"Arfa Daily" wrote in message
...

wrote in message
oups.com...
On Nov 8, 3:16 pm, "robb" wrote:
wrote in message

ps.com...


Yes. All agreed. This is basically the first set of tests that
I was
suggesting, which Tony has usefully now converted to specific
measurement
points for you.

Just to try to help you understand the points about the
regulators that we
were making. Without being too 'blanket', there are two types
of linear
regulator chips in common use. These are fixed, and variable.
The fixed ones
are typically three pin, IN - GND -OUT, and come in many
different voltages
eg 5v 8v 9v 10v 12v 15v and so on. They come in both positive
and negative
flavours. In general, they don't care too much whether their
output pin is
loaded or not, but they do have a minimum load figure specified
to
*guarantee* regulation. Also, disconnecting the output pin from
the board,
will likely isolate it from a small decoupling cap which will
usually be
present, and this might, under some circumstances, lead to the
regulator
oscillating.

The variable ones also often have just three pins, such as the
LM317, but
sometimes have more. The main pins that are generally of any
concern, are
IN - SET - OUT. The output voltage is determined by a potential
divider
comprising two resistors, one strung between OUT and SET and
the other
between SET and the circuit common ground. By altering the
ratio of the two
resistors, any voltage to within a couple of volts of the
input, can be set
as the output. Clearly, for this setup to work, the 'top'
resistor of the
voltage-set divider, *must* remain connected to the output pin.
What Tony
was saying, is that if you just disconnect the output pin from
the board,
you will also be disconnecting it from the v-set resistors,
which could lead
to all manner of nasties ...

The good thing about these regulators is that they are pretty
bomb-proof.
They are safe area protected, short circuit protected and
thermally
protected, and will go into a voltage foldback condition if any
anomalies in
their operating conditions are detected. That is not to say
that they don't
fail themselves. They do. But often, the measured conditions,
particularly
the parameters measured by the Mk 1 finger-thermometer, are a
big clue to
what's going on. Example. 5v fixed regulator - say a 7805 1 amp
version.
Measured output voltage 1.8v. Measured device temperature,
using Mk 1 finger
= Ouch. Conclusion ? More than an amp is being drawn, the
device has
overheated, and gone into thermal voltage foldback, to keep
within its SOA.
Example 2. Same regulator, same set up, same measured ouput
voltage.
Measured device temperature = cool. Conclusion ? The regulator
device itself
is faulty. Before there are lots of cries of anguish, a bit
simplistic, I
know, and yes, there could be other reasons for the measured
conditions, but
those two examples cover probably 90% of problems around those
regulator
types.

Arfa


Hi Arfa,
thanks for all the useful help and iinformatin i really do
appreciate your's and everyones helpful advice and information. I
started this project as a learning exercise guess its more a
workout.

Regarding the L387, since i had already removed it hat was asy
to test.

I am pretty sure it is bad from what you explained. that is 5V
regulators only output 5V .

when i applied a 5V/1A DC source to Vin/GND i measured 3.47V
between Vout/GND.
when i applied a 18 V / 3.5A DC source to the Vin/GND i measured
8.37V between Vout/GND

an LED load between Vout/GND caused the L387 to get very hot
quickly.

so a new L387 is in order so that i can complete the other tests
that you TM and others have suggested

thanks again for help,
robb