I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred

On Mon, 21 Jul 2014 11:18:07 -0700, Fred McKenzie wrote:

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred

46C? and 50C no biggie

transformers are passive devices and made out of 'material'

easily take 65C, that's 150F

you may be running your AC mains at 125Vac instead of the 115Vac, which
will make them run just that bit hotter to where you noticed.

"RobertMacy" wrote in message
newsp.xjc24hpo2cx0wh@ajm...
On Mon, 21 Jul 2014 11:18:07 -0700, Fred McKenzie wrote:

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred

46C? and 50C no biggie

transformers are passive devices and made out of 'material'

easily take 65C, that's 150F

you may be running your AC mains at 125Vac instead of the 115Vac, which
will make them run just that bit hotter to where you noticed.


One thing the OP failed to state is the voltage of his 20 amp power supply.
2000 volts at 20 amps is a much larger transformer then one might find in a
12 volt, 20 amp supply.

In article ,
"Tom Miller" wrote:

One thing the OP failed to state is the voltage of his 20 amp power supply.
2000 volts at 20 amps is a much larger transformer then one might find in a
12 volt, 20 amp supply.

I am using these power supplies for Ham Radio equipment. Voltage is
supposed to be 13.8, but I think most people just say 12.

A temperature of 125º F/52º C is hotter than you want to hold your hand
on, but you probably will not scream from the pain. Temperature is a
significant factor in reliability calculations. I was concerned about
the long term degradation of components.

I think I just answered my own question. The hottest power supply is 30
years old and the other is 20. Both appear to meet their electrical
specifications.

Fred

It happens that Fred McKenzie formulated :
In article ,
"Tom Miller" wrote:

One thing the OP failed to state is the voltage of his 20 amp power supply.
2000 volts at 20 amps is a much larger transformer then one might find in a
12 volt, 20 amp supply.

I am using these power supplies for Ham Radio equipment. Voltage is
supposed to be 13.8, but I think most people just say 12.

A temperature of 125º F/52º C is hotter than you want to hold your hand
on, but you probably will not scream from the pain. Temperature is a
significant factor in reliability calculations. I was concerned about
the long term degradation of components.

I think I just answered my own question. The hottest power supply is 30
years old and the other is 20. Both appear to meet their electrical
specifications.

Fred

Do they really meet their Specs?
Back then (30 years ago) the nominal in USA was more likely 110 volts
whereas today to follow Harmonisation rules it should be 120 which is
a like 9% increase. Of course there is a wide margin but you may be on
the high side. Just a thought.

"John G" wrote in message
. au...
It happens that Fred McKenzie formulated :
In article ,
"Tom Miller" wrote:

One thing the OP failed to state is the voltage of his 20 amp power
supply. 2000 volts at 20 amps is a much larger transformer then one
might find in a 12 volt, 20 amp supply.

I am using these power supplies for Ham Radio equipment. Voltage is
supposed to be 13.8, but I think most people just say 12.

A temperature of 125º F/52º C is hotter than you want to hold your hand
on, but you probably will not scream from the pain. Temperature is a
significant factor in reliability calculations. I was concerned about
the long term degradation of components.

I think I just answered my own question. The hottest power supply is 30
years old and the other is 20. Both appear to meet their electrical
specifications.

Fred

Do they really meet their Specs?
Back then (30 years ago) the nominal in USA was more likely 110 volts
whereas today to follow Harmonisation rules it should be 120 which is a
like 9% increase. Of course there is a wide margin but you may be on the
high side. Just a thought.

Sounds like an Astron AS20 or similar. The transformers are made with cost
in mind and they do run hot even at no load.

As long as it doesn't smoke, I would not be too worried.

On Mon, 21 Jul 2014 19:30:16 -0700, Fred McKenzie wrote:

...snip....
A temperature of 125º F/52º C is hotter than you want to hold your hand
on, but you probably will not scream from the pain. Temperature is a
significant factor in reliability calculations. I was concerned about
the long term degradation of components.

I think I just answered my own question. The hottest power supply is 30
years old and the other is 20. Both appear to meet their electrical
specifications.

Fred


Standard 'rule of thumb' the MTBF is halved for every 10C increase in
temperature.
(10C == 18F)

On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F
is and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like
degrees F, anymore. Please learn the Metric system.

--
Trevor Wilson www.rageaudio.com.au

"Trevor Wilson" wrote in message
...
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F
is and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like degrees
F, anymore. Please learn the Metric system.

--
Trevor Wilson www.rapeaudio.com.au


Why, because you are too ****ing stupid or lazy to do a simple conversion?

So why did you even take the time to make your stupid comments?

"Tom Miller" wrote in message ...


"Trevor Wilson" wrote in message
...
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F
is and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like degrees
F, anymore. Please learn the Metric system.

