Battery Backup for Power Fail

[bdickens]

I have a variant on an earlier question. I have a situation where, when the AC power fails, I need to turn on a DC powered pump and run it until the AC comes back on. It runs easily for 2 days from a 12 v car battery. I was tinkering with this but AC "tinkering" makes me nervous. So I need something that will turn on when the AC fails, and turn off when it resumes. Ideal might be to have a delay of a few minutes (power goes off a lot here). <p>Thanks

[ljbeng]

I am going to use a AC/DC power supply adjusted to 14V dc. I will have a 12V lead acid battery for backup. I am going to use the device below to charge battery when AC is on, switch over to the battery when it loses AC and cut the battery off when it gets low....<p>http://www.powerstream.com/DC-UPS-1212.htm

[Engineer1138]

Bill:
The simple solution sounds like you should switch your car battery with an AC relay. Connect the pump & battery as normal, with the relay contacts acting as a Normally Closed switch. Connect the 120VAC to the relay coil. Now, when the coil is energized (AC on), the battery/pump is open circuit. When AC power is lost, the relay closes and the pump runs. You may want to use a secondary time-delay relay in the battery circuit to delay switching on for whatever time interval you need.<p>Let me know if this isn't clear and I can put a quick schematic up on my site.<p>Like most such things, this can easily be handled with a microcontroller if you didn't want to use the TD relays

[jollyrgr]

Not to be a spoil sport but I would avoid the 120VAC relay since "tinkering" with AC makes you nervous.

Does this pump run off a DC power supply when AC power is available and all you want to do is switch between AC derrived DC and battery or do you simply want to turn on the DC when AC fails? Depending on how you have this setup it could be as simple as a couple steering diodes or a similar setup to Engineer1138's idea.

1138's idea is very sound but may not be to your liking because of the AC coil relay. Instead of using an 120VAC relay coil (and mixing low and high voltage circuits) use a wall wart and a DC relay. There is nothing wrong with Engineer1138's plan except I respect your fear of AC circuits. Using a wall wart makes everything extremely safe at a minimal cost. I'm not sure about you but I have numerous wall warts and they can be scavanged quite easily. Full retail at RS for a wall wart is a bit expensive but any clearance item with a wall wart will work if you MUST purchase one.

If the pump runs off a DC supply while AC is present, you can use the power supply to run the pump AND charge the battery. With a couple steering diodes and a "trickle charge" resistor you can accomplish the same task. See the schematic below. D1 is a blocking diode to prevent power from the battery going back into the power supply and you might be able to get away without it. When power is on D2 will be reverse biased and won't be turned on and won't be conducting. R1 will trickle charge the battery from the DC power supply. During a power failure there will not be any energy coming from the power supply. D2 will now start conducting and continue to run the pump. You didn't mention the pump's current consumption. Use diodes with about 25% or more current rating above what the pump draws.

<img src="http://img425.imageshack.us/img425/2132 ... tch2rs.png" alt=" - " />

<small>[ October 26, 2005, 04:00 PM: Message edited by: Jolly Roger ]</small>

[ljbeng]

Jolly Roger,

Could you expand on your drawing a little? It is very close to what I would like to do for my project except I need to shut the battery off if there is no AC and the battery is low. I wouldn't mind a relay even though that will drain the battery faster. Thanks.

[philba]

If the device being powered is at all fussy about the voltage level, I'd recommend using schottky diodes for their lower voltage drop. of course, high amp schottkys are expensive.

A relay has the problem of not having a smooth cut-over. Probably on the order of 10s of milliseconds of drop out. I suppose a big capacitor could solve that.

I, too, am interested in a circuit to cut out the battery when it reaches a set lower value. I'd have to experiment with this but I start with a comparator, one input being battery voltage divided down (adjustable via a pot) and the other input a voltage reference, also powered by the battery. Comparator output into a PMosfet in series with the + output to shut it off. When the battery V is above the reference, the mosfet is on, when it falls below, it turns off. I'd be worried about the circuit shutting off due to large load transients so I think it needs some work... Maybe I'll spice that circuit. I'm sure someone must have done this before.

<small>[ October 27, 2005, 09:29 AM: Message edited by: philba ]</small>

[philba]

OK, I played with the circuit in spice and sure enough, transient loads are a problem. I solved this by using a cap to buffer the input to the comparator so it doesn't shut off immediately if the voltage drops. Disclaimer, this has never been implemented, just simulated. the real world has a nasty way of intruding on simulations...
<img src="http://www.geocities.com/pcbs4less/batprotect.gif" alt=" - " />

A couple of points.
- I didn't have a voltage ref model, so I just used a 2.5 V voltage source (V3) as input to comparator +. An LM 336 would probably do just fine.
- I chose a random comparator but think that almost any cheap one would work. R1 and R2 form a voltage divider for 13V trigger point. I'd use a trimmer pot in between to allow some limited adjustment of the trigger voltage.
- C1 controls the delay on switching the mosfet off. Notice the Vtrigger line in the wave display. Sensitivity is controlled by R5. Making it smaller increases the sensitivity. I'd put a trimmer pot there as well.
- Hysteresis could be added to prevent chattering at the switching point. I think C1/R5 will prevent this but it needs to get looked at.

<img src="http://www.geocities.com/pcbs4less/batprotectwave.gif" alt=" - " />

[bdickens]

thank you everyone who responded. Some great information and effort here. Thanks again