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Old power supply issue -- any ideas ?

Started by Unknown October 27, 2013
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. 
     
On Saturday, October 26, 2013 11:15:11 PM UTC-4, daku...@gmail.com wrote:
> Could some electronics guru please help ?=20 >=20 > I have an old (about 7 - 8 years) +/-15=20 >=20 > Volt 2 A power supply that so far has=20 >=20 > been very reliable. It is a standard=20 >=20 > linear design, with the transformer is=20 >=20 > rated at +/- 20 Volts 2A max. The output >=20 > voltage is adjustable via pots at the >=20 > +/- terminals. Recently, I have been using=20 >=20 > it to run two CPU style fans that cools a=20 >=20 > experimental server machine. >=20 > I have noticed that recently the fans=20 >=20 > start at full speed, but then become=20 >=20 > slow. Each fan is rated at 12 Volt, 0.17A >=20 > (170 mA). So I checked with my Fluke=20 >=20 > the voltage outputs at various point.=20 >=20 > The transformer AC output is a steady +/-=20 >=20 > 20 Volt, but the DC output of the rectifier=20 >=20 > +/- 30 Volt!. Also, a voltage regulator >=20 > that comes after the rectifier is becoming=20 >=20 > extremely hot(even with a thick heat sink) >=20 > and when the fans slow down, the output >=20 > voltage drops to 8.4 Volts, as opposed to=20 >=20 > the preset value of 13.0 Volts. Any ideas >=20 > as to what the problem might be ? Obviously >=20 > that current being drawn by the fans is=20 >=20 > increasing, leading to the droop.
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't= see the rectified voltage holding up at 30V. Note the nominal 20VAC peak r= ectifies to sqrt(2)x for nominal 30VDC.
On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), the renowned
dakupoto@gmail.com wrote:

>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
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
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!.
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)
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.
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" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
On 27/10/2013 2:15 PM, dakupoto@gmail.com wrote:
> 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. > >
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.
On 2013-10-27, dakupoto@gmail.com <dakupoto@gmail.com> wrote:
> 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.
> +/- 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.
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, -- &#9858;&#9859; 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
Spehro Pefhany wrote:
> On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), the renowned > dakupoto@gmail.com wrote: > >> 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 > > 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 > > 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!. > > 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) > > 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. > > 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
-- 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
On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), dakupoto@gmail.com wrote:

>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. >
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
On Sunday, October 27, 2013 10:51:13 AM UTC-4, legg wrote:
> On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), dakupoto@gmail.com wrote: > > > > >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. > > > > > > > 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
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.
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.
On Sunday, October 27, 2013 6:49:51 AM UTC-4, Dave M wrote:
> Spehro Pefhany wrote: > > > On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), the renowned > > > dakupoto@gmail.com wrote: > > > > > >> 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 > > > > > > 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 > > > > > > 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!. > > > > > > 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) > > > > > > 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. > > > > > > 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 > > > > -- > > > > 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
That is indeed a possibility, but careful visual inspection does not appear to indicate any leaking.
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message 
news:ft3p69dinff1eb6l0dpn7pm23trkmn4vur@4ax.com...
> On Sat, 26 Oct 2013 20:15:11 -0700 (PDT), the renowned > dakupoto@gmail.com wrote: > >>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 > > 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 > > 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!. > > 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) > > 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. > > 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" > speff@interlog.com Info for manufacturers: > http://www.trexon.com > Embedded software/hardware/analog Info for designers: > http://www.speff.com
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.