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

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?

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 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

snipped-for-privacy@pandoraBOX.be (delete BOX)

PC.

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.

is

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