PSU voltage figures

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Recently I have been attaching several extra devices to my PC and I am
worrying that the PSU is beginning to get overloaded.

Is my PSU getting into trouble based on this data?

---------------- START DATA ------------
When the PC booted up and all the hardware devices were attached then a
screen in the BIOS showed that the nominal 12v voltage was 2.0 % down at
11.77 and the nominal 5v was 2.2% down at 4.89v.

When I booted into XP and using MBM I saw the figure for 12v went up to
4.93v (1.4% down) and 12v went down to 11.81 (1.6% down).

Then I saw som eother figures in MBM:

+3.3V was 3.31v

-12V was between -5.90V and -10.71V with an average of -7.74V. This seems
like a huge difference.

-5.0V was showing in MBM as 0.00V. Maybe is not used or does not exist.

---------------- END DATA ------------

So what do you reckon about my PSU?

 

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If MBM is the onboard voltage monitor, then you still do not
know what those voltages are. It is a monitor build with low
accuracy components. Once calibrated, then the monitor can
detect voltage changes. But you must still get the 3.5 digit
multimeter to take voltage measurements and to calibrate that
voltage monitor.

Typically the -12 volts has no real load. So what is
loading down the -12 volts. Or is that voltage monitor that
inaccurate? Insufficient information until reading are
obtained from a meter.

Aaron S wrote:
> Recently I have been attaching several extra devices to my PC and I am
> worrying that the PSU is beginning to get overloaded.
>
> Is my PSU getting into trouble based on this data?
>
> ---------------- START DATA ------------
> When the PC booted up and all the hardware devices were attached then a
> screen in the BIOS showed that the nominal 12v voltage was 2.0 % down at
> 11.77 and the nominal 5v was 2.2% down at 4.89v.
>
> When I booted into XP and using MBM I saw the figure for 12v went up to
> 4.93v (1.4% down) and 12v went down to 11.81 (1.6% down).
>
> Then I saw som eother figures in MBM:
>
> +3.3V was 3.31v
>
> -12V was between -5.90V and -10.71V with an average of -7.74V. This seems
> like a huge difference.
>
> -5.0V was showing in MBM as 0.00V. Maybe is not used or does not exist.
>
> ---------------- END DATA ------------
>
> So what do you reckon about my PSU?

 

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On Fri, 17 Sep 2004 17:44:55 +0100, Aaron S
<nomail@thankyou.com> wrote:

>Recently I have been attaching several extra devices to my PC and I am
>worrying that the PSU is beginning to get overloaded.
>
>Is my PSU getting into trouble based on this data?
>
>---------------- START DATA ------------
>When the PC booted up and all the hardware devices were attached then a
>screen in the BIOS showed that the nominal 12v voltage was 2.0 % down at
>11.77 and the nominal 5v was 2.2% down at 4.89v.
>
>When I booted into XP and using MBM I saw the figure for 12v went up to
>4.93v (1.4% down) and 12v went down to 11.81 (1.6% down).
>
>Then I saw som eother figures in MBM:
>
>+3.3V was 3.31v
>
>-12V was between -5.90V and -10.71V with an average of -7.74V. This seems
>like a huge difference.
>
>-5.0V was showing in MBM as 0.00V. Maybe is not used or does not exist.
>
>---------------- END DATA ------------
>
>So what do you reckon about my PSU?


That looks typical of a normally working PSU, there is
nothing there to be concerned about. -5V and -12V may vary
wildly since there is no load on them, it is not a problem.
That BOTH the 5V & 12V are similarly low, is actually
(usually) a good sign, opposed to one being too high and the
other too low.

However, voltage readings can't be taken this way and
considered accurate, because motherboard trace resistance
lowers the voltage by varying amounts on different boards.
In fact, extreme overclockers will often note (if they
bother to check) that their voltage levels are low on the
motherboard, but voltage at connection to motherboard is
fine... Power supply is only supposed to output at correct
voltage at it's connectors, not higher such that motherboard
reading is compensated for.

You need to use a multimeter to take voltage readings on the
backside of the ATX connector (where the wires enter it)
while system is running, to get valid output voltage
readings.

