"Mini-UPS" for wall wart

[Tommy Phillips]

I'm looking for a little advice. I have put my cable modem into a wiring box in the wall, and I'm pretty happy with it, except that I get very short power hits once in a while. Sometimes, the network connection drops, sometimes it doesn't.

So I'm thinking about putting a minimal circuit with a biggish capacitor between the wall wart and the modem to smooth out the drops.

I don't really want to put in a commercial UPS at this spot, because there is not room in the wall box, and I would have to re-wire the outlet that is in the box, and even the smallest UPS would be overkill for this application.

It has been a long time since I designed or built any circuits, so I want to get the benefit of all the experience and creativity on this board before I get into too much detail.

I'm thinking along the line of maybe a 1 F supercap, something to limit the initial charging current, and a fuse to protect the modem. I suspect that my ideas about this are not only vague, but incomplete.

The wall wart is labeled 11VDC, 800mA.

Any ideas about how I should proceed?

Thank you very much.

-Tommy Phillips

[philba]

super caps get pretty pricey when you get above 5V or so. However, you are on the right track. I'd do the following:
- come up with a guesstimate of how long the drop outs are for.
- figure out the device's current draw (and thus the implied resistance)
- determine the minimum input voltage for the cable modem to continue running
- using the cap discharge formula, you can determine how big a cap is needed to cover the drop out gap. I'd bet something well less than 1F would do.

I would add an inline Schottky rectifier between the wall wart and the cap to prevent the WW from drawing down the cap. Also, as you point out, you need to limit inrush current for the cap during initial charging. You could be looking at a blown WW otherwise.

[haklesup]

Actually super caps designed for car audio would be perfect right off the shelf with no additional circuitry. Some even have LED voltage displays built in. The cost of these has come down appreciably in the last few years, I assume because the market for them is saturating.

Looks like about $100 retail (best buy) and ~$50 on ebay up to 2.8F caps. Shop around, some of the smaller catalogs may have a deal. Fry's Electronics has on (1.2F) sale for $29 this weekend if you live near one.

http://www.eepn.com/Locator/Products/In ... leID=28446

Something less than 1F would probably do but the cost and package makes the car audio type attractive still. A couple of computer grade 33,000uF caps will cost almost that much and be harder to install mechanically.

Be sure to put a high value (1M-10M) bleeder resistor across it so it can discharge when you do intentionally turn it off (if one is not already built in). I bet those things can throw a good arc when shotred accidentally .

[MrAl]

Hi there,

Since your voltage is DC, you can also get away with using a back
up battery pack. The pack would be trickle charged when not
needed (series resistor and diode) and would kick in via another
diode when the input power dropped out.
The pack can be made from NiCd's, AA size or larger. If you have
a portable drill battery pack laying around you can use that.

You can use NiMH but you'll have to make sure the charge
current isnt too high. I've been experimenting with constant
low level charging of NiMH for long periods of time and where
the device isnt used very often and so far it's working out fine,
but that's only been tested for about a year now, using AA cells.
NiCds have been used this way for ages.

[Chris Smith]

The battery pack and diode Idea is the way to go.

Its small and efficient.

I put one on my digital clock about 12 years ago because of the same power glitches and then having to reset the clock, and so far I haven’t had to reset the time once in a dozen years now.

The one time I did reset the clock was when the power went out for most of the day.

Even AAA size batteries will go for many hours.

[dacflyer]

just make sure the battery pack is rechargable {:>P

[Geoff]

Does your cable company stay online when the drop-outs happen? I did something similar (Gel battery and 50watt inverter to run the cable modem and router), and could access the computers on my network until the UPSs ran out of juice, but the cable modem didn't have anyone to talk to because the cable company dropped out when the AC line on my street dropped out.

I think the battery/diode method is probably the best for a low power device like a cable modem.
Geoff

[dacflyer]

cable nodes here are on ups also...
we have fiber optic cable around here. and then it goes to nodes etc. and then distributed normally..
wish i had cable tv here.. antenne life sucks..lol

[stevech]

My cable company has parts of their field network that are UPSed and parts that aren't. Service outage depends on where the power failure occurs.

It's getting better with digital phone on cable, but they put that no-service on power failure warning in bold face type. And they don't ship the backup battery with their phone adapter. Probably on advise of lawyers.

[RDL2004]

Why not just take the easy and cheap way out?

http://www.newegg.com/Product/Product.a ... 6842117003

- Rick

[stevech]

Why not just take the easy and cheap way out?

http://www.newegg.com/Product/Product.a ... 6842117003

- Rick

fine, if your ISP has all of his head-end and outdoor system elements on UPS.

[haklesup]

Tommy, how long are these dropouts? The duration will dictate the proper solution

For long >5s brownouts a UPS or battery pack would be best.
for medium .2-2s the super cap or something a little smaller would be best.

for glitches in the <<1s range then a couple modist 470uF caps should be enough.

Observe the time it takes the LED pilot lamp on the router to fade out when powered off to get a feel for the decay time.

[Tommy Phillips]

I found a low-end Ultra UPS for $20. I couldn't convince myself to go to the effort of building a circuit when I could solve the problem for about $30 (including shipping.)

I appreciate all the advice. Thank you very much.

Now I just have to build a cabinet around the wiring to hid all the ugly.

-Tommy Phillips