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Old power supply issue -- any ideas ?

D

[email protected]

Could some electronics guru please help ?
I have an old (about 7 - 8 years) +/-15
Volt 2 A power supply that so far has
been very reliable. It is a standard
linear design, with the transformer is
rated at +/- 20 Volts 2A max. The output
voltage is adjustable via pots at the
+/- terminals. Recently, I have been using
it to run two CPU style fans that cools a
experimental server machine.
I have noticed that recently the fans
start at full speed, but then become
slow. Each fan is rated at 12 Volt, 0.17A
(170 mA). So I checked with my Fluke
the voltage outputs at various point.
The transformer AC output is a steady +/-
20 Volt, but the DC output of the rectifier
+/- 30 Volt!. Also, a voltage regulator
that comes after the rectifier is becoming
extremely hot(even with a thick heat sink)
and when the fans slow down, the output
voltage drops to 8.4 Volts, as opposed to
the preset value of 13.0 Volts. Any ideas
as to what the problem might be ? Obviously
that current being drawn by the fans is
increasing, leading to the droop.
 
B

[email protected]

Could some electronics guru please help ?

I have an old (about 7 - 8 years) +/-15

Volt 2 A power supply that so far has

been very reliable. It is a standard

linear design, with the transformer is

rated at +/- 20 Volts 2A max. The output

voltage is adjustable via pots at the

+/- terminals. Recently, I have been using

it to run two CPU style fans that cools a

experimental server machine.

I have noticed that recently the fans

start at full speed, but then become

slow. Each fan is rated at 12 Volt, 0.17A

(170 mA). So I checked with my Fluke

the voltage outputs at various point.

The transformer AC output is a steady +/-

20 Volt, but the DC output of the rectifier

+/- 30 Volt!. Also, a voltage regulator

that comes after the rectifier is becoming

extremely hot(even with a thick heat sink)

and when the fans slow down, the output

voltage drops to 8.4 Volts, as opposed to

the preset value of 13.0 Volts. Any ideas

as to what the problem might be ? Obviously

that current being drawn by the fans is

increasing, leading to the droop.
Click to expand...

You can start by disconnecting the fans and seeing how the voltage holds up.. If the loading was causing some kind of overtemperature foldback, I don'tsee the rectified voltage holding up at 30V. Note the nominal 20VAC peak rectifies to sqrt(2)x for nominal 30VDC.
 
S

Spehro Pefhany

On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), the renowned
Could some electronics guru please help ?
I have an old (about 7 - 8 years) +/-15
Volt 2 A power supply that so far has
been very reliable. It is a standard
linear design, with the transformer is
rated at +/- 20 Volts 2A max. The output
Click to expand...

Is it a 20-0-20 output rated at 2A per output?
(an 80VA transformer)
voltage is adjustable via pots at the
+/- terminals. Recently, I have been using
it to run two CPU style fans that cools a
experimental server machine.
I have noticed that recently the fans
start at full speed, but then become
slow. Each fan is rated at 12 Volt, 0.17A
(170 mA). So I checked with my Fluke
Click to expand...

Okay, so about 4W of fans.
the voltage outputs at various point.
The transformer AC output is a steady +/-
20 Volt, but the DC output of the rectifier
+/- 30 Volt!.
Click to expand...

That is normal. ~sqrt(2) * AC voltage minus
a diode drop, (AC voltage will be a bit
higher with light loading than at full load).
Also, a voltage regulator
that comes after the rectifier is becoming
extremely hot(even with a thick heat sink)
Click to expand...

It should be dissipating about 6 watts.
and when the fans slow down, the output
voltage drops to 8.4 Volts, as opposed to
the preset value of 13.0 Volts. Any ideas
as to what the problem might be ? Obviously
that current being drawn by the fans is
increasing, leading to the droop.
Click to expand...

Does the supply have a current limit function?
If so, and it's set too low, you'd see that.

What happens if you disconnect one or the other
of the fans?



Best regards,
Spehro Pefhany
 
S

Sylvia Else

Could some electronics guru please help ?
I have an old (about 7 - 8 years) +/-15
Volt 2 A power supply that so far has
been very reliable. It is a standard
linear design, with the transformer is
rated at +/- 20 Volts 2A max. The output
voltage is adjustable via pots at the
+/- terminals. Recently, I have been using
it to run two CPU style fans that cools a
experimental server machine.
I have noticed that recently the fans
start at full speed, but then become
slow. Each fan is rated at 12 Volt, 0.17A
(170 mA). So I checked with my Fluke
the voltage outputs at various point.
The transformer AC output is a steady +/-
20 Volt, but the DC output of the rectifier
+/- 30 Volt!. Also, a voltage regulator
that comes after the rectifier is becoming
extremely hot(even with a thick heat sink)
and when the fans slow down, the output
voltage drops to 8.4 Volts, as opposed to
the preset value of 13.0 Volts. Any ideas
as to what the problem might be ? Obviously
that current being drawn by the fans is
increasing, leading to the droop.
Click to expand...

