Need Help reducing noise from an AC inverter

Discussion in 'General Electronics Chat' started by GroundHawg, Mar 31, 2008.

  1. GroundHawg

    Thread Starter New Member

    Mar 31, 2008
    HELPP!!!! I'm not really that knowledgeable in circuitry, and am having a serious problem with noise in my truck's DC wiring induced by a 2500W AC inverter that im using. I am getting lots of noise in my CB and scanner any time that the inverter is on and it gets worse with each AC item that i power on. Unfortunately I need the power on constantly for my computer et al. Is there a way to filter this noise directly at the inverter pos & neg posts to avoid the battery cables from becoming antenna's? Don't know how to deal with the approx 150amps that could be drawn there....without killing myself that is...

    Thought I might try a capacitor from pos to neg post and one from pos to ground directly at the inverter input posts but don't know how to figure size and all that or even if that would be effective or possible. Any suggestions would be really appreciated.

    CB is on a 30 amp circuit and scanner a 10 amp. Noise shows up in the 42 Mhz range on the scanner and on all CB channels (26-28Mhz) so Im guessing that there is some being radiated and some conducted, does this sound correct??

    Many thanks for your advice in advance.....
  2. Wendy


    Mar 24, 2008
    The classic pi filter might work. You can buy them at most auto/audio shops as simple kits.


    This schematic was pulled off another thread I made a long time ago. Basically it isolates the battery and, in this case, the invertor. But like I said, you can buy them premade. They are to clean up stereos, but in this case they can be put between the inverter and the rest of your electrical system.
  3. GroundHawg

    Thread Starter New Member

    Mar 31, 2008
    Thanks for the info Bill I'll check it out. You wouldn't happen to have any idea of what sizes for the Cap & inductor & conductor in an application like this would be, would you?
    What I'm unclear on is that since only the AC or noise would be conducted to ground through the cap and the wires, does the circuit still have to be sized to handle the full 200amp potential at the inverter input or can it be sized for the voltage and a smaller current than that..? I use a 4ga wire from the battery to the posts and am not even sure how i would make up something like what you described with that kind of size wiring....? Perhaps an ignorant question....but would really like to learn this stuff.

    Thanks again.
  4. Wendy


    Mar 24, 2008
    With RF you don't need really large caps, although you might try them. For that kind of current you might have to wind your own coils though. If you're in an experimenting mood you could try the secondary coils on power transformers, though 200 Amp is pretty hefty.
  5. SgtWookie


    Jul 17, 2007
    Wow, that will have to be a couple of honking big inductors! :eek:

    What gauge (AWG) and type (stranded/solid) wire are you using to connect your inverter to the battery, and how long a run is it? Even 4 AWG isn't large enough for 200 amps, unless the run is REALLY short. See this chart:

    Solid conductors don't carry high frequency signals as well as stranded conductors do.

    Scroll down to the bottom of the page the above link points to, and plug your numbers in; what your wire size is and the length of the run, for 12v @ 200 amps. The voltage drop is partly why there is noise.

    Your power inverter works by switching power to a transformer on and off very quickly, in order to approximate a 60Hz output at 115VAC. The switching on and off enables the converter to be efficient, but it can be a big source of electrical noise if your supply wiring isn't good. The kind of noise that the inverter makes will be tough to get rid of, because it is noise across a very broad band.

    Actually, the filter Bill posted is known as a "T" (or "tee") filter, that's made with two inductors in series with a cap to ground. A "pi" filter is a cap to ground, then an inductor, and then another cap to ground. Both filters can accomplish the same goal, but you might be better off with a "pi" filter if only because your inductors would have to be so large.

    But first, try the Vdrop calculator, and see what you come up with.
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    A quality inverter will have sufficient on-board capacitance to run the inverter, otherwise you may need a close external capacitor to draw from. They make these mainly for audio amplifiers at 1/2 to 1F range.

  7. mrmeval

    AAC Fanatic!

    Jun 30, 2006
    You might try something like these.

    We ship them with our equipment and require they are installed to keep the crap out of our stuff.

    Those supplies are notorious for radiating noise, that won't be fixed with line filters. You'll need to shield the entire box which is not trivial.
  8. GroundHawg

    Thread Starter New Member

    Mar 31, 2008
    Wow, thanks for all the replies!

    I do see now that probably my biggest concern is the length of run vs wire gauge and load. As far as the 200amps being actual...??...?? I assumed max draw for the system and even that I now see was overstated as the circuit is fused with parallel 80amp fuses using stranded wire. So, 160amp is the actual max load (since I haven't fried the fuses). I didn't know that the voltage drop from the cabling was so drastic but I see now from the calculator (thanks sgtwookie) that mine is pretty bad with the set-up i've got.

    I'll try moving the install closer to the battery or the power cap idea (was wondering if that might be a good idea anyway, as the cap may help filter some noise as well, eh? Thanks scubasteve_911!)

    Mrmeval, I did try the ferrite snap-on at the CB Radio as my first attempt to reduce the noise with little success, so that's when I began thinking that my wiring was radiating the bad freq's as well.

    With the antenna unplugged I get far less noise but still have a nasty sporadic buzzing through the power line (thanks for the input, though!!!).

    Fixing the wiring will be my first effort, though, as I believe now that may be my biggest source. I used a 1500W inverter on the same set-up with very little noise and since installing this new 2500W unit have had the noise problem. Just never thought that the wiring could be such a major concern (duh...?). I appreciate all your responses and I'll let you know how it comes out!