DC Backup for aquarium pump

Discussion in 'Power Electronics' started by Brad Bellomo, Aug 19, 2017.

  1. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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    I am building a battery backup for an aquarium pump, which runs on 24V DC from an adapter plugged into a standard AC American outlet. The easy, expensive and inefficient solution is to buy a UPS, which has a DC battery, an inverter to convert to AC which runs the adapter to convert back to DC. I don't want to lose 40% of my backup power to conversion in an emergency.

    I've seen plans online without details, probably because this is a simple project and I am just not familiar with this type of circuit. A 12V DC battery (runs pump half speed) is maintained on a trickle charger and connected to a DPDT relay, which when power goes out, cuts power to the adapter and connects power from the battery. When the power comes back on, the relay disconnects the battery and restores the adapter. Most designers feel the need to include fuse(s) or some sort of timer for circuit protection, but I don't understand how or why.

    I haven't used a DPTP relay before, and am not sure which spec I need. I know I need to run DC power from the battery through the NO terminals. I can run either AC power from the mains through the COIL terminal of the relay, or 24V DC power from the the adapter or any voltage from a seperate adapter. The pump draws 40 watts on 24 volts DC. So should I buy a DC or AC relay? Can you give me an example of a relay similar to what I should buy?

    Assuming I don't run current from the mains through the NC terminal, I don't understand the need for any circuit protection. I assume the NC and NO are never both closed on a DPDT relay, but if they were, worst case is running 24V accross the terminals of a large 12V battery for a brief period of time.
     
  2. jbike

    New Member

    Feb 2, 2016
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    I would go with a 12v DC relay, powered by a separate 12v 500 mA DC wall wart. Power goes out, the coil is not powered, and the NO contact closes to the common terminal, completing the circuit from the battery to the pump. The relay only needs SPDT contacts. A relay like https://www.amazon.com/Ehdis-Pre-wi...7939&sr=1-14&keywords=12vdc+relay+low+current
    would work. A fuse of 4 amps or so might be good protection in case of a short.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Why not use a 24vdc relay on the present power supply for the relay, then the relay is not powered when it switches off over to 12vdc.
    Max.
     
  4. jbike

    New Member

    Feb 2, 2016
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    I would go with a 12v DC relay, powered by a separate 12v 500 mA DC wall wart. Power goes out, the coil is not powered, and the NC_1 contact closes to the common_1 terminal, completing the circuit from the battery to the pump. NO_2 contact is closed to the common_2 when power is on, so your 24 volt normal power runs through those contacts. The The relay could be https://www.amazon.com/Uxcell-a1106...rd_wg=jjTio&psc=1&refRID=Y8NHRY5NANCCFTPFS9XV
    would work. A fuse of 4 amps or so might be good protection in case of a short.
     
  5. Reloadron

    Distinguished Member

    Jan 15, 2015
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    You can use any of the SPDT (Single Pole Double Throw) out there and there is likely hundreds to choose from. You are just switching the source power between Battery or the Mains Supply. Placing a pair of 12 Volt SLA (Sealed Lead Acid) 7.0 Amp Hour batteries in series would likely run the pump for several days (assuming generic aquarium pump).

    The relay will have a Comm, N/O and N/C (Normally Open and Closed) contact. The N/O goes to the mains power supply + Out and the N/C goes to the Battery +. All of the common - (Negative) lines are tied as a common. Depending on the output of the 24 VDC Mains Supply it could possibly be used to charge/maintain the batteries or just buy a 24 VDC small battery trickle battery charger or maintainer. I would also run with a relay having a 24 VDC coil as they are very common.

    Ron
     
  6. jbike

    New Member

    Feb 2, 2016
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    A simple diode might be all you need, you might connect the battery ground and the 24vdc pump negative together. The battery 12 volts connects to anode of the diode, with cathode connected to 24vdc positive of the pump. That way, power fails and current flows though the diode to the pump. When power is restored, 24 volts can't backflow to the battery because of the diode.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    ..... or you could use just a hefty diode between the 12V and the 24V rails (anode to 12V).

    Edit: Jbike beat me to it.
     
  8. tcmtech

    Distinguished Member

    Nov 4, 2013
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    I would just put to 12 volt batteries together to make 24 volt system then tie that in in parallel with the 24 VDC power pack. Depending on the power paces actual output voltage you my or many not need to use a diode. If its output at load is around 25 - 27 volts I wouldn't even worry about the diode and it would let the power pack float charge the battery as well.

    Below that you would need the relay to disconnect the battery to which the battery and power pack could share common ground point and you would only need a SPDT relay for the switching.
     
  9. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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    I'd prefer not to use 2 batteries, as batteries are expensive and there is really no need to run the pump full power. If I were willing to spend more money to gain more flow, I'd rather just build another backup for another pump which gives redundancy in case anything goes wrong.

    Am I missing something, or is the only advantage of this idea to save the cost difference between a SPDT and DPDT relay and possibly a diode? Actually a SPDT should work fine if I use a diode regardless of voltage.
     
  10. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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    What is the advantage of a separate 12v 500 mA DC wall wart and 12v DC relay? Can't I run 12V of current through the NO of a 24V relay? Isn't this an extra 6 watts of constant draw?
     
  11. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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  12. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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    Any advantage to a SPDT instead of a DPDT? Or can I use either?
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    This is what I suggested, post #3.
    Why do you need another supply?
    Max.
     
  14. jbike

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    Feb 2, 2016
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  15. jbike

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    Feb 2, 2016
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    I would just use the diode, no relay. But depending on your design, you could use a DPDT relay instead, but it would be more complicated and costly.
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No. You want one with a much higher current rating (I'd suggest at least 20A), in case it has to cope with the start-up current of the pump (which is several times the normal running current). A Schottky diode is the preferred type, since it has a lower voltage drop than a normal silicon diode.
     
  17. Reloadron

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    Jan 15, 2015
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    Question about the pump. Will the 24 VDC pump start with 12 VDC?

    Ron
     
  18. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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    Yes, it will start with much less than 12 VDC. This is probably a much smaller pump than people think. The wall wart that powers it says 2.5 amps maximum output.
     
  19. tcmtech

    Distinguished Member

    Nov 4, 2013
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    If it's a 2.5 amp power pack a common 50 volt or higher rated 3 amp diode would be more than enough.
     
  20. Brad Bellomo

    Thread Starter New Member

    Aug 18, 2017
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