When you say the regulator is becoming very hot, do you actually mean
that the heat sink is? If the regulator is becoming much hotter than the
heat sink, then the problem may be that the heatsink is no longer in
good thermal contact with the regulator.

Sylvia.
 
J

Jasen Betts

Could some electronics guru please help ?
I have an old (about 7 - 8 years) +/-15
Volt 2 A power supply that so far has
been very reliable.
Click to expand...
+/- 30 Volt!. Also, a voltage regulator
that comes after the rectifier is becoming
extremely hot(even with a thick heat sink)
and when the fans slow down, the output
voltage drops to 8.4 Volts, as opposed to
the preset value of 13.0 Volts. Any ideas
as to what the problem might be ? Obviously
that current being drawn by the fans is
increasing, leading to the droop.
Click to expand...


if the heatsink isn't also getting very hot it sunds like the
regulator is incorrectly mounted or possibly the regulator booster
transistor has failed, all that anyone can really be sure of is that
youu awr describing a broken PSU.

if you can't tap +12V from somewhere in the server buy a cheap 12V
powersupply, either reatil or online,
 
L

legg

Could some electronics guru please help ?
I have an old (about 7 - 8 years) +/-15
Volt 2 A power supply that so far has
been very reliable. It is a standard
linear design, with the transformer is
rated at +/- 20 Volts 2A max. The output
voltage is adjustable via pots at the
+/- terminals. Recently, I have been using
it to run two CPU style fans that cools a
experimental server machine.
I have noticed that recently the fans
start at full speed, but then become
slow. Each fan is rated at 12 Volt, 0.17A
(170 mA). So I checked with my Fluke
the voltage outputs at various point.
The transformer AC output is a steady +/-
20 Volt, but the DC output of the rectifier
+/- 30 Volt!. Also, a voltage regulator
that comes after the rectifier is becoming
extremely hot(even with a thick heat sink)
and when the fans slow down, the output
voltage drops to 8.4 Volts, as opposed to
the preset value of 13.0 Volts. Any ideas
as to what the problem might be ? Obviously
that current being drawn by the fans is
increasing, leading to the droop.
Click to expand...

Brand name and model?

Have you measured the output ripple voltage? Could be a stability
issue under the pulsed load presented by the fans. Stability may
degrade as tempertures increase/reduce.

You say you're adjusting the previously fixed 15V to 13V.
Is this adjustment made when loaded or unloaded?

Some common linear circuits will limit earlier if the regulated output
is adjusted low, without other compensation.

If you've lost a phase in the rectifier (+/-20V suggests half wave
rectification, ratherthan fullwave), there will be increased input
ripple and degraded load regulation. Dried out caps can also cause
input regulation effects and output stability issues.

If you were happy with it in the past, it's probably worth fixing,
something that might not be as easy with a modern replacement.

RL
 
D

[email protected]

Brand name and model?



Have you measured the output ripple voltage? Could be a stability

issue under the pulsed load presented by the fans. Stability may

degrade as tempertures increase/reduce.



You say you're adjusting the previously fixed 15V to 13V.

Is this adjustment made when loaded or unloaded?



Some common linear circuits will limit earlier if the regulated output

is adjusted low, without other compensation.



If you've lost a phase in the rectifier (+/-20V suggests half wave

rectification, ratherthan fullwave), there will be increased input

ripple and degraded load regulation. Dried out caps can also cause

input regulation effects and output stability issues.



If you were happy with it in the past, it's probably worth fixing,

something that might not be as easy with a modern replacement.



RL
Click to expand...

I am a bit puzzled by some of your questions.
You say:
If you've lost a phase in the rectifier (+/-20V >suggests half wave rectification, rather than >fullwave), there will be increased input ripple
and degraded load regulation. Dried out caps can
also cause input regulation effects and output >stability issues.
Click to expand...
The transformer has a 20 - 9 - 20 V 2 Amp AC
output, and the rectifier output is +/- 30 V DC,
taking into account the diode drop sqrt(2) x 20 V.
May I know why you say that it is half wave
rectification, given that the input AC is a
standard 1 phase AC ?
I have adjusted the output voltage with and
without the fans connected, and the result is
the same, fans run at full speed for e.g. 45
minutes, and then slow down.
 
D

[email protected]

--



Check the capacitors located close to the output terminals (after the

regulators). One or more might be leaking seriously and overloading the

regulator.



Cheers,

Dave M
Click to expand...

That is indeed a possibility, but careful visual
inspection does not appear to indicate any leaking.
 
R

Rick

Spehro Pefhany said:
On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), the renowned


Is it a 20-0-20 output rated at 2A per output?
(an 80VA transformer)


Okay, so about 4W of fans.


