Upgrading a MFJ-1112 DC power strip

Discussion in 'Homebrew and Kit Projects' started by KJ6ZOL, Jun 29, 2021.

ad: L-HROutlet
ad: l-rl
ad: HRDLLC-2
ad: MessiPaoloni-1
ad: L-Geochron
ad: L-MFJ
ad: Left-2
ad: abrind-2
ad: Left-3
  1. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    I am building a tiny house intended to be off grid, and to connect all the various AC circuits to the 12v main battery I thought a MFJ-1112 DC power strip would work fine.


    Then my uncle, who has a lot of electrical knowledge and is helping me with the electrical system, took a look inside and was shocked at what he found.


    The + and - sides are connected via a single flimsy serial wire connection directly to the DC input. There are two tiny ceramic disk caps connecting each side to ground, and a single tiny cap as a shunt. The three caps are only marked with "104Z". My uncle handed it back to me and said it was worthless. His response was to buy a bunch of parts at Home Depot with the intention of making a homebrew bus out of a couple cheap terminal blocks and a bunch of various lugs and crimp connectors.

    I personally am wondering if I could simply cut out the wires and caps from the 1112 and make something much sturdier. I'm thinking 12 gauge stranded steel wire of the type Mouser sells and a lot of semi-HV caps to use as shunts and such and probably a few varistors to act as circuit protection, or possibly even a fuse.

    The electrical system in this tiny home will use 20A Siemens circuit breakers for all AC plugs with a maximum draw at any one time probably not more than 16A or so. (There will be a separate DC circuit for my ham rigs.) If you were looking at this with the use I intend, how would you beef it up?
  2. AD5HR

    AD5HR Ham Member QRZ Page

    That device is only rated for 15 Amps total, so #18 wire is all you need
    at this Amperage. (as the wires are "open air)
    The caps are to attenuate any RF on the wiring.
    This product will work just fine within it's ratings, but unless you only
    operate QRP, 15 Amps won't cut it for a 100 Watt rig.

    I don't understand your comment about connecting to A.C circuits,
    this product is only rated for 15 Amps D.C. only.
    DO NOT connect this device to any 120V A.C. circuit !!!
    DO NOT use steel wire for any current- carrying wiring !!!
    Buy a device rated for the current you plan to need,
    and buy it with some "head-room".
  3. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    Oops, I forgot to mention that the wires from this will be connected to the 12vdc inputs of several inverters, whose 120vac outputs will go to breakers.

    Maybe I was not clear. I want to chop out the #18 wires and tiny caps and rewire this for a higher tolerance. I was thinking of maybe putting electrolytics rated for maximum 24vdc across the + and - terminals of the six outlets, then use #12 wire to connect them to the input, then place a varistor across the leads of the input. The - terminals would be wired to external ground with a ceramic cap between the - leads and ext ground.

    There would be four 12vdc output leads going to four inverters, a direct 12vdc lead going to a 40 amp dc breaker for ham equipment, and a dc lead going to a 15 amp dc breaker intended for several small uses like dc lights and a water pump. There would also be a 100 amp dc breaker between the battery output and the input of this.

    The thing is, I was unable to find any suitable dc bus item at all, except ones for heavy industrial use and Rigrunners. I bought this because it was all I could find. I have decided to ask if anybody has any idea as to how to calculate the microfarad ratings for the electrolytic and ceramic ground caps, and what varistor I should buy.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would say no go.

    The wires would need to be much larger. The best that you can do with that power strip, Solder all of the connections properly and use it as MFJ intended.

    You can build something better that has proper fuse ratings for protection. Inverters are best connected directly to the power source where voltage drop is minimal.

    Good Luck on your project. Your idea does sound good, But may not work as expected.
    Last edited: Jul 1, 2021
  5. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    OK, I have done a LOT more exposition on my plans, complete with (admittedly crudely drawn) schematics.

    Here is the current electrical layout of the MFJ-1112. The dots are the output terminals.


    As you can see, the wires connecting the output terminals are 18 ga, and even worse they are completely uninsulated and apparently made of aluminum. On top of that, there are three tiny ceramic disk caps that my tester rated as 200 nf (yes, nanofarads). One cap apparently acts as a shunt between the + and - rows and the other two are connected from the rows to external ground.

