DIYbanter - View Single Post

Dimitrij Klingbeil

On 06.05.2016 21:14, wrote:
On Friday, May 6, 2016 at 3:02:30 PM UTC-4, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a
minute or two the power supply starts working. I was told by
someone here or on the basics group that there is probably a bad
electrolytic capacitor causing the problem. I also have a CNC
machine that has a servo amp that acts the same. The machine must
be on for a while and when the cabinet that holds the servo amps
gets warm enough inside the amp start working and will work fine as
long as the machine is kept powered up. If the machine is turned
off for an hour or so and then powered back up the amp still works.
But if the machine is off overnight the amp acts the same as above.
I replaced the amp so there is no problem running the machine but I
would like to repair this amp so I can have a spare on the shelf.
Anyway, it has only 3 physically large electrolytic caps. I
unsoldered one lead from each cap and measured the capacitance with
my Extech multimeter. The two 10 uf caps measured 11. something uf
and the 33 uf cap measured 37. something. Could these caps be bad
anyway? There are also three blue colored dipped caps that have
printed on them: .068K 250. I don't what the dot in the .068K means
but I don't think it's a decimal point. Maybe it means lead free.
There are also a couple 47 uf wound polymer caps. Could either of
these types of caps need to be warm before they work properly? I
have no schematic for this amp and have not been able to get one.
Furthermore, the company that makes this servo amp has ground the
markings off of any device with more than three leads. Thanks,
Eric

Yes, caps that read good value can still be bad. Most electrolytics
that are bad will show high ESR readings (bad) before they show low
capacitance. ESR meters are cheap enough that anyone who screws
with electronics should have one. Despite the advice of others, I
always remove the capacitor to test ESR with an "in-circuit" ESR
tester.

Also, a good percentage of wonky electrolytics will show signs of
venting at the top of the cap, if not an actual electrolyte leakage,
then a discernible bulge. If you see any bulged caps, change them.

Most small disc and film caps will not become temperature sensitive.
For a device to be warm up sensitive like your device it's usually a
weak electrolytic , so concentrate there.

If there's only a few electrolytics, change them all with good
quality (I like Panasonic) caps and they'll probably run.

Good advice. But there is one more small-ish pitfall for the unwary. If
a power supply refuses to start when cold, it's usually an electrolytic
cap. But in many cases it's not one of the big caps but the small one
that is located in the start-up / auxiliary section.

Mains power supplies usually have an auxiliary section that is used to
power the internal needs of the primary side switching circuitry. This
section is powered by a small auxiliary winding on the transformer in
steady-state. But because it needs to start up first, and because at
first the main transformer is not yet operating, it is precharged with a
high value resistor (or sometimes a string of 2 or 3 resistors in
series) directly from the rectified mains. There is a small (47 uF or
similar, 25 to 35 V typically) electrolytic. At first it gets precharged
through the resistor up to the point where the controller chip can start
up. Once the controller starts, it would rapidly drain the small cap, so
the main switching circuitry has to come on line fast in order to top up
the the auxiliary supply cap and keep it from discharging.

Now this small cap only provides little reserve. It's usually sized very
spartanically and it lasts for just one try and only if the start-up
timing of the switching circuitry is not delayed for any reason. If it
drains before the switcher can fully start, the switcher will stop and
wait for another retry at a later time.

Often this small cap is considered "non-critical" and carelessly placed
by the power supply designer into whatever corner had whatever little
free space. This may happen to literally be a "corner" of a heatsink!

When this cap dries out either from overheating of just being selected
from the cheapest manufacturer, it will rise in ESR and thus no longer
provide the peak current that the controller needs in order to start.
This either delays the start to the point where the cap is drained or
causes it to outright abort prematurely because the auxiliary supply
rail tanks right at the first few switching cycles.

When warmer it has lower resistance, thus it can provide higher peak
currents and have better chances of starting the supply successfully.
Also if the supply is working continuously for many years and never
switched off, the condition of this capacitor is of no consequence
during operation. Even if somebody just cut it out of a working power
supply, there are usually some small ceramics on the rail and the
auxiliary transformer winding will keep them topped up, so that the
supply would quite likely keep on working. But if the supply ever gets
switched off, even for a very short time, it will never start up again.

If you see this type of behavior, find and replace the cap in the
auxiliary section. You can normally identify it as being physically
small, a low voltage type (from 25 to 50 V) and being connected to a
sizeable precharge resistor that gets supplied from the high voltage
DC rail.

If you don't have an exact replacement, it's normally OK to use the
next higher capacitance as long as the voltage rating fits. If you
find that yours sits in the corner of a heatsink, use a low ESR high
temperature rated replacement (no matter what the original has been)
and some reasonable reserve capacitance-wise won't hurt either.

Dimitrij