Voltage regulator instability

[John Popelish]

John Popelish wrote:

It is only drawing a 20mA current steady state, so I thought heat would
not be a problem.

At 20 mA, it should work. What is the current drawn during the start
up transient, and how long does the transient last?

(snip)

I don't have any adapters of that size. Besides the spec sheet says the
regulator handles up to 30 or so Volts.

That means the voltage will not instantly destroy anything, but you
still have to make sure the temperature does not reach a destructive
level. If you can put your knuckle against the side of the regulator
while it is operating, it is doing fine, from a thermal standpoint.
If it makes you jump, not so much.

 

[suputnic]

At 20 mA, it should work. What is the current drawn during the start
up transient, and how long does the transient last?

Sorry been away setting up and testing these things. The transient
current is 500 - 600 mA and it lasts about a second. Hard to tell with
a multimeter. Two of my 3.3V regultors are measuring 2.7V, have they
been damaged by being added to a 12V adapter with no caps you reckon?
Or could they be originally faulty? The caps have made all the
differemce BTW, the heater killer adapter now is stable.

(snip)

That means the voltage will not instantly destroy anything, but you
still have to make sure the temperature does not reach a destructive
level. If you can put your knuckle against the side of the regulator
while it is operating, it is doing fine, from a thermal standpoint.
If it makes you jump, not so much.

Temperature is fine.

 

[John Popelish]

John Popelish wrote: (snip)

Sorry been away setting up and testing these things. The transient
current is 500 - 600 mA and it lasts about a second. Hard to tell with
a multimeter. Two of my 3.3V regultors are measuring 2.7V, have they
been damaged by being added to a 12V adapter with no caps you reckon?

I think that is possible, since large output swings might
have also caused large output current spikes into this sort
of load and caused damaging temperature spikes in the regulator.

Or could they be originally faulty?

We will never know, but new devices generally have a very
low fault rate.

The caps have made all the
differemce BTW, the heater killer adapter now is stable.

Good news.

 

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Sorry been away setting up and testing these things. The transient
current is 500 - 600 mA and it lasts about a second.

I wouldn't consider something that lasts a second being a transient.
Granted, you don't have a current probe to make an accurate
measurement, but a second is forever in the land of electromigration.