DC DC converter vs AC DC converter

Discussion in 'The Projects Forum' started by DC_Kid, Oct 19, 2017.

  1. DC_Kid

    Thread Starter AAC Fanatic!

    Feb 25, 2008
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    I have a small dilemma

    I have a 24vdc batt powered system. some stuff runs off of 24v, some 12v, and some I need to inverter up to 120vac60Hz. I have all the components but ran into a situation that I am not sure what the best approach is.

    One item is a MiniX PC that runs on 12vdc. It came with its own 120vac to 12vdc power supply. But I also have a 24v-to-12v DC DC converter module. This module however will burn 20mA with no load.

    So, what's my best option, use the 120vac/12vdc that came with the PC, plugged into a true sine 89% efficient inverter, of just go all dc and burn the 20mA.

    I assume these tiny power supplies that come with devices like this are crude dc-dc converters too, so they too probably burn some mA w/ no load.

    As a side note, the 24vdc I have are two big 100AH 12v batts in series. The idea to use only 24vdc is so that the batts drain off at about the same rate. An option I have is to wire the PC to just one batt. The PC power supply is rated 12v 36watt, but I do not think the PC pulls 3A (its a tiny fanless PC) . Online posts say this PC is approx 10w, but with my two cameras the total is around 12w, so lets say 1A. I can test later as needed, but 1A sounds about right from the data I have.

    With wiring direct to one batt I then have one less component to worry about, but then the batts will not discharge at same rates. The whole system runs on batts for about 8hrs a day, and after that it goes back onto a batt charger.

    Or yet another option, LM7812 linear, I just not sure how much power this wastes.

    Any suggestions?
     
    Last edited: Oct 19, 2017
  2. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,067
    1,059
    Power the miniX with the bottom battery, one day(even), and the top battery the next day(odd).
     
  3. DC_Kid

    Thread Starter AAC Fanatic!

    Feb 25, 2008
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    It's basically hard wired one way, so I am not sure how it could swap to the other batt.

    Because the batts are rather pricey I would like to cycle them the same way.

    I do not know the real efficiency of the Dc-Dc module I have, got it from ebay(HK) and it's in a 2x2 potting heatsink, looks like cast AL used in automotive electronics industry.

    I have 3 options:

    1) use the 120vac-12vdc power supply that came with
    2) use the dc-dc module I have
    3) use LM7812 LDO linear

    Option #1 is plug-&-play, the other options I have to make connectors and would need to pot a linear reg.

    #1 however has an 11% hit in the power loss equation because my inverter is only 89% efficient (approx 1.32watt wasted just in the inverter alone for real power use, plus whatever waste the OEM power supply has, so maybe round over to 1.5w waste just for #1 option).

    #2 looks like ~0.5watt wasting(min) no load, I have not done any real testing with 1A on the output side.

    #3 may or may not be good because Vin = 2 x Vout, the bigger the diff the more wasted power.

    Option #2 at ~0.5w seems good when looking at just the power #'s.

    Granted, I can still run the 8hrs I need to with these losses, but it just means with more losses I need to go further with batts DoD on every cycle.
     
    Last edited: Oct 19, 2017
  4. Sensacell

    Moderator

    Jun 19, 2012
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    Every conversion step implies power loss.

    Use the minimum steps.
    24V to 12V DC-DC converter is one step, inverter to 120 VAC plus AC power unit is 2 steps, more loss.
     
  5. DC_Kid

    Thread Starter AAC Fanatic!

    Feb 25, 2008
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    I might see this as not being accurate in all cases, but not a bad way.

    If say a dc-dc is real crappy on efficiency, then a 2-step might actually be better, although both kinda crappy.
     
  6. Sensacell

    Moderator

    Jun 19, 2012
    1,982
    613
    And altogether crappier than the crappier of the two! (Crap X Crap = Really Crap)

    I could not resist.
     
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