 

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kony <spam@spam.com> wrote:

> That looks typical of a normally working PSU, there is
> nothing there to be concerned about. -5V and -12V may vary
> wildly since there is no load on them, it is not a problem.
> That BOTH the 5V & 12V are similarly low, is actually
> (usually) a good sign, opposed to one being too high and the
> other too low.

Kony, can I ask you or your colleagues here a related question. I have got
*five* IDE hard drives attached to this PC (plus a CD burner). There is a
general shortage of mounting spaces for all the HDDs but I have found a
workable solution which seems ok.

My PSU has *three* peripheral power connectors (each with a set of wires
leading to a 4 pin Molex connector). If it is relevant, this is the actual
PSU though there doesn't seem a lot of info there. http://snipurl.com/954t

On account of the locations of the HDDs I have had to "daisy chain" three
(and maybe soon four) of the HDDs off *one* 4 pin Molex power connector. Is
this sort of arrangement ok? Maybe the wires in each 4 pin Molex connector
goes back to the same output terminals on the PSU? If you see what I mean.

Or alternatively, does one set of 4 pin Molex wires go back to a different
winding on the transformer than another set? If so, then I don't want to
overload one part of the transformer or PSU circuitry by attaching too many
devices to just one power connector.

 

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Often not discussed, but if we review the manufacturer's
specs for Molex connector pins, well those exapansion cords
are assembled by violating the manufacturer specs. For
example, if the manufacturer says wire must be 18-22 AWG, then
that means even 23 AWG wire if not acceptable for reliable
operation.

Two 22 AWG wires may be pushed into a crimp connector
designed only for one wire. Normally not a problem even
thought it outrightly violates the manufacturer spec.
However, on at least one occassion, I found this resulted is
peripheral failure. The wire power wire was intermittent.

Best to verify that voltage at each peripheral maintains
same voltage on all power supply connectors. Another reason
why multimeters are an important tool. A low voltage would
imply a defective crimp inside the Molex connector. I solved
that connector problem by soldering those crimped wires into
that crimp connection.

Aaron S wrote:
> Kony, can I ask you or your colleagues here a related question. I
> have got *five* IDE hard drives attached to this PC (plus a CD
> burner). There is a general shortage of mounting spaces for all
> the HDDs but I have found a workable solution which seems ok.
>
> My PSU has *three* peripheral power connectors (each with a set of
> wires leading to a 4 pin Molex connector). If it is relevant,
> this is the actual PSU though there doesn't seem a lot of info
> there. http://snipurl.com/954t
>
> On account of the locations of the HDDs I have had to "daisy
> chain" three (and maybe soon four) of the HDDs off *one* 4 pin
> Molex power connector. Is this sort of arrangement ok? Maybe
> the wires in each 4 pin Molex connector goes back to the same
> output terminals on the PSU? If you see what I mean.
>
> Or alternatively, does one set of 4 pin Molex wires go back to
> a different winding on the transformer than another set? If so,
> then I don't want to overload one part of the transformer or PSU
> circuitry by attaching too many devices to just one power
> connector.

 

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On Fri, 17 Sep 2004 20:02:19 +0100, Aaron S
<nomail@thankyou.com> wrote:


>Kony, can I ask you or your colleagues here a related question. I have got
>*five* IDE hard drives attached to this PC (plus a CD burner). There is a
>general shortage of mounting spaces for all the HDDs but I have found a
>workable solution which seems ok.
>
>My PSU has *three* peripheral power connectors (each with a set of wires
>leading to a 4 pin Molex connector). If it is relevant, this is the actual
>PSU though there doesn't seem a lot of info there. http://snipurl.com/954t
>
>On account of the locations of the HDDs I have had to "daisy chain" three
>(and maybe soon four) of the HDDs off *one* 4 pin Molex power connector. Is
>this sort of arrangement ok? Maybe the wires in each 4 pin Molex connector
>goes back to the same output terminals on the PSU? If you see what I mean.
>
>Or alternatively, does one set of 4 pin Molex wires go back to a different
>winding on the transformer than another set? If so, then I don't want to
>overload one part of the transformer or PSU circuitry by attaching too many
>devices to just one power connector.

