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

[Silvia Janssen]

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

 

[CWatters]

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

[email protected] (delete BOX)

 

[Silvia]

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.

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