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Experience with LD1085 voltage regulator?

Discussion in 'Electronic Basics' started by mark risher, Feb 29, 2004.

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  1. mark risher

    mark risher Guest


    I'm having difficulty with a new LD1085V adjustable voltage regulator,
    and hope some kind individual in this group can help.

    I hooked it up as the ST Micro datasheet indicates, with 10uF caps to
    ground, a 120_o resistor from OUT to ADJ, and a pot from ADJ to
    ground, but I'm not getting adjustable voltage. Instead, I always get
    5.3V at the OUT, representing a ~1V drop from Vin, regardless of the
    setting on my trimmer pot. My "load," for testing purposes, is a 1k

    The voltage across R1 (from OUT to ADJ) reads in the millivolt range.

    Is this chip toasted, or am I missing something?

    Thank you much,
  2. I suspect that you have it connected incorrectly.

    for the TO-220 package, if you hold the part with the labelled side
    towards you, and the pins hanging down, the pins are, left to right,
    adjust, output, and input.

    Peter Bennett, VE7CEI
    peterbb (at)
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  3. mark risher

    mark risher Guest

    Thank you for the reply, but I checked the connections, and I do have
    the output and input correct. Into the rightmost pin (input) I have a
    +6 and cap to ground; into the middle pin, a resistor to the

    Other thoughts? What would the failure mode of this device be?

    Thank you.
  4. I was just having problems with Voltage regulators, and nothing anybody
    suggested was any help.

    Someone suggested that it might be because of resonance triggered by
    transients in the input voltage, or some such analog witchery.

    In any case, I tried running the voltage regulators (7805 and 317)
    from a 9V battery, instead of the transformer I had been using, and it
    worked fine.

    Which proved to mome that the problem was with choosing the right
    protective caps and diodes on the input, instead of my wiring. (Which,
    to me, means its black magic).
  5. mark risher

    mark risher Guest

    I'm running off a 6V battery pack, but I'll try your suggestion. Do you
    recall which caps were recommended, or how to pick better ones? I'm using a
    10uF, like the datasheet recommends...

    Thank you,
  6. This is what I got:

    3. Use a 1000uf 25 volt cap on the input.

    4. Add a .1 uf 25 volt cap on the output.

    5. Add a 1n4004 or 1n4007 diode from the output to the input, just
    in case you short the input while the output is connected to a battery.

    Which I'm not sure parses clearly.

    But a 6V input raises another possibility.

    Does the 1085 have a minimum input voltage? Some require the input voltage
    to be at least 2V above the output.

    Others require a minimum current draw in order to work.

    The most dangerous man, to any government, is the man who is able to think
    things out for himself without regard to the prevailing superstitions
    and taboos. Almost inevitably he comes to the conclusion that the
    government he lives under is dishonest, insane and intolerable, and so,
    if he is romantic, he tries to change it. And even if he is not romantic
    personally he is apt to spread discontent among those who are.
    - H.L. Mencken
  7. I had trouble with low drop out regulators (not this one, in
    particular) as long as I tried to find output caps that had the right
    combination of capacitance and ESR, over all operating conditions. I
    had good luck with a low ESR stacked film cap of 1 uf in series with a
    1 ohm film resistor, very close to the regulator. Then mine worked
    even with a few more microfarads spread around close to loads in
    parallel with this network but separated by some distance.

    Based on what I see on the data sheet:
    I think this approach might do well here, also.

    The input capacitor is less critical. Ordinary aluminum electrolytics
    usually work well, and bigger is usually better.
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