Connect with us

A UPS -- RFI question

Discussion in 'Electronic Basics' started by Haines Brown, Oct 19, 2003.

Scroll to continue with content
  1. Haines Brown

    Haines Brown Guest

    I have an old UPS that supports my computer and monitor (55%
    load). When I print from my laser printer, which is on the same
    outlet, but not plugged into the UPS), the UPS switches itself off,
    and of course, my computer crashes.

    I removed the UPS and experimented with a lamp, and the UPS worked
    fine. It kept the lamp (15% load) running even when the UPS was
    disconnected from the line. However, if I try this using an old sparky
    hand drill as load (also 15%), when I pull the UPS' own power plug, it
    shuts itself down and has to be restarted by toggling the power

    I believe I've eliminated the issue of load by reproducing the problem
    under lighter load conditions. Also, the only connection between the
    UPS and the printer is that they are on the same circuit. If the
    printer pulled the circuit voltage down, handling that situation is
    what a UPS is designed for.

    My theory is that RFI is the culprit, but I assume the UPS (a very old
    Best Fortress) should filter out the RFI. If it is unable to do that,
    there's no point of thinking about a ferrite choke on the printer
    power line.

    Anyone have any ideas on this? Does a (fairly new Lexmark) laser
    printer likely have dirty line output? Is a UPS supposed to filer such
    trash? Do filters age and become ineffective?
  2. SHAUN

    SHAUN Guest

    Haines, how is this UPS? Eventually, the IC's go bad. The average run life
    of an UPS is ten years. and that is if you have preventive and planned
    maintenance done! Let me contemplate on this for a quick minute and get
    back to you. By the way that's what I do for a living, please visit our
  3. Haines Brown

    Haines Brown Guest

    It's not as old as I thought. Purchased it in May 96, so that makes it
    over seven years old. I'm surprised the ICs would deteriorate in that
    time, but based on your estimations, it appears to have reached the
    end of its life. I just assumed it would last forever. I put
    (expensive) batteries in a couple years ago. I can probably find some
    good use for them, although they are getting old by this time, too.
    Thanks, but don't get back by e-mail. My system is
    disfunctional. Within three weeks I had mouse finally go, hard disk go
    south (an expensive SCSI, which I may be able to get replaced under
    warranty), video card (nVidia, the second time I've replaced that card
    under warrantee), and now, some three weeks later, my UPS. On top of
    that, I can't get my e-mail working :-(
  4. Len

    Len Guest

    I have a small 500va monster power UPS and if I get any 440Mhz type
    radios near it it starts to beep then will shut down if I do not remove
    the RF, 1 type of radio is a handheld motorola bussiness band 450Mhz
    radio about 2-5 watts output and it does this within 2-3 feet of the
    ups, the other is a low end cobra FRS radio about a foot from the UPS, I
    have also just found out my 2.4 Ghz spread spectrum vtech cordless
    phones will do the same thing if brought to within a foot of the UPS.

    I figure what is happening is that the UPS is seeing the RF and it is
    messing up the 60 cycle so the UPS thinks the incoming power is dirty
    and shuts down the input.

    REMOVE the NOCRAP in my address to reply.
  5. Haines Brown

    Haines Brown Guest

    Len, very interesting. I was not too impressed by my RFI theory, but
    your examples lend a little credibility. A QRP transmitter a couple
    feet away is not putting much RF into the power line, while my printer
    is directly connected to it, and might indeed generate more. If this
    is so, I'll find some discussion of it by googling, I'm sure.

    I don't know the quality of your UPS, but I assume that quality has
    got something to do with RF sensitivity.

    I do see on line that laser printers are susceptible to EMF and RFI
    interference, and for that reason cables are shielded. However, I'm
    speaking here of interference in the other direction.
  6. SHAUN

    SHAUN Guest

    Haines wrote
    Oh don't worry, I meant I will be posting an answer to the group for you.
    Can you give me the specs on it I am working on finding you a solution right
    now. Also I may be able to get you an APC or Toshiba for a great price! and
    have shipped to you or installed for you!
  7. I do not have a real knowledge about UPS devices, but I once noticed a
    similar thing with linear regulators and it quite possible to assume that
    some of their kind are used in the control electronics os a UPS. Sometimes a
    regulator will fail when an IF or HF signal is somehow fed into its output.
    The time I noticed it first, I was connecting a digital voltmeter to a
    simple device powered through a linear regulator (no idea which, it was two
    years ago) and when the voltmeter was connected a HF signal from its
    electronics caused the regulator to almost completely switch off. The
    voltage was supposed to be 5V, but it turned out to be no more than one (no,
    it was not a current meter). Placing a small capacitor in parallel to this
    troublesome voltmeter did however solve the problem. Consider placing a
    small filter of 2 capacitors with less than 0.5 microfarads each and two
    inductors (a dozen of windings on a toroid core) in the power input of the
    printer to prevent HF from exiting.

    | |
    --- ---
    --- ---
    | |

    Note also that some UPS devices immediately shut down when they detect an
    'earth fault', an event very easy to trip when both the PC and the printer
    are grounded, but over separate grounds. In this case, disconnect the
    printer's separate ground connection from where it is connected now and
    connect it via one of the screws that hold the metal case of the PC's power
    supply in place.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day