DC converters

EstragonEstragon Registered Users Posts: 3,589 ✭✭✭✭✭
I want to buck nominal 12v bank voltage down to 9v to run a cellphone booster.  The AC power brick says 3A, which I assume would be a max output?  I don't have the unit at hand to measure actual draw, but presumably the brick would be chosen for some overhead capacity.

I found tons of small converters for 5v (for phones etc), but nothing for 9v in the size needed and reasonable cost, so I got some cheap switch mode converters with variable output which might work.  The question is would putting them in series work?  They're rated for 3A, but I'm a bit concerned their 3a rating may be optimistic.  I'd prefer not to have to add cooling fan, but that's an option.

I suspect running in parallel would be unwise, as resistance differences might make one carry much of the load or possibly even backfeed the other.  In series, the first might be set to buck from whatever bank voltage to say 11v, and the second from 11v to 9v.  Would this work, and if so, would the incremental bucking reduce heating in each?
Off-grid.  
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,009 admin
    In general, a buck converter needs voltage drop across the input and output leads to work correctly. You may have issues if your input is not , at least, 1.5 volt higher input vs output. Bucking from ~10.5 to 16+ volts input (typical range of a 12 volt battery bank) down to 9 volts should be fine.

    Here is one 12 volt to 9 volt at 3 amp converter (links to EBay, so link will go bad):

    https://www.ebay.com/itm/Waterproof-DC-DC-Converter-12V-Step-Down-to-9V-Power-Supply-Module-3A-27W-A365/401647536168
    https://www.ebay.com/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Module-7-32V-to-0-8-28V-12A-300W-USA/182602817113

    Waterproof DC-DC Converter 12V Step Down to 9V Power Supply Module 3A 27W A365  $400-$5.00

    Here is a 12 amp buck converter for $4.00 (not much information on page):

    https://www.ebay.com/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Module-7-32V-to-0-8-28V-12A-300W-USBT/132836947885

    DC-DC CC CV Buck Converter Step-down Power Module 7-32V to 0.8-28V 12A 300W USBT

    And some more similar links (last two even have V/A meter):

    https://www.ebay.com/itm/LM2596-2X-DC-DC-Adjustable-Step-Down-Power-Module-7V-35V-to-1-25V-30V-3A/273388909512
    https://www.ebay.com/itm/Adjustable-Buck-Converter-Digital-DC-Voltage-Regulator-Step-Down-Supply-Module/332826902314
    https://www.ebay.com/itm/DC-DC-Adjustable-Buck-Converter-Stabilizer-Step-Down-Voltage-Reducer-W-DIY-Case/282559541237

    Not a big fan of EBay--But a good place to start a search. Many (most/all?) of the above are shipping from Hong Kong / China (I think, I did not look closely).

    -Bill


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 3,589 ✭✭✭✭✭
    Thanks Bill.  I got some of the LM2596 type converters (your 3rd link) from Amazon.  Unfortunately Ebay/Amazon selection in Canada is much more limited than US versions, and getting small value shipments is a PITA :neutral:  

    I'll be in the US in January though, so if these don't work, can get something bigger then.

    I also have some small heatsinks I could add.  I assume it's the chip itself that would heat up?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 29,009 admin
    edited December 2018 #4
    Whatever is the "switching element" (generally the large 3 legged devices) is where to add the heat sink(s) if needed.

    More than likely, the device will draw substantially less than the 3 amp rating of their power supply (fingers crossed).

    Have good air circulation and protection from dust (no clogged filters, not an open top where dust can settle on device). If you have a spare "larger" metal box that can be closed (sealed with gaskets is another issue--generally need to drill a small weep hole at the low point of the box to let out any condensation--Update/carified. -Bill) would be nice (if you have dusty/inclement conditions).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,266 ✭✭✭✭
    edited December 2018 #5
    My experience has been that you don't want to run ebay DC-DC converters at their rated amps.   Buy a 5A+ one if you need 3A.  I certainly wouldn't use one that says "Output current: 3A Peak, recommend the use of within 2A. (Over 2A, please enhance heat dissipation.)".

    Both series and parallel connection have issues (avoid both).

    I am available for custom hardware/firmware development

  • EstragonEstragon Registered Users Posts: 3,589 ✭✭✭✭✭
    I'm hoping the booster uses materially less than the 3adc rating of the power brick, allowing the use of the 3a converter with a heat sink.  If not, will get a bigger converter when I'm south later in Jan as you suggest.

    As a matter of interest though, what are the issues with running in series?  I know they need voltage headroom to buck, but are there other problems?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 29,009 admin
    With digital switching power supplies, as long as the output is stable (perhaps add a filter capacitor to the output of the first buck converter), it should be fine (note: This are not really connected in series in that you have a + and - output from the first supply, and the second supply - connects to the first supply - , and the load too--Everybody needs the same ground reference).

    Buck regulators only regulate voltage below (~1.5 volts or more) below the input voltage.

    Boost regulators only regulate voltage over the input voltage.

    To create a stable voltage that is around the input voltage (such as 10.5 to 16 volts input, and 12.0 volts output), you need a buck/boost converter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,266 ✭✭✭✭
    edited December 2018 #8
    > what are the issues with running in series?

    The almost certain problem with two DC-DC converter in series  from a single battery is the shared ground - the two supplies need different output ground references to be connected in series.

    And series connection doesn't help your amps problem.

    I am available for custom hardware/firmware development

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