All wires of same color join at same place on the circuit
board, not a different transformer winding or anything like
that. The issue is generally how much current the wire
itself can sustain, and 4 HDDs is better avoided but
possible. In any event, measurements can be taken with a
multimeter at the end, to see if voltage has dropped
significantly.

Otherwise, this is not any more of a load or overload than
if you had the drives all connected differently, the total
load remains (almost, very close to) the same.

Ideally with so many drives, and presumably a lot of data on
them, you might consider a higher wattage power supply.
Even though voltages look ok, data is often worth more than
the cost of PSU replacement. You didn't mention the rest of
the system, how old or power hungry it is, but generally a
box with that many drives (and considering you may've
upgraded other parts as well) is better paired with a 350W
PSU if not higher... particularly with an Athlon or P4 box,
instead of something less demanding like a P3.

 

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In article <9567B48C69E1617E5@130.133.1.4>, Aaron S says...
> Recently I have been attaching several extra devices to my PC and I am
> worrying that the PSU is beginning to get overloaded.
>
> Is my PSU getting into trouble based on this data?
>
> ---------------- START DATA ------------
> When the PC booted up and all the hardware devices were attached then a
> screen in the BIOS showed that the nominal 12v voltage was 2.0 % down at
> 11.77 and the nominal 5v was 2.2% down at 4.89v.
>
> When I booted into XP and using MBM I saw the figure for 12v went up to
> 4.93v (1.4% down) and 12v went down to 11.81 (1.6% down).
>
> Then I saw som eother figures in MBM:
>
> +3.3V was 3.31v
>
> -12V was between -5.90V and -10.71V with an average of -7.74V. This seems
> like a huge difference.
>
> -5.0V was showing in MBM as 0.00V. Maybe is not used or does not exist.
>
> ---------------- END DATA ------------
>
> So what do you reckon about my PSU?
>
Unless you've verified those details with a multimeter then they're not
reliable. As a system gets warmer, internal resistance increases thus
lowering internal voltages.

--
Conor

Opinions personal, facts suspect.

 

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> On account of the locations of the HDDs I have had to "daisy chain" three
> (and maybe soon four) of the HDDs off *one* 4 pin Molex power connector.
Is
> this sort of arrangement ok? Maybe the wires in each 4 pin Molex
connector
> goes back to the same output terminals on the PSU? If you see what I
mean.

That's just fine. There is a limit as to how much current you can push
down those wires (they're not very large), but you're not going to come
anywhere near that limit with just three or four hard drives. I wouldn't
worry until I had at least 8 drives daisy-chained.

> Or alternatively, does one set of 4 pin Molex wires go back to a different
> winding on the transformer than another set? If so, then I don't want to
> overload one part of the transformer or PSU circuitry by attaching too
many
> devices to just one power connector.

They all go back to the same spot. The only consideration (and this is
why the GF6800U cards need connectors on two different sets of wires) is
that because PSU wires are typically pretty thin wire, you're not going to
be able to get all of the 12V amperage that the PSU can supply down one set
of wires without a good bit of voltage drop.

steve

 

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On Fri, 17 Sep 2004 17:44:55 +0100, Aaron S <nomail@thankyou.com> put
finger to keyboard and composed:

>Recently I have been attaching several extra devices to my PC and I am
>worrying that the PSU is beginning to get overloaded.
>
>Is my PSU getting into trouble based on this data?
>
>---------------- START DATA ------------
>When the PC booted up and all the hardware devices were attached then a
>screen in the BIOS showed that the nominal 12v voltage was 2.0 % down at
>11.77 and the nominal 5v was 2.2% down at 4.89v.
>
>When I booted into XP and using MBM I saw the figure for 12v went up to
>4.93v (1.4% down) and 12v went down to 11.81 (1.6% down).
>
>Then I saw som eother figures in MBM:
>
>+3.3V was 3.31v
>
>-12V was between -5.90V and -10.71V with an average of -7.74V. This seems
>like a huge difference.
>
>-5.0V was showing in MBM as 0.00V. Maybe is not used or does not exist.
>
>---------------- END DATA ------------
>
>So what do you reckon about my PSU?

