains supply must use a
properly certified and tested power supply. The developer of the
telecom device prefers not to have to go to the trouble and expense of
performing this step and therefore leaves that up to the power supply
developers.

 

[Dave Plowman (News)]

The poster asked why they use mains transformer power supplies versus
switching power supplies.

Read it again.

 

[Gary Tait]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high
voltage to the telco switch?

Thomas

My telco provided broadband modem (Elastic Networks) has an electronic
SMPS supply, as did my old D-link router. My Linksys router and SMC
switch have iron trasnformers. My NAS has an SMPS.

Dialup modems would have AC to make it easier to make dual voltages for
RS232. Otherwise linear supplies tend to produce less "noise" than
SMPS.

 

[Marra]

What is the reason that many/most telco related equipment like analog
modems, dsl modems, SIP adapters etc use A/C (iron core) power adapter
instead of D/C adapters?

I'm hunting down the mass of inefficient power adapters I have, and
I'm replacing many of the hot running transformer types with more
efficient switched ones, and I now have two that are A/C, the SIP
adapter and the DSL router/modem.

I have checked around my stash of old stuff, and with the exception of
an old ZyXEL analog modem, all of the modems have A/C adapters, and
also the DSL modems and SIP adapters, basically anything that connects
to the phone company or have analog phones attached.

Is it perhaps an insulation issue or added safety from feeding high voltage
to the telco switch?

Thomas

Coz they are cheap.
Also a seperate adaptor is often easier to get through approvals.