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View Full Version : OT: Could I run my modem, router etc off my PC's PSU?



gellfex
08-17-2011, 11:37 PM
I sit here staring at a shelf full of all the small computing peripheral devices we need, a router, modem, 2 VOIP modems, my DECT6 phone charger and a scanner, all sucking on warm power bricks all day.

Now, I charge my phone & MP3 off the PC's USB instead of wall plug bricks, wouldn't it make more sense and be way more efficient to run them all off the PSU? Wouldn't all you need is the right voltage DC-DC transformer to plug into the molex drive connectors for each unit? As long as the PSU has enough watts, why isn't this done?

The piles of AC power bricks we accumulate is getting ridiculous, are there any other brick consolidation solutions out there?

Black_Moons
08-17-2011, 11:44 PM
Good idea, though Id be a little wary, Not all devices actualy use incomming power ground as ground, Some might have a full bridge for example, causing a 0.6v drop to power ground on its signals (or causing all the current to flow via signal ground to the PC if its connected)

And generaly yea, would cause ground loop problems to some degree with the PC and current going through signal ground and power ground (Voltage drop across signal ground is the results, power consumption is reflected as an input signal. Ok untill it gets bad for digital stuff, Will affect audio/analog stuff at MUCH lower levels.

With proper design (Balanced audio, proper I/O buffers with large ground diffrential immunity) it would work fine.. but then, not much has 'proper design' these days, stuff has 'cheap design'

Evan
08-18-2011, 12:45 AM
It can be done but you run the risk of introducing noise on the power buss. Just bringing the power outside the case will allow it to act as an antenna for outside emf as well as any glitches that might be introduced by the devices it powers. Using bypass capacitors at each device would help with that as well as shielding the power leads to the computer case. A lot of effort is expended in the design of computer parts that run on the power busses to keep the power clean. There are noise bypass capacitors all over the motherboard for just that reason.

The best approach would be to buy or build a healthy 18 volt supply and then use a bank of individual regulators to tailor the outputs to each device.

Some devices will have "smart" chargers that limit charging current and times based on temperature and/or special charge cycle curves. That especially applies to lithium powered devices and you should use the correct charger for those devices or you risk damaging the batteries.

macona
08-18-2011, 03:30 AM
If you use a DC/DC power converter to run the item you should have no issues. They isolate and filter.

gellfex
08-18-2011, 12:19 PM
Any idea why there doesn't seem to be any products on the market to do this?

macona
08-18-2011, 12:47 PM
Because no one uses modems? ;)

Way too complex for most people to deal with. Good DC/DC converters with decent current handling capabilities are not cheap either. Plus with a lot of things being USB or Firewire, power is supplied down the line. Only higher current items like full size hard drives, printers, etc, need wall warts.

Evan
08-18-2011, 01:58 PM
Also, USB power is not connected directly to the 5 volt buss. It is current regulated at a maximum of 500 ma per host device and devices connected to it are supposed to meet strict standards for power on current surge. That means they have specified limits on the total capacitance permitted to be presented when the device is connected. If the host detects excess surge or constant power demand it shuts off or limits the power.

The host is a special chip in the computer that provides the data and power interface to the power and logic in the computer. This prevents excessive noise and current swings that could affect the 5 volt buss.

Black_Moons
08-18-2011, 02:12 PM
If you use a DC/DC power converter to run the item you should have no issues. They isolate and filter.

Depends on the filter, Transformer based ones usally will, But low voltage DC/DC modules are very often NOT isolated as they are just inductor based, and often use the input voltage to run the controller.

Also, while SMPS would filter output noise from the input, they also emit tons of noise on thier input and output.

gellfex
08-18-2011, 03:42 PM
As expected, nothing's simple. I just get fed up with the chaos. Look how long it took to standardize cells to the micro usb, and not everybody is aboard even now!

I have a dream....where all low voltage devices run off a standard, or even 2, say USB and 12v, and all connectors are standard, none of this reversed polarity nonsense or 5 different voltages and both AC & DC on similar plugs.

I have a milk crate full of wall warts of all voltages from 2-40 and every connector you ever dreamed of. And I still don't have a "ground on the pin 9v dc" for my postal scale without a soldering iron.

Black_Moons
08-18-2011, 03:45 PM
I have a milk crate full of wall warts of all voltages from 2-40 and every connector you ever dreamed of. And I still don't have a "ground on the pin 9v dc" for my postal scale without a soldering iron.

Digikey sells barrel connectors (of all sizes) that come in a nice 'twist apart, solder, twist togethor' format, so you can make a proper looking cable with connector, insted of a slice half way down the cable..