yes, sure. COM and LPT ports is not provide power.

The modem most certainly got an internal rectifier bridge/voltage regulator, my guess would be an LM7805 or something like that. A 9VDC power source will work fine too. Depending on the regulator, you might even be able to feed it with lower voltages, and lessen the heat dissipation.

You can always open the modem to verify it, you're looking for a 3 legged component, probably with a heat sink, located close to the power connector.

Or are you wondering if it needs a power supply at all? In that case, yes, it does.

See if you can scrape up a free doorbell or thermostat transformer of the off-center tapped type, that has 8V one side, 16V the other and 24V across, quite often the 8V is still working and it blew on the 16V side. Thrift stores in my city generally have a large selection of wallwarts and I do see AC ones.

kaputnik wrote:

The modem most certainly got an internal rectifier bridge/voltage regulator, my guess would be an LM7805 or something like that. A 9VDC power source will work fine too. Depending on the regulator, you might even be able to feed it with lower voltages, and lessen the heat dissipation.

You can always open the modem to verify it, you're looking for a 3 legged component, probably with a heat sink, located close to the power connector.

Or are you wondering if it needs a power supply at all? In that case, yes, it does.

Sorry for not making it clear at all, but yes my question was if DC power source work? I connected a 9vdc source and it doesn't seem to work although some lights turn on, I get nothing on the terminal (ATZ etc) except for an echo (which even happens when I turn it off), but on the first serial character I send it makes a little blip sound. It also seems to pick up my call but it just makes a few non-modem-sounding noises and keeps the call on (keyboard blips occationally when I type something in).

Will I be in better luck if I use a 9vac psu or is it safe to assume this thing is dead?

BitWrangler wrote:

See if you can scrape up a free doorbell or thermostat transformer of the off-center tapped type, that has 8V one side, 16V the other and 24V across, quite often the 8V is still working and it blew on the 16V side. Thrift stores in my city generally have a large selection of wallwarts and I do see AC ones.

I am unable to figure out what you mean, can you please give me a picture of the kind of device I should look for?

This kind of thing... https://www.lowes.ca/doorbell-chimes/utilitec … r_12163915.html

They turn up cheap at yard sales etc. Need to wire it to a barrel plug and mains cord and stick it in a box though.

Wallwart= nickname for power adapter that is built onto the plug.

When electronics call for low-voltage AC:

• It might indeed just be that the rectification & smoothing is done on the device instead of the wall wart for no special reason, in that case feeding DC to the device is totally fine. Example: the Nintendo Entertainment System (NES) takes 9VAC but runs absolutely fine on DC.
• In some cases AC is needed to derive 'special' (higher/negative) voltages through the use of capacitive voltage multipliers or charge pumps. Feeding DC to these usually results in seemingly fine but erratic operation because some voltages will be correct while others won't. Example: the Atari 1050 disk drive converts the 9VAC input internally to regulated 5 and 12VDC, but the 12V is derived from a voltage doubler, so by using it on DC the drive would appear to work, but error out when trying to read/write data because only one of the two power rails is being generated correctly.
• AC could be used for timing purposes, for example as a timebase for a real time clock or as a zero crossing reference for thyristor control of line voltage, but in these cases the transformer is likely to be built-in.

I don't have much experience with external modems, my guess is that it could be possible that the AC might be used to generate something like the RS232 signaling voltages, so I'd try it with 9VAC before assuming it's dead.

aditya3098 wrote:

Hi,
I found an old D-Link DFM-562E++ modem with a serial port. It says it requires a 9VAC power supply. Is this actually needed, and is it common for modems to need an AC power source?

It was common for modems to utilize 9VAC, and if it says 9VAC then that what it wants. I would not even try with 9VDC.

Most likely thing why 9VAC is necessary is to generate +12V and -12V for internal operation and RS-232 port signal voltages, and 9VAC does just those voltages after rectification, while 9VDC does not.