Go Down

Topic: Critique my circuit: Poor man's UPS (Read 839 times) previous topic - next topic

Jack Christensen

Jan 26, 2012, 05:00 am Last Edit: Jan 26, 2012, 05:05 am by Jack Christensen Reason: 1
I have a project that I want to keep running for at least a few hours if the utility power fails. My initial thought was to use a UPS unit designed for a computer, but that's relatively expensive, and it turns out that it doesn't run all that long, I get an hour or so with APC's smallest unit. I imagine they're optimized for larger loads and shorter periods of time.

So I came up with this idea. I've only done initial testing; it seems to work. The next test will be to see how long it will actually run the circuit.

Would appreciate any input from the forum.

Circuit theory: With power supplied via J1, Q1 is kept saturated, pulling Q2's base to +9V and preventing it from conducting. Without input power at J1, R2 pulls Q2's base to ground, causing it to conduct and power the load from the battery. Q2's VCE was < 50mV when powering the load from the battery, so I was pretty happy with that. Didn't have any Schottky diodes handy, but I may try one for D2, just to squeeze a little more out of the battery. Maybe D2 can be eliminated altogether.

Thanks in advance!
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

macegr

If you're OK with the diode voltage drop already, then those two diodes alone are enough to do what you need.
Unique RGB LED Modules and Arduino shields: http://www.macetech.com/store

Jack Christensen

#2
Jan 26, 2012, 01:38 pm Last Edit: Jan 26, 2012, 01:39 pm by Jack Christensen Reason: 1

If you're OK with the diode voltage drop already, then those two diodes alone are enough to do what you need.


I should have mentioned, the wall wart I'm using actually supplies 400-500mV less than six fresh AAs. So with the two-diode approach, even with the wall supply on, the load would be powered from the batteries until they drop below the wall wart voltage. Not sure how much of the battery's capacity that would represent, but I wasn't sure I wanted to lose that.

I'm considering redesigning the load circuit to operate at 3.3V instead of 5V. Then I can use 4 x AA cells, and if I stick with the same wall wart, then the two-diode approach is a winner.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

Constantin

If voltage drop across the diodes is a concern, you could also go the mosFET approach. At least that's what I remember the folk over at picoUPS do.  

That pre-engineered solution may be better (even if it's oversized) since it'll also charge a 12V battery (i.e. set and forget). You get 12V out, could use a switchmode VR from dimension engineering to deal with the Vdrop after that. That's the route I'm going to take on a phone base station in my house that will also serve as a USB phone charger.

kf2qd

Why not use some recharge-ables and leave them in the circuit all the time. At least then you won't have the system failing because someone forgot to change the batteries. Then you power supply just has to be regulated to the proper voltage for the batteries. NiCads peak out at approx 1.25 V/cell and run through a good part of their discharge cycle approx 1.2 V/cell so you coud regulate to 5V and supply that 5V after the regulator, straight to the arduino and the batteries.

Go Up