--
Trevor Wilson www.rapeaudio.com.au


Why, because you are too ****ing stupid or lazy to do a simple conversion?




OK, convert the very accurate and reproducable measurement of 62.25cm, to
the Imperial system, and you get 24inches and 39/64ths.

That's a bit of a joke, surely, that should not be present in the 21st
century.


Gareth.

Gareth Magennis laid this down on his screen :

"Tom Miller" wrote in message ...


"Trevor Wilson" wrote in message
...
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F is
and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like degrees
F, anymore. Please learn the Metric system.

-- Trevor Wilson www.rapeaudio.com.au


Why, because you are too ****ing stupid or lazy to do a simple conversion?




OK, convert the very accurate and reproducable measurement of 62.25cm, to the
Imperial system, and you get 24inches and 39/64ths.

That's a bit of a joke, surely, that should not be present in the 21st
century.


Gareth.

No it converts to 24.507874 inches if you us the same maths notation.

"John G" wrote in message
. au...

Gareth Magennis laid this down on his screen :

"Tom Miller" wrote in message ...


"Trevor Wilson" wrote in message
...
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot
transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F
is and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like degrees
F, anymore. Please learn the Metric system.

-- Trevor Wilson www.rapeaudio.com.au


Why, because you are too ****ing stupid or lazy to do a simple conversion?




OK, convert the very accurate and reproducable measurement of 62.25cm, to
the Imperial system, and you get 24inches and 39/64ths.

That's a bit of a joke, surely, that should not be present in the 21st
century.


Gareth.

No it converts to 24.507874 inches if you us the same maths notation.




The point is that the Imperial system commonly uses discrete steps that are
only accurate to those steps.

The metric system does not suffer this limitation.


Gareth.

**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F is
and I have no interest either, but I suspect it is less than 75 C.

It's such a big deal to convert, isn't it?

Start by subtracting 32. That's 93, which you can round off to 90. Then take
5/9 of that -- 50, which is close enough. You can do it in your head in a few
seconds.

There's no need to memorize the formula, if you remember that 0 and 32 degrees
are the same point, and 1 degree F is 9/5 degree C.


[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements,
like degrees F, anymore. Please learn the Metric system.

First, it's spelled arcane (hidden, mysterious). Which isn't the word you
want -- you want archaic.

Metric is not capitalized.

"Gareth Magennis" wrote in message
...


"Tom Miller" wrote in message ...


"Trevor Wilson" wrote in message
...
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot
transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees F
is and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like degrees
F, anymore. Please learn the Metric system.

--
Trevor Wilson www.rapeaudio.com.au


Why, because you are too ****ing stupid or lazy to do a simple conversion?




OK, convert the very accurate and reproducable measurement of 62.25cm, to
the Imperial system, and you get 24inches and 39/64ths.

That's a bit of a joke, surely, that should not be present in the 21st
century.


Gareth.

Why not use 24.50 inches? Minding the significant figures.
It only bothers lazy people. But at least stay within one system.

Then there's pounds, shillings, pence, etc. Oh, at least you have the Euro.

On 22/07/2014 8:48 AM, Tom Miller wrote:

"Trevor Wilson" wrote in message
...
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot
transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely,
though I prefer to see less than 75 C. I have no idea what 125 degrees
F is and I have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like
degrees F, anymore. Please learn the Metric system.

--
Trevor Wilson www.rapeaudio.com.au


Why, because you are too ****ing stupid or lazy to do a simple conversion?

**There is no need to do the conversion. Hardly anyone bothers with
Imperial measurements anymore.


So why did you even take the time to make your stupid comments?

**I answered the question, correctly and succinctly. I also attempted to
educate the original poster in how to deal with an international
audience, by pointing out that a measurement system which is used in one
small corner of the planet may not be understood by the vast majority.

--
Trevor Wilson www.rageaudio.com.au

Trevor Wilson was thinking very hard :
On 22/07/2014 4:18 AM, Fred McKenzie wrote:
I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer. I disconnected the
secondary windings of one, but results were the same.

With their covers installed, neither power supply feels excessively hot.
The one that got to 125, gets to 95 Degrees F on top of its cover with
about two Amps load.

One cause might be not enough iron in the core. I suppose there could
be some circulating currents in the core as well. Regardless, does
anyone know if they are hot enough to be a problem? Or should I just
not worry about it?

I checked several other power supplies, and none had transformers above
95 Degrees F.

Fred


**Most power transformers will operate at 100 degrees C quite safely, though
I prefer to see less than 75 C. I have no idea what 125 degrees F is and I
have no interest either, but I suspect it is less than 75 C.

[ASIDE] This is an international group. Almost none of the planet's
population (less than 5%) bothers with archane measurements, like degrees F,
anymore. Please learn the Metric system.