Your data are meaningless unless you can be certain that MBM is
telling you the truth. As others have said, buy yourself a DMM and
measure the actual voltages. Failing that, believe what the BIOS tells
you, rather than MBM, because the BIOS is written to match the
motherboard hardware. Incidentally, you didn't mention what the BIOS
had to say about the -5V and -12V rails.

The -5V rail is not bussed to PCI slots, so if the motherboard doesn't
use it for anything, then it may as well not exist. OTOH, the -12V
rail *is* specified for PCI use, so something is wrong there. Contrary
to what others have said, the -12V rail should not fluctuate anywhere
near as much as you have observed, not even under no-load conditions.
I doubt that your PSU is faulty, however. Rather, I suspect that MBM
does not understand how your motherboard is laid out.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

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On Mon, 20 Sep 2004 23:38:49 -0400, "LostSoul"
<snowball12@adelphia.com> wrote:

>Try this power calcultor an add in what u got
>

Even though your intentions may have been good, this is not
a binary newsgroup, posting such attachments is not
appropriate. Instead, please host the file on any server
available to you, and link to it, thanks.

 

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w_tom <w_tom1@hotmail.com> wrote:
>
> Often not discussed, but if we review the manufacturer's
> specs for Molex connector pins, well those exapansion cords
> are assembled by violating the manufacturer specs. For
> example, if the manufacturer says wire must be 18-22 AWG, then
> that means even 23 AWG wire if not acceptable for reliable
> operation.
>
> Two 22 AWG wires may be pushed into a crimp connector
> designed only for one wire. Normally not a problem even
> thought it outrightly violates the manufacturer spec.
> However, on at least one occassion, I found this resulted is
> peripheral failure. The wire power wire was intermittent.

What you write is interesting. Reading about specifications for the Molex
connector makes me ask two questions

(1) Do you know what current the Molex 4-pin connector is rated for?

(2) Approx what current capacity does 18 to 22 AWG wire have? We don't use
AWG to refer to our wires here in the UK.

 

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On Tue, 21 Sep 2004 20:31:32 +0100, Aaron S <nomail@thankyou.com> put
finger to keyboard and composed:

>w_tom <w_tom1@hotmail.com> wrote:
>>
>> Often not discussed, but if we review the manufacturer's
>> specs for Molex connector pins, well those exapansion cords
>> are assembled by violating the manufacturer specs. For
>> example, if the manufacturer says wire must be 18-22 AWG, then
>> that means even 23 AWG wire if not acceptable for reliable
>> operation.
>>
>> Two 22 AWG wires may be pushed into a crimp connector
>> designed only for one wire. Normally not a problem even
>> thought it outrightly violates the manufacturer spec.
>> However, on at least one occassion, I found this resulted is
>> peripheral failure. The wire power wire was intermittent.
>
>What you write is interesting. Reading about specifications for the Molex
>connector makes me ask two questions
>
>(1) Do you know what current the Molex 4-pin connector is rated for?

About 9A or 12A per pin.

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=121585
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=128349

>(2) Approx what current capacity does 18 to 22 AWG wire have? We don't use
>AWG to refer to our wires here in the UK.

http://www.dse.com.au/cgi-bin/dse.storefront/415095320579414a273fc0a87f9c06bb/Export/catalogs/DTS0000048


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

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I am not going to dig through the Molex data book. However
I recall those Molex pins were rated only for something like 5
amps per pin.

Franc is a well respected source of information. However I
recall having to use two pins in a Molex connector because my
current draw was something less than 10 amps - too much for a
single connector pin. 10 amps would also be too much for the
22 AWG wire connected to that pin. 22 AWG would be about .65
mm. If I remember, 10 amps on a 22 AWG wire would leave the
wire at something approaching 100 degree C.

These numbers should also be useful to put the original post
question into perspective.