    Here is what I am planning:


    The 12 ga wires would be solid copper. Across the two input leads would be a varistor capable of handling 40vdc. I am open to making this a standard fuse, but between the battery and this would be a 100 amp 12vdc circuit breaker. Then I would place an electrolytic across each set of + and - output terminals that would be rated around 40vdc. A ceramic disk cap would connect the - line to external ground.

    My problem, first off, is that I don't know what the uf ratings of the caps should be, and I can't find a way of calculating it online despite beating up Google. The varistor is a whole other worm, and I may solve it by simply using a large auto fuse.
    The issue here is that I have one 12vdc 100Ah battery, and six inputs to connect to it-four inverters and two sets of DC circuits. My uncle was worried that connecting six circuit inputs directly to the output wires of the battery could cause a fire and/or other undesirable outcomes. So I looked online, and found Rigrunners and gazillion dollar heavy industrial equipment. The 1112 was the only thing I could think of that even remotely did what I wanted-a simple DC-DC power strip. My uncle opened it up and just shook his head.

    Each inverter and DC circuit would be connected to a single set of DC output terminals.

    There are OM's here who have far more experience in this than I do, so suggestions on how to approach the basic problem-connecting 12vdc inputs of multiple inverters and two sets of direct DC power circuits to a deep cycle AGM battery-are welcome and will all be considered.
  6. W9GB

    W9GB Ham Member QRZ Page

    Marine Shops, like West Marine , have the Parts and Assemblies
    you are likely looking for (Electrical DC Distribution)

    Marine Systems (Boats) routinely use High Amperage Batteries for power.
    You appear to be FIXATED on usage of Capacitors. WHY?
    You have not stated a Specific Need for Them.
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    How much power do your inverters require ? What size are they ?
  8. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    The four inverters:

    1) 400w, for small AC freezer and mini fridge. Freezer and fridge are older models and use less electricity. Keep in mind that I have hooked a Kill-A-Watt to all this stuff.

    2) 450w, for general electrical stuff-laptop, clocks, etc.

    3) 600w, for small window AC and small (450w) space heater.

    4) 1600w, for cooking appliances. The size of this one is dictated by a convection oven.

    Keep in mind that everything will be on power strip switches and switched off or unplugged when not in use. The only things that will be going ALL the time are the freezer and fridge. Power usage will fluctuate wildly depending on what is being used and when.

    I was thinking of capacitors in terms of regulating the flow of electricity. The problem with learning electronics from Google and old textbooks bought at Goodwill is that admittedly I don't know much. I was thinking of this as a learning opportunity.

    I have a West Marine near me, in Rocklin, Calif. I have gone there before, to buy the DC circuit breakers I needed. I will have to make another visit and try to see what I can come up with. I will also peruse their website.
  9. KJ6ZOL

    KJ6ZOL Ham Member QRZ Page

    I looked at the West Marine high amp distribution page and there does seem to be a couple items there worth looking into. It looks as if their bus items usually have single screw connectors instead of the + and - connectors that attracted me to the MFJ.

    OK, honesty time: if I don't need capacitors to regulate the flow of electricity through this bus bar, then exactly HOW do I do it? As I noted, there will be a 100A circuit breaker that the battery feeds into right away, then the main leads of the bus bar will be hooked to the output of the breaker. Then, I will need some sort of shunt system to regulate the flow of electricity within the bus system. After I wrote that last sentence, I suddenly thought RESISTORS. From the little I know about shunting, it involves resistors, and I am still thinking of a varistor across the input leads.

    As I noted, I am essentially a newbie to electronics, most of my "experience" has been simply replacing components in old radios and what was called "appliance operation" by longtime OM's. I WANT to learn, that's the whole point of why I get the ideas for projects that I do. For example, I recently bought an old 5" portable TV at a yard sale to use as a monitor for SSTV signal reception, eventually. I have components for an Arduino-based GPS-fed clock too.

    It's a better use of the internet than cat memes and racy pictures, that's for sure.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Not counting Losses, And there will be some. 10-20 % or so. Maybe more.

    1600 Watts will be about 133 amp at 12 volts.

    600 Watts, 50 amps.

    450 Watts 38 amps

    400 Watts 33 amps.

    Do the math, Remember PIE ? Even a 400 Watt inverter will be cutting it close using that power strip. The connectors will not handle a big load like that for very long.

    Do it and try. I would have a smoke detector to warn when to turn on the video camera to record the meltdown. Then you can post the video here, Make sure to record audio also. :)

    Have Fun.
    W9WQA likes this.

Share This Page