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My requirement is to keep running supply of low power devices like DSL modems, etc. Most of these devices input 12V/9V DC so i don't see the value in using ordinary UPS which converts DC to AC then back to DC, lots of conversion lots of power loss lots of money involved. I want a DC UPS output which can be directly input into these devices. DC UPS diagram

I am not pro, so I need a circuit diagram for a DC UPS, that:

  1. Takes 12V input.
  2. Gives 12V output to devices.
  3. If input current is available it will also charge the battery.
  4. If input current is not available, will provide current from battery to attached devices.
Hightower
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Imran Naqvi
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    @ImranNaqvi, This site is really good for getting technical advice. This is not a great place to come to get someone to do a complete project for you. Try starting on your own and come back if you have issues. – Kortuk Mar 10 '11 at 13:14
  • So, really you just want to connect the devices directly to the battery, and then have an intelligent charger on the battery? – Brian Knoblauch Mar 10 '11 at 14:34
  • @Imran Naqvi: you might consider accepting some of the answers to the questions you have asked... – Richard Mar 10 '11 at 17:09
  • @Kortuk, Thanks for your response, i am new to electronics but i don't want that someone should do it for me, it can't be. I just want some directions. – Imran Naqvi Mar 11 '11 at 06:37
  • @Brian Knoblauch, yes you are right. – Imran Naqvi Mar 11 '11 at 06:37
  • @Richard, ofcourse i will if i get one. – Imran Naqvi Mar 11 '11 at 06:38

4 Answers4

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First of all you need to realize that a 12V SLA battery is not, ever 12V, if it's charging then it sits around 14.3V (but that's dependent on chemistry and temperature) when it's discharging then it can be as low as 10V.

The most robust solution is to make sure that all the equipment can tolerate 10-15 V, because that will allow you to get rid of any output regulator which will waste some energy while on battery.

Almost all electronics that says 12V will easily tolerate 10-15V, much of it will be happy with 8-24V as well, the main exception are computers that feed the 12V input directly to harddisks, those devices really like a regulated 12V.

A good charger will be needed that regulates the float voltage according to temperature and also limits the charging current.

One solution would be:

  • A beefy AC/DC off-the-shelf mains supply, a 19V power brick for a laptop could do.
  • A Buck DC/DC converter handling charging, but a simple LM317 could also be used.
  • A Buck DC/DC converter handling the on-line output regulation.
  • A switch (2 FETs) which switches over to battery if the output of the online-converter drops out of regulation.

Buck converters tend to be simpler and more efficient than buck/boost converters, so that's a good reason to prefer those in a design, but if you really want a regulated 12V output then there is no way to avoid a buck/boost converter and the plan becomes:

  • A beefy AC/DC off-the-shelf mains supply, a 19V power brick for a laptop would do nicely.
  • A Buck DC/DC converter handling charging, but a simple LM317 could also be used, especially if the battery is stored at a fixed temperature, (look up "SLA charger lm317).
  • A switch (an opamp controllring two 2 FETs) which switches the input of the output regulator from the primary input DC over to battery if it drops below the battery voltage or simply two diodes.
  • A Buck/boost DC/DC converter handling the output regulation.

A good buck/boost converter topology is SEPIC, because you only need one FET and a single coil, so it's cheaper than two converters back-to-back: http://dren.dk/carpower.html the linked design will output the same voltage no matter what the input voltage is (8-24 V)

... or, if you are lucky, you can just buy one: http://www.mini-box.com/micro-UPS-load-sharing

dren.dk
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    BTW, I tried putting too many volts into a HDD using a modified ATX PSU. They will run up to around 14V - but past that point, the disk goes too fast and the drive keeps resetting. About 18V was when some magic smoke was released... (It was an old 5 GB ATA drive. I didn't verify functionality because I didn't want to blow up a motherboard.) – Thomas O Mar 10 '11 at 15:38
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My suggestion would be to use an above-voltage battery - like a 24V, 36V or 48V battery. The voltage can be stepped up to charge it, and stepped down to run devices. I'd highly recommend a switching boost regulator for the charging (use a dedicated charging IC) and a buck converter for the discharging. Make sure any converter is rated to handle the maximum voltage of the battery - for example a 48V battery may reach 58V when fully charged! Also, be careful of voltages over about 60V (dc; 30V for ac) - they are generally considered hazardous as they can cause electric shock and require proper wiring standards.

Thomas O
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As a starting point I would research voltage regulators and search for a battery that meets your needs. There are different types that could be used such as linear(cheap but inefficient) and switching(little more pricey but efficient). Making sure that the battery will only output current when there is no input voltage is pretty simple. As long as the input voltage is greater than that of the battery, the battery will act as a power sink, and the input will charge the battery. Otherwise the battery will act as a power source. It might also be useful for you to know that a 12 volt car or deep cycle battery is typically around 12.5 volts when it is fully charged. That means that you could set your input voltage at around 12.5 volts to charge the battery and set the voltage regulator output at 12 volts.. Hope this helps.

Richard
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My approach would select a 24 volt battery or two car batteries or similar in series. Then regulate that 24 volts to produce your 12 volt output. Then buy or build a AC charger for the battery(ies). http://www.batterystuff.com/battery-chargers/12-volt/marine-chargers/GEN2.html

kenny
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