Franc Zabkar wrote:
> On Tue, 21 Sep 2004 20:31:32 +0100, Aaron S <nomail@thankyou.com> put
> finger to keyboard and composed:
>> What you write is interesting. Reading about specifications
>> for the Molex connector makes me ask two questions
>>
>> 1) Do you know what current the Molex 4-pin connector is rated
>> for?
>
> About 9A or 12A per pin.
>
> http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=121585
> http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=128349
>
>> (2) Approx what current capacity does 18 to 22 AWG wire have?
>> We don't use AWG to refer to our wires here in the UK.
>
> http://www.dse.com.au/cgi-bin/dse.storefront/415095320579414a273fc0a87f9c06bb/Export/catalogs/DTS0000048
>
> - Franc Zabkar

 

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On Tue, 21 Sep 2004 19:23:19 -0400, w_tom <w_tom1@hotmail.com> put
finger to keyboard and composed:

>However I
>recall having to use two pins in a Molex connector because my
>current draw was something less than 10 amps - too much for a
>single connector pin.

My old Molex catalogue states that the 0.062" (1.58mm) connector
series had a max current rating of 5A per pin. The larger 0.093"
(2.36mm) series are rated from 7.5 to 12A per pin depending on the
number of circuits.

The 8981 series 4-pin disc drive headers were rated at 10A max per
pin.

See http://www.molex.com/pdm_docs/ps/b9clpatf_ps.pdf

> 10 amps would also be too much for the
>22 AWG wire connected to that pin. 22 AWG would be about .65
>mm. If I remember, 10 amps on a 22 AWG wire would leave the
>wire at something approaching 100 degree C.

The URL below specifies a nominal current rating of ~3A for 22 AWG
wire. The above Molex connector spec allows 6A for this gauge. <shrug>

> These numbers should also be useful to put the original post
>question into perspective.
>
>Franc Zabkar wrote:
>> On Tue, 21 Sep 2004 20:31:32 +0100, Aaron S <nomail@thankyou.com> put
>> finger to keyboard and composed:
>>> What you write is interesting. Reading about specifications
>>> for the Molex connector makes me ask two questions
>>>
>>> 1) Do you know what current the Molex 4-pin connector is rated
>>> for?
>>
>> About 9A or 12A per pin.
>>
>> http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=121585
>> http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=128349
>>
>>> (2) Approx what current capacity does 18 to 22 AWG wire have?
>>> We don't use AWG to refer to our wires here in the UK.
>>
>> http://www.dse.com.au/cgi-bin/dse.storefront/415095320579414a273fc0a87f9c06bb/Export/catalogs/DTS0000048
>>
>> - Franc Zabkar


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

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Those numbers are more in line with what I remember.
However I never encountered 'larger' pins. Is this something
new?

Power supplies (not PC type) use standard configurations and
standard connectors. Even location of connector on power
supply is standardized. Problem is that power supplies now
output many times more power for the same configuration. But
that configuration must use same connector pins. IOW that
would explain why Molex now has a larger pin for the same
connector.

Franc Zabkar wrote:
> My old Molex catalogue states that the 0.062" (1.58mm) connector
> series had a max current rating of 5A per pin. The larger 0.093"
> (2.36mm) series are rated from 7.5 to 12A per pin depending on the
> number of circuits.
>
> The 8981 series 4-pin disc drive headers were rated at 10A max per
> pin.
>
> See http://www.molex.com/pdm_docs/ps/b9clpatf_ps.pdf
>
> > 10 amps would also be too much for the
> >22 AWG wire connected to that pin. 22 AWG would be about .65
> >mm. If I remember, 10 amps on a 22 AWG wire would leave the
> >wire at something approaching 100 degree C.
>
> The URL below specifies a nominal current rating of ~3A for 22 AWG
> wire. The above Molex connector spec allows 6A for this gauge. <shrug>

 

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w_tom wrote:
> Those numbers are more in line with what I remember.
> However I never encountered 'larger' pins. Is this something
> new?