Actually it is 51.66 deg C which is not to hard to figure and afterall
most of us down here have learnt over the years to cope with whatever
the underdeveloped world chooses to use. :-?

On Monday, July 21, 2014 5:04:09 PM UTC-4, Trevor Wilson wrote:
[ASIDE] This is an international group. Almost none of the planet's

population (less than 5%) bothers with archane measurements, like

degrees F, anymore. Please learn the Metric system.

While I agree in general with the sentiment, there is no actual advantage of C over F in doing calculations.

C and F are both European temperature systems based on 100 degrees. For C there are 100 degrees between the temperature at which water freezes and boils; for F there are 100 degrees between the usual hottest and coldest temperatures in Europe at the time.

On 23/07/2014 2:37 AM, Tim R wrote:
On Monday, July 21, 2014 5:04:09 PM UTC-4, Trevor Wilson wrote:
[ASIDE] This is an international group. Almost none of the
planet's

population (less than 5%) bothers with archane measurements, like

degrees F, anymore. Please learn the Metric system.

While I agree in general with the sentiment, there is no actual
advantage of C over F in doing calculations.

C and F are both European temperature systems based on 100 degrees.
For C there are 100 degrees between the temperature at which water
freezes and boils; for F there are 100 degrees between the usual
hottest and coldest temperatures in Europe at the time.


**I agree that there is no specific advantage in Celsius, except, of
course, that it dove-tails neatly with Kelvin measurements. SI units, on
the whole, however, are universally accepted and logical for humans to
work with. No one of any consequence bothers with Imperial measurements
any longer.

--
Trevor Wilson www.rageaudio.com.au

"Fred McKenzie"

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.


** FYI there is a much simpler, low tech way to find the internal temp of a
transformer.

1. Measure the primary winding's resistance when at room temp.

2. Make the same measurement ( quick as you can) when the tranny's running
temp has stabilised.

3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.

Eg.

Room temp = 4 ohms

Hot temp = 5 ohms

Increase factor = 0.25

0.25 x 254 = 63.5 degrees C

Add on the actual room temp to get final result.

If the number is over 100C there is cause for concern.



..... Phil

Phil Allison wrote:

"Fred McKenzie"

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.


** FYI there is a much simpler, low tech way to find the internal temp of a
transformer.

1. Measure the primary winding's resistance when at room temp.

2. Make the same measurement ( quick as you can) when the tranny's running
temp has stabilised.

3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.

Eg.

Room temp = 4 ohms

Hot temp = 5 ohms

Increase factor = 0.25

0.25 x 254 = 63.5 degrees C

Add on the actual room temp to get final result.

If the number is over 100C there is cause for concern.

spiffy trick. thanks.

In article ,
"Phil Allison" wrote:

"Fred McKenzie"

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.


** FYI there is a much simpler, low tech way to find the internal temp of a
transformer.

1. Measure the primary winding's resistance when at room temp.

2. Make the same measurement ( quick as you can) when the tranny's running
temp has stabilised.

3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.

Eg.

Room temp = 4 ohms

Hot temp = 5 ohms

Increase factor = 0.25

0.25 x 254 = 63.5 degrees C

Add on the actual room temp to get final result.

If the number is over 100C there is cause for concern.

Phil-

This is useful information, but my results did not agree with IR
measurements.

I think my problem was that it is hard to measure low values of
resistance accurately. For my "hot" transformer, cold resistance
measurement of the primary was around one Ohm on a Fluke digital meter.
However shorting the meter leads produced 0.4 Ohms.

After several hours, primary resistance measurement was 1.2 Ohms.
Subtracting 0.4 from both measurements, the increase factor was
(0.8-0.6)/0.6 = 1/3. Calculated rise was 84.7º C, plus original ambient
24.4º C gives 109.1º C.

The IR thermometer measured the top of the transformer laminations.
Initial temperature was 24.4º C. Final temperature was 53.5º C. I
could hold my hand on the transformer without screaming, so I am fairly
sure the temperature was nowhere near 109.1º C!

I think a 4-terminal resistance measurement would provide more realistic
readings.

Fred

"Fred McKenzie"
"Phil Allison"
"Fred McKenzie"

I have a couple of 20 Amp linear DC power supplies with hot
transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.


** FYI there is a much simpler, low tech way to find the internal temp of
a
transformer.

1. Measure the primary winding's resistance when at room temp.

2. Make the same measurement ( quick as you can) when the tranny's
running
temp has stabilised.

3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.

Eg.

Room temp = 4 ohms

Hot temp = 5 ohms

Increase factor = 0.25

0.25 x 254 = 63.5 degrees C

Add on the actual room temp to get final result.

If the number is over 100C there is cause for concern.


This is useful information, but my results did not agree with IR
measurements.

