Problem with switching off load on output of voltage regulator. Regulator switches to lower voltage.

Discussion in 'Misc Electronics' started by Silvia Janssen, Dec 12, 2003.

  1. Hi,

    I've connected an industrial PC to an LT1085 voltage regulator which
    converts a transformer output of 27V to the 24V that is needed for the PC.

    It all works fine, except when I shut down the PC with its own
    on/off-switch.
    In that case the output voltage of the LT1085 drops down to about 3 Volt and
    can only be recovered by switching the transformer that forms the input of
    the LT1085 off and on again.

    The LT1085-application notes state that this is caused by the safe area
    protection of the LT1085, but I would like to know whether there is a way to
    prevent this from happening.
    Would I need a regulator that can handle larger currents? Or is there
    another solution?

    Switching of the transformer causes my whole installation to switch off,
    while I only want to switch off the PC to recover from crashes etc.

    Thanks for any advice,

    Silvia
     
    Silvia Janssen, Dec 12, 2003
    #1
    1. Advertising

  2. Silvia Janssen

    CWatters Guest

    "Silvia Janssen" < THIS.nl> wrote in message
    news:brc8jt$sk1$...

    > I've connected an industrial PC to an LT1085 voltage regulator which
    > converts a transformer output of 27V to the 24V that is needed for the PC.


    Is that a typing error? Do you mean "the _5V_ that is needed for the PC"?
    The LT1085 is a 5V regulator isn't it?

    > It all works fine, except when I shut down the PC with its own
    > on/off-switch.
    > In that case the output voltage of the LT1085 drops down to about 3 Volt


    That doesn't sound right. You would not expect the voltage to _fall_ when
    you _remove_ the load unless something is very wrong. When you shut down a
    PC it normally results in a burst of disc activity that increases power
    consumption briefly. Perhaps your regulator is overloaded and this extra is
    the final straw?

    > Would I need a regulator that can handle larger currents? Or is there
    > another solution?


    It might not be the current that's the problem but the power required could
    be.

    How much power does the PC need?

    The data I've seen for the LT1085 says it can provides 7,5A at 5V. If your
    PC is taking say more than a few Amps I hope you have a big heat sink
    and/or a fan on the LT1085 !

    Lets say the PC need 7A max. That's less than the 7.5A limit on the
    regulator BUT....

    I calculate that the power dissipation in the regulator would be...

    (27V - 5V) x 7A = 154 Watts !

    The LT1085 is available in various packages. These have max thermal
    resistance specs of between 1 and 3 degrees per watt. (even even connected
    to a perfect/ideal heat sink).

    2 degrees x 154 = 300C

    I think it's getting too hot.

    The real problem is that the LT1085 is a linear regulator. I think if you
    are taking more than a few amps you will need to look at using a switching
    regulator to reduce losses in the regulator and allow a smaller practial
    heat sink.

    Colin

    (delete BOX)
     
    CWatters, Dec 12, 2003
    #2
    1. Advertising

  3. Silvia Janssen

    Silvia Guest

    "CWatters" <> wrote in message
    news:<RinCb.74530$-ops.be>...
    >
    > "Silvia Janssen" < THIS.nl> wrote in message
    > news:brc8jt$sk1$...
    >
    > > I've connected an industrial PC to an LT1085 voltage regulator which
    > > converts a transformer output of 27V to the 24V that is needed for the

    PC.
    >
    > Is that a typing error? Do you mean "the _5V_ that is needed for the PC"?
    > The LT1085 is a 5V regulator isn't it?
    >


    No, it's no typing error. It's a 24 Volt PC.
    But now I see I omitted the CT at the end of the componenttype.
    It's a Linear Technology LT1085 CT. Maybe that's what makes the difference.
    The LT1085CT is an adjustable voltage regulator.
    I can control the output voltage with a pair of resistors.

    Sorry that I put you on the wrong foot here.

    > > It all works fine, except when I shut down the PC with its own
    > > on/off-switch.
    > > In that case the output voltage of the LT1085 drops down to about 3 Volt

    >
    > That doesn't sound right. You would not expect the voltage to _fall_ when
    > you _remove_ the load unless something is very wrong. When you shut down a
    > PC it normally results in a burst of disc activity that increases power
    > consumption briefly. Perhaps your regulator is overloaded and this extra

    is
    > the final straw?
    >


    It could be that that's the problem. But I really switch it off hard with
    the
    on/off switch, so I guess there is no last activity possible on the PC.
    The Application notes of the component state that it can shut down to a
    lower
    voltage when the load is removed. This is done because of the 'safe area
    protection'.
    In that case you need to cycle the input voltage down and bring it back up.
    There is no other solution stated in the application notes.

    I would like to know how I circumvent this 'safe area protection' and keep
    it from
    activating. I noticed that the LT1085CT has a maximum voltage drop of 25
    Volt. When I remove
    the load, can that create a drop to 0 Volt, which is a difference of 27 Volt
    ?
    Or am I talking absolute nonsense here? My electronic knowledge is sketchy,
    I'm
    a computer programmer mainly.

    I also noticed that this component has a maximum current of 0.5 A when the
    voltage
    difference between in- and output is 20V. Normally its maximum current is
    3A.
    My PC draws about 1A. Could that be the problem when I shut it down?

    Or is this a problem specific to adjustable regulators and should I just get
    a
    fixed regulator?

    I really appreciate the time you took to write this answer. Sorry that I
    forgot to
    specify the component type precisely enough.

    Thanks very much,

    Silvia
     
    Silvia, Dec 12, 2003
    #3
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Replies:
    0
    Views:
    321
  2. Replies:
    2
    Views:
    753
    mook Johnson
    Jan 7, 2006
  3. supervinx

    Switching PSU shutting off under every load

    supervinx, Mar 30, 2010, in forum: Electronic Repair
    Replies:
    11
    Views:
    554
    supervinx
    Apr 1, 2010
  4. Eugbug

    Switching Regulator IC with Voltage Controlled Output

    Eugbug, Nov 20, 2012, in forum: General Electronics Chat
    Replies:
    8
    Views:
    340
    Eugbug
    Dec 22, 2012
  5. ketchumwt

    Delay-off switching of a 115v 2amp load

    ketchumwt, Oct 20, 2013, in forum: General Electronics Chat
    Replies:
    9
    Views:
    224
    (*steve*)
    Oct 21, 2013
Loading...

Share This Page