All of the 4-pin connectors on ATX power supplies use larger pins.
This may be what Molex calls: 2.13mm or 0.084" The female sockets will
accept a 0.093 solid pin. The male Molex pins are compressible and
fit the 0.093 sockets snugly.
>
> Power supplies (not PC type) use standard configurations and
> standard connectors. Even location of connector on power
> supply is standardized. Problem is that power supplies now
> output many times more power for the same configuration. But
> that configuration must use same connector pins. IOW that
> would explain why Molex now has a larger pin for the same
> connector.
>
> Franc Zabkar wrote:
>
>>My old Molex catalogue states that the 0.062" (1.58mm) connector
>>series had a max current rating of 5A per pin. The larger 0.093"
>>(2.36mm) series are rated from 7.5 to 12A per pin depending on the
>>number of circuits.
>>
>>The 8981 series 4-pin disc drive headers were rated at 10A max per
>>pin.
>>
>>See http://www.molex.com/pdm_docs/ps/b9clpatf_ps.pdf
>>
>>
>>> 10 amps would also be too much for the
>>>22 AWG wire connected to that pin. 22 AWG would be about .65
>>>mm. If I remember, 10 amps on a 22 AWG wire would leave the
>>>wire at something approaching 100 degree C.

10 amps on a 22 AWG wire 18" long produces a watts loss of ~0.25 W. over
the entire length of the wire. It's hard to see how that would produce
100 degrees C. Try it yourself. 6 A is a more reasonable number.
>>
>>The URL below specifies a nominal current rating of ~3A for 22 AWG
>>wire. The above Molex connector spec allows 6A for this gauge. <shrug>

Virg Wall
--
A foolish consistency is the
hobgoblin of little minds,........
Ralph Waldo Emerson
(Microsoft programmer's manual.)

 

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On Wed, 22 Sep 2004 11:40:23 -0400, w_tom <w_tom1@hotmail.com> put
finger to keyboard and composed:

> Those numbers are more in line with what I remember.
>However I never encountered 'larger' pins. Is this something
>new?

I think this article answers most questions:
http://www.dansdata.com/spcables.htm


- Franc Zabkar
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On Wed, 22 Sep 2004 19:14:46 GMT, VWWall <vwall@DEADearthlink.net> put
finger to keyboard and composed:

>10 amps on a 22 AWG wire 18" long produces a watts loss of ~0.25 W. over
>the entire length of the wire. It's hard to see how that would produce
>100 degrees C.

According to ...

http://www.dse.com.au/cgi-bin/dse.filereader?4151fcd200eed316273fc0a87f9c070b+EN/catalogs/DTS0000048

.... 21 AWG copper wire has a max resistance of 0.05 ohms per metre. At
10A this equates to a dissipation of 5W/m. That's about 2.3W in total
for an 18" cable.


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Franc Zabkar wrote:

> On Wed, 22 Sep 2004 19:14:46 GMT, VWWall <vwall@DEADearthlink.net> put
> finger to keyboard and composed:
>
>
>>10 amps on a 22 AWG wire 18" long produces a watts loss of ~0.25 W. over
>>the entire length of the wire. It's hard to see how that would produce
>>100 degrees C.
>
>
> According to ...
>
> http://www.dse.com.au/cgi-bin/dse.filereader?4151fcd200eed316273fc0a87f9c070b+EN/catalogs/DTS0000048
>
> ... 21 AWG copper wire has a max resistance of 0.05 ohms per metre. At
> 10A this equates to a dissipation of 5W/m. That's about 2.3W in total
> for an 18" cable.

#22 wire is ~16 ohm/1000' or 0.024 for 10' At 10 A, this is 2.4 W. I
should have used my slide rule and checked for decimal position! It
still seems low for a 70 degree C temp rise. Thus 3 A limit would seem
prudent.

Virg Wall
--
A foolish consistency is the
hobgoblin of little minds,........
Ralph Waldo Emerson
(Microsoft programmer's manual.)

