12V DC automatic transfer switch?

Discussion in 'General Electronics Chat' started by random-username, Aug 12, 2015.

  1. random-username

    Thread Starter New Member

    Aug 12, 2015
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    This is my first post here so bare with me.

    I'm trying to build my own powerbank for my 12v modem because the area I live in experiences frequent and prolonged cutouts. The reason I don't want to get a normal UPS is because of the power it's going to waste by converting 12v to 220v and back to 12v.

    I want to set this transfer switch (whatever it's called) up in a way that it switches to the 12v lead acid battery when the power goes out and back to the DC adapter when the power comes back up. I'm not looking for a high speed switch and I'm fine with losing internet connectivity for a couple of minutes, all I care about is making this switch automated.

    So far, I have a 12v lead acid battery, 12v lead acid automatic charger, 12v modem+AC adapter and a DC to DC adjustable voltage regulator.

    How do I proceed?

    Regards,
    MJ
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    May just need a small relay..

    What else will run on this powerbank?.. Just keeping a modem up is pointless without a computer to browse porn and a fridge to keep the beer cold..

    You need to size the "switch" based on the loads it will be "switching"
     
  3. random-username

    Thread Starter New Member

    Aug 12, 2015
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    It's a 9A 12v battery, I won't be running a fridge on it any time soon. I just want to power the modem, I have several laptops around the house and a small generator to power the fridge and charge the batteries in case the powerout is longer than 6 hours.
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    So how much power does your modem require..
    Easiest would be a form c relay with the coil attached to line voltage.. C terminal to the modem 12V+ and 12+ plus from the normal power supply to the NO terminal and 12V+ from the batteries to the NC terminal.
    When AC drops out the contacts will switch over to the battery voltage..
     
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  5. random-username

    Thread Starter New Member

    Aug 12, 2015
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  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why not use two diodes? connect the cathodes together, one anode to the dc adapter, and one to the battery +. connect the cathodes to your modem. when the dc adaptor drops voltage, the battery will already be connected. whitchever has higher voltage will power the modem.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Only potential issue with that is he needs to ensure his 12V battery based source is at a lower voltage than the 12V adapter or the battery will be used when not needed..
     
  8. #12

    Expert

    Nov 30, 2010
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    It seems likely to work because wall warts typically run high under less than full rated load, still a measurement is imperative in this case.
     
  9. Reloadron

    Active Member

    Jan 15, 2015
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    The simplest solution is likely to just use a relay. However, my experiences with doing this lend to the need of a large capacitor. The problem was when mains power failed the switching time of the relay comes into play. Here in the US a switching time (the contacts mechanical time) of 16 mS represents about 1 cycle of input power and frequently the modem wanted to reboot itself. Not a big deal or problem if you can live with that. Option 2 is electronic transfer where the modem won't miss a beat and makes for a much smoother power transition. A Google of 12 VDC Backup Power Supply should bring up a dozen hits a circuits which transfer and keep the battery charged with a float charge.

    Just My Thoughts
    Ron
     
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  10. dl324

    Distinguished Member

    Mar 30, 2015
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    How much current can the charger provide? If it can charged the battery and provide the half amp the modem needs, you could just connect the battery to the modem power input. When you lose power, switch over is automatic.

    You just need to make sure the modem can tolerate the charging voltage of around 14V max.
     
  11. random-username

    Thread Starter New Member

    Aug 12, 2015
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    Thanks for the input guys, here are some further info:

    - The Battery charger has a rated output of 13.8V and 1A.
    - The modem's AC adapter has a real output of 12.4V
    - I'm fine with the modem rebooting in between switches.
    - The modem can run on 13.8v for a prolonged period without frying the circuits.
    - I'm more inclined towards building this system my self rather than purchase a commercial one, since I already have most of the components.
    - I already tried powering the modem and charging the battery with the same adapter, but the battery got drained and the voltage dropped so much that the modem turned off. (It should theoretically work, since the modem uses 0.45A at most, the battery should be charged at at least 0.55A).

    I liked the relay idea the most, would you mind drawing a schematic? Does it also require soldering or a circuit board?
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Something fishy here. You must have had something misconnected. This is exactly the configuration that is the simplest, providing that the "charger" is not confused by the continuous load of the modem. Can you post a link to the specs on the charger?

    This would work well provided that the power supply is a very-well regulated DC 1A power supply which can be adjusted to exactly 13.65V, which is the ideal float voltage for a 12V Sealed Lead Acid battery at ~25deg C.
     
    Last edited: Aug 13, 2015
  13. random-username

    Thread Starter New Member

    Aug 12, 2015
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  14. MikeML

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    What a WORTHLESS supplier. No useful data about what the product does, or how it works, whatsoever!

    I suspect that this charger is the kind that can only recharge a battery that is completely isolated from any load.
     
  15. random-username

    Thread Starter New Member

    Aug 12, 2015
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    That's what I figured, it's exactly why I would like to set up some sort of automated switch that switches between the battery and AC adapter.

    Is there some sort of transfer switch that has 2 inputs and one output. With one of the inputs being primary and the other a backup/secondary?
     
  16. dl324

    Distinguished Member

    Mar 30, 2015
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    Determine what the charger is doing connected to just the modem and then the modem with battery. Measure the output voltage of the charger for each case. If the charger doesn't work with both connected, buy or make one that will.
     
  17. Reloadron

    Active Member

    Jan 15, 2015
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    Yeah, if you want simple then go with a DPDT relay with a mains coil voltage. Long as you don't mind if the reboots if the relay switch over is too slow.

    Ron
     
  18. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is how I would do it, using a modern approach. Only two parts: a PMOS power transistor and a Schottky diode, much cheaper than a relay. Note that it never drops the voltage to the Modem. Doesn't matter how slowly the Wall-Wart power goes away...

    104.gif
     
  19. random-username

    Thread Starter New Member

    Aug 12, 2015
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    Thanks Ron, I can get my hands on a DPDT in a couple of hours and since I've never used one before would you mind telling how to connect everything to the relay?

    I understand the the coils must be powered by a power supply rated to its voltage and plugged into the mains to indicate when the power is on or off. But what about the other 6 pins?

    My thoughts so far: - Common gets both the negative terminals of the 2 DC inputs
    - NC is connected to the positive terminal of the AC adapter
    - NO is connected to the positive terminal of the battery

    What about the rest of the 3 pins? (Com-NO-NC)
     
  20. random-username

    Thread Starter New Member

    Aug 12, 2015
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    To be honest, I prefer using the relay method, that way I can protect the modem from power spikes. I would also like to run the modem on the AC adapter that came with it and not push my luck with the 13.8V power supply in case it fries.

    Thanks for the input guys, and I'll keep you updated!
     
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