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Switching power supply problem for cheap LED meter?

Discussion in 'Electronic Design' started by Paul E. Schoen, Apr 27, 2007.

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  1. I just ran into a strange problem. We have been using a cheap ($12-$20) LED
    panel meter, which runs on nominal 9 VDC, and cannot read its own supply.
    Originally we dropped 12 VDC from a switching supply through 4 diodes to
    get about 9 VDC, and fed the AC voltage we were monitoring through a
    transformer and then a bridge rectifier, capacitor, and some resistors to
    the 199.9 mVDC maximum for the meter. After some field failures, we used a
    12 V to 9 V, 1 watt DC-DC converter, and had no more problems.

    Recently, we wanted to replace some expensive line voltage operated AC
    meters with a cheaper alternative, and I was able to mount one of the cheap
    meters in a 1/8 DIN case along with a 5W 12 VDC switcher and the 12V to 9V
    DC-DC used previously. It worked well, but it seemed a waste to convert 120
    VAC to 12 VDC and then to 9 VDC, so I found some small 10W switchers with 9
    VDC output. When we powered the meters directly, the readings bounced
    around from 200 to 500 with a 450 VAC signal. The supply voltage read just
    about 9 VDC, very solid, with only about 5 mV ripple, and it did not help
    to add capacitance from 0.005 uF to 200 uF, and a 100 ohm load did nothing.

    Using a 9 VDC battery worked fine, as did a cheap 9VDC wall-wart, and a
    variable supply from about 7V to 10V. I made a simple linear supply from a
    10 volt transformer, bridge rectifier, 1000 uF capacitor, and a 7808
    regulator with 1 uF bypass capacitors, and it works fine.

    I didn't look at the power with a scope, but I can't imagine even a
    switching supply with extreme waveform glitches, and there does not appear
    to be any leakage from input to output. The DC-DC is also a switcher, but
    does not cause the problem. We have now designed a new PCB to use the
    simple linear design, and it will ultimately be cheapest, but I'd like to
    know why the 9V switcher caused such extreme fluctuations of readings. It's
    a simple ICL7107 or similar DPM.

  2. Fred Bloggs

    Fred Bloggs Guest

    Was that 200-500 counts out of 2000 or so, or do you mean 200-500VAC on
    the reading of the 450VAC? A lot of the DC-DC converters are
    minimalistic junk producing stray high frequency magnetic fields all
    over the place. Because of the high frequency, it does not take much of
    a field to corrupt nearby circuits. Doing things like twisting leads and
    experimenting with relative orientations of the components gives away a
    through the air problem without actually making measurements. Then again
    with high frequency noise, all the specs for power supply and common
    mode noise rejection are out the window too.
  3. That supports a problem I had with a system last
    year. It was a +/- 15V analogue measuring system,
    (linear supplies), looking at things in a system
    powered from a 48V switcher (from a reputable mfr).

    At switchon the analogue readings were all over the
    place, and I subsequently discovered that the *whole*
    of the +/- 15V circuitry had huge 40KHz spikes in it.
    It turned out that that reputable? 48V switcher was
    splurging 40KHz common mode spikes everywhere.

    A toroidal common mode choke and capacitors to the
    0V of the +/- 15V sent the spikes away.
  4. Hawker

    Hawker Guest

    Others posted some possible scenarios, but I also want to say do you
    have a ground potential issues? Is your 9V switcher isolated? Have you
    also looked for load stability? You say it is a 10W switcher. Some cheap
    switchers need to be in continuous mode (not DisconTin) mode for good
    regulation. At light load it may not be that way.

    My 2 cents
  5. Joop

    Joop Guest

    On Fri, 27 Apr 2007 05:45:43 -0400, "Paul E. Schoen"
    Most (el cheapo) 199.9 mVDC meters need 9V independend power.
    Don't mess about, just provide it.

    A simple circuit idea to create such floating power can be found here
    (bottom of the page)


  6. Thanks for the info. That looks like a "flying capacitor" isolation supply,
    which may work well for very low current LCD meters, but maybe not for my
    LED type. Also, I still need to obtain my supply from a 120 VAC source.

    I think the problem must be common mode noise that is perhaps capacitively
    coupled from the switching supply high voltage section through the
    transformer and to the output. I'm not going to try common mode chokes or
    other cures, because the linear supply works like a champ and is the lowest
    cost, simplest solution, and will not suffer from obsolescence of the
    switching supply.

    Looking at the supply, the transformer seems to be wound with layers over a
    common EI core. This probably makes for better coupling and higher
    efficiency than coils wound side by side or on opposite ends of a toroid,
    but provides a direct capacitive path for the 180 volt peak transients to
    couple to the output and wreak havoc with the meter. I think it may
    actually be causing glitches in the meter's digital circuitry, causing
    incomplete conversions, and random displays varying several hundred counts.

  7. legg

    legg Guest

    The dual slope conversion obtains it's immunity to 60 or 50 Hz linear
    interference through selection of components syncronizing the
    conversion rate .

    Hf noise generated by the switcher is not compensated and can affect
    the early terination of either conversion slope period unpredictably.
    You could probably get an indication of what you're dealing with on a
    scope and judge the effectiveness of any orientation change, added
    filtering or grounding/screening hardware.

  8. Rick G

    Rick G Guest

    I have read all the posts about your problem. All the responders seem
    to sugest exotic dc-dc conversion method, dual slope yada yada. I
    don't get the feeling that you understand dc-dc converter operation.
    Seems to me your looking for a BUCK type converter. One that throttles
    the output down to a lower level etc. 12v to 9v, 120V to 12V. But what
    I don't seem to hear about is your converter inductor current. This
    you need to set limits on so that it doesn't saturate the field, (loss
    of magnetic flux density). Also your period time is determined by some
    RC time constant. Anyway I have built a few converters in my day and
    never experenced your problem. The output remained at a dc level
    despite the load because of the output capacitor. The only problem of
    high loads is that it may exceded the supply wattage and output
    deteriates. You can easily make a good converter from Linfinity
    SGx842/43 pwm chips. The peak coil current should be at 1.5x your load
    current. Most PWM converter topology use FET to sink current through
    the coil. Just feedback the peak voltage across the sense resistor in
    series with the coil. Converters usually have a current sense line
    which stops the converter when voltage reaches 1V. Also there is a
    voltage feedback line that controls the output voltage. The voltage
    feedback maybe more than 1V, the ones I'm familar with stop the
    converter when 3V is present. Usually this is from the mid-point of a
    voltage divider. So output voltage is determined by the divider ratio.
    In any case you should not see the bouncing you described.

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