 

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> According to ...
>
>
http://www.dse.com.au/cgi-bin/dse.filereader?4151fcd200eed316273fc0a87f9c070b+EN/catalogs/DTS0000048
>
> ... 21 AWG copper wire has a max resistance of 0.05 ohms per metre. At
> 10A this equates to a dissipation of 5W/m. That's about 2.3W in total
> for an 18" cable.

That resistance is measured at negligible current draw. As current goes
up, so will the resistance. Pretty soon (especially on such small wire),
you hit a point where even a small increase in current starts to cause LARGE
resistances - which mean significant voltage drops across the cable.

With 21-gauge aluminum wire at 10 amps, you're going to have a voltage
drop of around 0.85 volts just over a 2-foot length of cable. That means
you're dissipating 8.5 watts of power over the length of the cable, and it
means that you're really only getting 11.15 volts at the far end.

steve

 

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Steve Wolfe wrote:
>>According to ...
>>
>>
>
> http://www.dse.com.au/cgi-bin/dse.filereader?4151fcd200eed316273fc0a87f9c070b+EN/catalogs/DTS0000048
>
>>... 21 AWG copper wire has a max resistance of 0.05 ohms per metre. At
>>10A this equates to a dissipation of 5W/m. That's about 2.3W in total
>>for an 18" cable.
>
>
> That resistance is measured at negligible current draw. As current goes
> up, so will the resistance. Pretty soon (especially on such small wire),
> you hit a point where even a small increase in current starts to cause LARGE
> resistances - which mean significant voltage drops across the cable.

Copper has a temperature coefficient of 0.00393. This means the
resistance will increase by about 4/10% per degree rise in temperature.
Even if the wire did reach 100 degrees C, this is only a 32% increase,
which will reduce the current for a constant voltage input.
>
> With 21-gauge aluminum wire at 10 amps, you're going to have a voltage
> drop of around 0.85 volts just over a 2-foot length of cable. That means
> you're dissipating 8.5 watts of power over the length of the cable, and it
> means that you're really only getting 11.15 volts at the far end.

Don't know why we're talking about 21 or 22 gauge wire. (Sorry for my
misplaced decimal point). :-( The ATX/ATX12V Design Guide shows AWG 18
wires for the 12V and 5V leads. Looking at several ATX PSUs, I find the
leads are made up of 49 strands whose combined circular mils equal
~1624, which corresponds to 18 gauge wire. This has a resistance of
6.385 ohms/1000 feet. At 10 A, this is a drop of 0.128 V over a 2 foot
length with a loss of 1.28 W, with ~119 W delivered at the cable end.

Most electrical codes allow 18 gauge wire in extension cords rated at 10
A, even with rubber insulation. I've drawn 10 A from the 12 V rail of
an ATX PSU during a test, and the wire didn't get near 100 degrees C.

Virg Wall
--
A foolish consistency is the
hobgoblin of little minds,........
Ralph Waldo Emerson
(Microsoft programmer's manual.)

 

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In order to get two wires into one Molex connector, those
power splitters use 22 AWG. Aaron S said "had to "daisy
chain" three
(and maybe soon four) of the HDDs off *one* 4 pin Molex power
connector."

VWWall wrote:
> Don't know why we're talking about 21 or 22 gauge wire. (Sorry for my
> misplaced decimal point). :-( The ATX/ATX12V Design Guide shows AWG 18
> wires for the 12V and 5V leads. Looking at several ATX PSUs, I find the
> leads are made up of 49 strands whose combined circular mils equal
> ~1624, which corresponds to 18 gauge wire. This has a resistance of
> 6.385 ohms/1000 feet. At 10 A, this is a drop of 0.128 V over a 2 foot
> length with a loss of 1.28 W, with ~119 W delivered at the cable end.
>
> Most electrical codes allow 18 gauge wire in extension cords rated at 10
> A, even with rubber insulation. I've drawn 10 A from the 12 V rail of
> an ATX PSU during a test, and the wire didn't get near 100 degrees C.
>
> Virg Wall

 

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On Thu, 23 Sep 2004 10:28:32 -0400, w_tom
<w_tom1@hotmail.com> wrote:

> In order to get two wires into one Molex connector, those
>power splitters use 22 AWG. Aaron S said "had to "daisy
>chain" three
>(and maybe soon four) of the HDDs off *one* 4 pin Molex power
>connector."

