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

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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Price and reliability.

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Reliability

"hr(bob) snipped-for-privacy@att.net" wrote in news: snipped-for-privacy@k79g2000hse.googlegroups.com:

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...

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On Thu, 31 May 2007 20:39:36 GMT, Thomas Tornblom put finger to keyboard and composed:

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.

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.

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

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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.

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.

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

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.

Thomas

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.)

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.

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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.

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.

"James Sweet" wrote in news:wI18i.41$FU4.18 @trndny02:

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

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adapter

of

connects

voltage

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.

Read it again.

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Thomas Tornblom wrote in news: snipped-for-privacy@Hax.SE:

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.

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