That is normal. ~sqrt(2) * AC voltage minus
a diode drop, (AC voltage will be a bit
higher with light loading than at full load).


It should be dissipating about 6 watts.


Does the supply have a current limit function?
If so, and it's set too low, you'd see that.

What happens if you disconnect one or the other
of the fans?



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
[email protected] Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com
Click to expand...

7-8 years ago is the era of bad capacitors with high ESR. Look for swollen
tops on electrolytic or better check them with an ESR meter.
 
G

George Herold

Could some electronics guru please help ?
I have an old (about 7 - 8 years) +/-15
Volt 2 A power supply that so far has
been very reliable. It is a standard
linear design, with the transformer is
rated at +/- 20 Volts 2A max. The output
voltage is adjustable via pots at the
+/- terminals. Recently, I have been using
it to run two CPU style fans that cools a
experimental server machine.
I have noticed that recently the fans
start at full speed, but then become
slow. Each fan is rated at 12 Volt, 0.17A
(170 mA). So I checked with my Fluke
the voltage outputs at various point.
The transformer AC output is a steady +/-
20 Volt, but the DC output of the rectifier
+/- 30 Volt!. Also, a voltage regulator
that comes after the rectifier is becoming
extremely hot(even with a thick heat sink)
and when the fans slow down, the output
voltage drops to 8.4 Volts, as opposed to
the preset value of 13.0 Volts. Any ideas
as to what the problem might be ? Obviously
that current being drawn by the fans is
increasing, leading to the droop.
Click to expand...
Hmm I'm not sure how you can assume it's the fans? Try a resistor as load maybe.

How about a nice Phihong wall wart switcher.
http://www.digikey.com/product-detail/en/PSC12R-120/993-1052-ND/2384503

You can find them cheaper.

George H.
 
S

Spehro Pefhany

7-8 years ago is the era of bad capacitors with high ESR. Look for swollen
tops on electrolytic or better check them with an ESR meter.
Click to expand...

Possible, but they generally didn't use those caps in linear power
supplies- they were consumed in motherboards where they got to eat HF
ripple at high current.
 
L

legg

I am a bit puzzled by some of your questions.
Click to expand...

Yeah? Well, make and model isn't such stretch.
You say:
The transformer has a 20 - 9 - 20 V 2 Amp AC
output, and the rectifier output is +/- 30 V DC,
taking into account the diode drop sqrt(2) x 20 V.
May I know why you say that it is half wave
rectification, given that the input AC is a
standard 1 phase AC ?
Click to expand...

It could have been a single 20V winding with a full wave bridge. In
that case, you couldn't just lose a phase; it would be all or nothing.

If just one of the 20-0-20 windings is not being processed, you'd
still get 30V from the single phase, but ripple would drop from 120hz
to 60hz and various other problems would start to show up.
I have adjusted the output voltage with and
without the fans connected, and the result is
the same, fans run at full speed for e.g. 45
minutes, and then slow down.
Click to expand...
You mean 4 to 5 minutes? Thats electronic thermal time constant, but
the reason for thermal stress and effect on regulation or limiting
hasn't been identified.

45minutes is an eternity - thats a magnetics and housing thermal time
constant, maybe even runaway effects.

Do you adjust the output voltage using a supplied on-board adjustment
pot, or are you doing it on your own, using another method?

RL
 
J

[email protected]

"Obviously
that current being drawn by the fans is
increasing, leading to the droop. "

You seem to have answered your own question right there. Hot and low voltage outout = overload. There is a slight possibility that a zener on the outpuit is getting leaky when warmed up, but the odds of that are slim and none and slim is nowhere to be found. Usually when that happens and overvoltage condition happneed to cause it.

The long and short of it is anything with moving parts is suspect first. One of those fan motors is starting to bind up when warm. Actually most motors are the opposite. When the bearing get shitty they run better when hot, so I suspect you have taken their life cycle beyong that, to the point wheredust and shit gets in between the bearing surface and the shaft. That is when most of them start exhibiting these tendencies. On the other hand it could just be so worn out that somehow part of the armature is starting to rub part of the stator. You didn't mention exactly what kind of motors they are.

If they are brushless slotless DC motors using drive chips, there may be some electrolytics going bad. They don't get leaky but they **** up the signal from the Hall effect devices or whatever it uses and makes the motor workagainst itself. Like a car engine that jumped time in a way. Of course that only applies to certain chips and configurations.

if you don't think it's the motor(s) take a car taillight or something that'll pull about an amp and run it for a while. If the voltage doesn't drop, look at the motors.
 
J

josephkk

Not to mention the fans will slow if the volts drop to 8.4v
Click to expand...

Old brush motor fans for sure, i won't make that bet on electronically
controlled fans (BLDC).

?-)
 
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