Some do use 22 AWG, but not necessarily, I have some with
18AWG, not to mention that the PSU leads themselves also are
(usually) 18AWG and have two wires per connector on all
connectors until the last.

 

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w_tom wrote:
> In order to get two wires into one Molex connector, those
> power splitters use 22 AWG. Aaron S said "had to "daisy
> chain" three
> (and maybe soon four) of the HDDs off *one* 4 pin Molex power
> connector."
I have several power splitters with 18 gauge wire. All of the "in line"
Molex connectors on my PSUs have two #18 gauge wires per pin with no
problem. I will admit that if one uses 22 guage wire it ought not be
used to supply high current devices. A standard hard drive requires ~3
A at start-up; this is what power splitters were designed to supply. I
suggest a little re-arrangement of the drives, or finding someone who
knows how to use a soldering iron, would avoid using any wires beyond
their capacity.

I agree with you that "crimped" connector pins can be a problem. Even
modern appliances seem to be using these. I had a toaster which had a
crimped mains line connection which got so hot it melted the plastic
part to which it was attached. Your famous 3 1/2 digit DVM will show up
this situation. ;-)

>
> VWWall wrote:
>
>>Don't know why we're talking about 21 or 22 gauge wire. (Sorry for my
>>misplaced decimal point). :-( The ATX/ATX12V Design Guide shows AWG 18
>>wires for the 12V and 5V leads. Looking at several ATX PSUs, I find the
>>leads are made up of 49 strands whose combined circular mils equal
>>~1624, which corresponds to 18 gauge wire. This has a resistance of
>>6.385 ohms/1000 feet. At 10 A, this is a drop of 0.128 V over a 2 foot
>>length with a loss of 1.28 W, with ~119 W delivered at the cable end.
>>
>>Most electrical codes allow 18 gauge wire in extension cords rated at 10
>>A, even with rubber insulation. I've drawn 10 A from the 12 V rail of
>>an ATX PSU during a test, and the wire didn't get near 100 degrees C.
>>
>>Virg Wall


--
A foolish consistency is the
hobgoblin of little minds,........
Ralph Waldo Emerson
(Microsoft programmer's manual.)

 

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Those Molex pins, if I remember, are not speced and
therefore will not properly crimp two 18 AWG wires. Largest
sockets were only for one 18 AWG. Previously problems were
encountered even with two 22 AWG wires.

Have extensive experience with products where the assembly
house took the "screw the specs - it works for me" attitude.
Violating the manufacturer specs for wire size and number of
wires does cause numerous failures. A previous intermittent
was traced to a Molex connector that used two 22 AWG wires in
a Molex socket rated for one. The solution was to convert the
whole defective crimp into a conductive solder lump.

Two 18 AWG wires can be pushed into that square hole? Maybe
if wire insulation is trimmed back to be outside the white
nylon Molex body. But that crimp is designed to crimp a bare
wire AND to mechanically clamp onto wire insulation (strain
relief). The later required so that movement does not cause
connector failure. But two 18 AWG wires cannot be held by
their insulation - meaning intermittent failure. Again, all
this from distant memory. What do I remember the most? All
those failures due to a 'screw the specs' attitude.

VWWall wrote:
> I have several power splitters with 18 gauge wire. All of the "in line"
> Molex connectors on my PSUs have two #18 gauge wires per pin with no
> problem. I will admit that if one uses 22 guage wire it ought not be
> used to supply high current devices. A standard hard drive requires ~3
> A at start-up; this is what power splitters were designed to supply. I
> suggest a little re-arrangement of the drives, or finding someone who
> knows how to use a soldering iron, would avoid using any wires beyond
> their capacity.
>
> I agree with you that "crimped" connector pins can be a problem. Even
> modern appliances seem to be using these. I had a toaster which had a
> crimped mains line connection which got so hot it melted the plastic
> part to which it was attached. Your famous 3 1/2 digit DVM will show up
> this situation. ;-)