** It never will as the internal temp of a tranny is higher than the surface
temp - the test gives the average temp of the copper wire - which is
crucial for insulation purposes.

The same test is used in international standards for transformers etc to
establish safe temp rise.


I think my problem was that it is hard to measure low values of
resistance accurately.

** Not if you pass a known current through the winding and measure the
voltage drop.

Eg: Connect a 10ohm, 10W resistor in series with the winding and adjust a
bench PSU each time to give 10V across the resistor - so you have 1amp. The
voltage across the winding is then the same as its resistance in ohms.

Since you compute the ratio of two readings, absolute accuracy of the
resistor and the voltmeter do not matter.


..... Phil

On Sat, 02 Aug 2014 23:58:37 -0400, Fred McKenzie wrote:

In article ,
"Phil Allison" wrote:

"Fred McKenzie"

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.


** FYI there is a much simpler, low tech way to find the internal temp of a
transformer.

1. Measure the primary winding's resistance when at room temp.

2. Make the same measurement ( quick as you can) when the tranny's running
temp has stabilised.

3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.

Eg.

Room temp = 4 ohms

Hot temp = 5 ohms

Increase factor = 0.25

0.25 x 254 = 63.5 degrees C

Add on the actual room temp to get final result.

If the number is over 100C there is cause for concern.

Phil-

This is useful information, but my results did not agree with IR
measurements.

I think my problem was that it is hard to measure low values of
resistance accurately. For my "hot" transformer, cold resistance
measurement of the primary was around one Ohm on a Fluke digital meter.
However shorting the meter leads produced 0.4 Ohms.

After several hours, primary resistance measurement was 1.2 Ohms.
Subtracting 0.4 from both measurements, the increase factor was
(0.8-0.6)/0.6 = 1/3. Calculated rise was 84.7º C, plus original ambient
24.4º C gives 109.1º C.

The IR thermometer measured the top of the transformer laminations.
Initial temperature was 24.4º C. Final temperature was 53.5º C. I
could hold my hand on the transformer without screaming, so I am fairly
sure the temperature was nowhere near 109.1º C!

I think a 4-terminal resistance measurement would provide more realistic
readings.

Fred

Yes, you got the 4-terminal one absolutely right. A meter that can use
kelvin clips and the resulting measurements would be much better. This is
normal for resistances under 10 ohms and mandatory under 1 ohm (for 3
reliable digits). For lower resistance or more resolution it gets
progressively worse.

?-)

josephkk wrote:
On Sat, 02 Aug 2014 23:58:37 -0400, Fred McKenzie wrote:

In article ,
"Phil Allison" wrote:

"Fred McKenzie"

I have a couple of 20 Amp linear DC power supplies with hot transformers.

With no load, one gets up to 115 Degrees F after several hours. The
other gets up to 125 Degrees F. Measurements were made with an IR
thermometer looking at the top of the transformer.


** FYI there is a much simpler, low tech way to find the internal temp of a
transformer.

1. Measure the primary winding's resistance when at room temp.

2. Make the same measurement ( quick as you can) when the tranny's running
temp has stabilised.

3. Calculate the increase as a decimal factor and multiply that by 254 to
get the temp rise in degrees C.

Eg.

Room temp = 4 ohms

Hot temp = 5 ohms

Increase factor = 0.25

0.25 x 254 = 63.5 degrees C

Add on the actual room temp to get final result.

If the number is over 100C there is cause for concern.

Phil-

This is useful information, but my results did not agree with IR
measurements.

I think my problem was that it is hard to measure low values of
resistance accurately. For my "hot" transformer, cold resistance
measurement of the primary was around one Ohm on a Fluke digital meter.
However shorting the meter leads produced 0.4 Ohms.

After several hours, primary resistance measurement was 1.2 Ohms.
Subtracting 0.4 from both measurements, the increase factor was
(0.8-0.6)/0.6 = 1/3. Calculated rise was 84.7? C, plus original ambient
24.4? C gives 109.1? C.

The IR thermometer measured the top of the transformer laminations.
Initial temperature was 24.4? C. Final temperature was 53.5? C. I
could hold my hand on the transformer without screaming, so I am fairly
sure the temperature was nowhere near 109.1? C!

I think a 4-terminal resistance measurement would provide more realistic
readings.

Fred

Yes, you got the 4-terminal one absolutely right. A meter that can use
kelvin clips and the resulting measurements would be much better. This is
normal for resistances under 10 ohms and mandatory under 1 ohm (for 3
reliable digits). For lower resistance or more resolution it gets
progressively worse.

My mind was blown when I just looked up the street prices of commercial
kelvin clip test leads with banana jacks.

Even the B&K stuff was like $160. Pomona? well over $200. Keithley kelvin
probes? $300+

I'm surprised the audiophile folks haven't come up with some sort of play
on this stuff yet.