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Topic: Simple UPS for 2 iPhones and 1 house phone (Read 3818 times) previous topic - next topic


Mar 25, 2013, 03:26 pm Last Edit: Mar 25, 2013, 03:43 pm by Constantin Reason: 1
Hi all,

About a year ago, a neighbor's tree interrupted our power service for the better part of a day. Since the APC power supply that was supposed to keep the phone lines and cell phones charged had several annoying features, I decided to cast out for my own UPS for the house phone as well as the cell phones. The APC UPS is built for a totally different purpose - Lots of circuits, inductors, etc. to produce the semblance of a sine wave from a DC power source. The crazy thing is of course that for most applications all that AC power is turned back into DC, with the added bonus of an annoying recurring beep, vast inefficiency at lower power operation, and hence very short runtime despite the use of two 7Ah 12V SLA batteries inside (cabled in series)

So I decided to take a different route, electing to use a PICO-UPS charger board (green) for the battery and a small 5x5cm board (blue) of my own design to distribute power to the phones. The black phone jack on the left of the board is for the house phone (6.3V) while the dual USB jack provides 5VDC and features the resistor divider that adafruit was so kind to document to enable Apple users to recharge their iPhones. All outputs are protected with individual PTCs, feature individual LEDs (to alert me to a PTC shutting down a circuit) and two Dimension Engineering DC-DC power supplies provide the respective DC outputs. An external wall wart provides 15VDC @ up to 1.6A to charge the battery and provide power to the phone board.

I really like the Pico-UPS board so far thanks to its small size and very cool operation. My only improvement suggestion would be to change the power inlet from a .187" blade design to something compatible standard wall warts (i.e. 5.5mmx2.1mm center positive jack). But that is a totally minor issue. I am also super happy with the Dimension Engineering DC-DC converters (here and here), among others. They are well-designed, stable, and allow excellent adjustment of the output voltage. Yet to be implemented is a safety feature (i.e. 2A inline fuse on the battery line) and an enclosure.

Yes, there cheaper ways to do all this. Our friends at Dimension engineering have since released more power supplies, etc. But the 3A charger has ample capacity to charge two iPhones at 1A each while also allowing the house phone base (DECT) to continue to operate. Unlike the APC approach, there is no beeping in UPS operation (just some silent LEDs), the conversion efficiency is on the order of 75%, and the battery will be super easy to replace as needed in the future. FWIW, Pico UPS also offers units with programmable outputs, allowing enterprising users to run devices like routers off a similar setup with a stable voltage.

Lest it come across that way, I am not trying to bash APC and their lines of of products. Rather, I believe that there is a market for UPS's that provide efficient power at a regulated 12VDC and 5VDC output to complement what is already out there. Unfortunately (and this will likely kill the idea) industry in turn would have to agree on (a) standardized connector(s) to allow direct hookups and hence for devices to bypass the need for AC-DC-AC conversion. However, imagine the benefits for all sorts of electronic devices, from lower potential EMI to longer run time for the same size battery.


Thanks for sharing this write-up and links.  Can you share what is on your blue board, perhaps with a schematic?  Very interested to see what you are doing there.

Thanks, Geoff
"There is no problem so bad you can't make it worse" - retired astronaut Chris Hadfield


Mar 29, 2013, 03:39 am Last Edit: Mar 29, 2013, 03:41 am by Constantin Reason: 1
Very simple circuit, I suspect that looking at the board file is easier though than the schematic (sorry!)

FWIW, the empty rectangles on the board picture are the PTCs. The resistors are show their resistance (not name). All components are through-hole by design so I wouldn't have to bother with a stencil or fiddly soldering.



I liked this post as I do not like LM78L05's which sends full voltage out if it loses ground for any reason.

Does this device (DC-DC)do this too?

Also I see
The picoUPS-100 also has a built-in, two stage battery charger unit.
Do you know its cut in and cut out voltages or are there any details available on that?

Thanks for the post


Have a look at the data sheets I linked to above re: the Pico UPS and the Dimension Engineering gear, I think they should answer your questions.

The Dimension Engineering DC-DC converters are pin-compatible (but larger) than traditional LM780x voltage regulators and they do not need external capacitors (though I still gave them one on the input side). What I like about these DC-DC converters is that they are easy to adjust with a high-quality rheostat. They appear to be very stable re: output, i.e. little to no difference in voltage whether an iPhone is drawing an ampere of power or there is just the trickle of power running through the LEDs.

The datasheet for the Dimension Engineering unit indicated a range from 5-33V. The alleged conversion efficiency for my application appears to be about 85% and 90%+, respectively.

Jack Christensen

Lest it come across that way, I am not trying to bash APC and their lines of of products. Rather, I believe that there is a market for UPS's that provide efficient power at a regulated 12VDC and 5VDC output to complement what is already out there.

Spot-on. APC makes fine products but they are designed to supply relatively large amounts of power for relatively short intervals. Turn that upside down and (perhaps unsurprisingly) they just aren't efficient. I'd always used APC gear, but for low power and long intervals I found that Minuteman UPS units do somewhat better, although there is still a lot of room for improvement.

Appreciate the info on Pico UPS and Dimension Eng.


I note that MinuteMan used to make a UPS with ~12V unregulated battery output. However, that model appears to be discontinued and much of the electronica out there that is UPS-protected really wants regulated 12VDC and 5VDC inputs (i.e. like a ATX power plug into a motherboard). In an ideal world, the manufacturers on both sides (i.e. CPU as well as UPS) would come to an agreement for a standardized plug that offers several voltage outputs and a means to communicate between the devices to establish just what needs to be sent, how much can be sent, etc.

I really like the Pico line of products, as they also offer the option of efficiently running all sorts of standard PC motherboards off of unstable power sources like boat, car, and like systems.

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