Searching \ for '[EE] PC Power Supplies - stray voltage on +5V' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/power.htm?key=power
Search entire site for: 'PC Power Supplies - stray voltage on +5V'.

Exact match. Not showing close matches.
PICList Thread
'[EE] PC Power Supplies - stray voltage on +5V'
2006\11\13@181826 by Howard Winter

face
flavicon
picon face
I tried building a device into a PC which was powered from the +5V line on one of the disk drive power connectors, and found that it was very
unreliable in starting up - more often than not it didn't!

So I put a meter on the connector and found that when the PSU has mains connected, but is turned off, there was 1.8V on the 5V line, so powering
the PC down and up does not power the device I mentioned down properly.  From cold it seems to work if you connect the power lead and then press
the power-button within a couple of seconds - more than that and it locks up the device.

I tried another PSU and that was showing 0.8V and had the same problem.  I know there is a separate 5V Standby supply, but I would have thought
that the main 5V should drop to zero when power is off, shouldn't it?

Has anyone else encountered this?  Am I likely to find a PSU that does behave "properly"?

If not I'll have to use the +12V and put my own 5V regulator in-line, which will be annoying but will solve it (the +12V line does drop to zero).

Cheers,


Howard Winter
St.Albans, England


2006\11\13@202143 by Kevin Timmerman

flavicon
face
I just checked a system with an Antec power supply. It drops to 25 mV
within a few seconds and stays there.  The 12V drops right down to zero.

Have you tried a small resistor on the 5V line?  100 ohm 1/2 W would
be a good start.


At 11:11 AM 11/13/2006, you wrote:
{Quote hidden}

>

2006\11\13@215838 by Tony Smith

picon face
{Quote hidden}

You need to put a load on the 12V rail, it won't start up otherwise.  

A lot of people put a 10W resistor across it so it draws a half amp or so.
A light bulb works too.  Occasionally I'll just plug in a old hard drive.

Tony

2006\11\14@050838 by Howard Winter

face
flavicon
picon face
Tony,

Thanks for the reply...

On Tue, 14 Nov 2006 13:58:10 +1100, Tony Smith wrote:

> You need to put a load on the 12V rail, it won't start up
otherwise.  
>
> A lot of people put a 10W resistor across it so it draws a
half amp or so.
> A light bulb works too.  Occasionally I'll just plug in a old
hard drive.

Sorry, I didn't make it clear that the rest of the PC is still
there and connected to the PSU, so there is plenty of load on
all the power lines.

Cheers,


Howard Winter
St.Albans, England


2006\11\14@051736 by Howard Winter

face
flavicon
picon face
Kevin,

Thanks for the reply...

On Mon, 13 Nov 2006 20:23:33 -0500, Kevin Timmerman wrote:

> I just checked a system with an Antec power supply. It drops to 25 mV
> within a few seconds and stays there.  The 12V drops right down to zero.

Hmmm... maybe I should get one of theirs.  I seem to remember they do make decent-quality PSUs...

> Have you tried a small resistor on the 5V line?  100 ohm 1/2 W would
> be a good start.

No, I didn't think load was the problem.  I didn't make it clear in my original post, I'm installing this in a PC *with the rest of the PC still there*!  I'd
have thought that the motherboard, two hard drives, floppy drive, and three DVD writers would provide a load at least this big, wouldn't they?  Or
would the low voltage mean that they'd be effectively high resistance?  (I have no idea what ICs "do" when the voltage falls way below spec.)

I'll try your suggestion and see what happens.

Cheers,



Howard Winter
St.Albans, England


2006\11\14@060841 by Mohit M. (Lists)

picon face
> and three DVD writers
Just curious... what do you do with three of these?

:-)
Mohit.

----- Original Message -----
From: "Howard Winter" <spam_OUTHDRWTakeThisOuTspamH2Org.demon.co.uk>
To: "Microcontroller discussion list - Public." <.....piclistKILLspamspam@spam@mit.edu>
Sent: Tuesday, November 14, 2006 3:35 PM
Subject: Re: [EE] PC Power Supplies - stray voltage on +5V


{Quote hidden}

> --

2006\11\14@063301 by Alan B. Pearce

face picon face
>> and three DVD writers
>Just curious... what do you do with three of these?

he has to generate an income somehow ...

2006\11\14@064757 by Kevin Timmerman

flavicon
face
My test was on a complete system, not just the supply. I think that
the Vf of the various components in the PC may allow the caps to
retain a rather high residual charge. A resistor would certainly
fully discharge the caps. If the resistor doesn't work, there must be
some leakage somewhere (WOL?). Low cost PC power supplies may not
have drain resistors on the LV side, maybe not even the HV side.

{Quote hidden}

2006\11\14@071736 by Peter P.

picon face
IMNSHO use a proper reset chip and/or brownout detector. Adding resistive loads
to the psu outputs may cause it to fail to start under certain conditions.
Startup current is a critical parameter of PC type PSUs because of the very high
filter capacitances on the boards and in the PSU, and because of the various
SMPSUs on the main board, which are usually not current limited. Adding 0.5A
load on the 12V lead of a substandard PSU that already needs to start up 4
drives on the 12V line (but which are known to the PSU makers to start drawing
current a couple of seconds after the main voltage is up, when the BIOS turns
them on) may make it behave strangely.

If you need 5V to work then specify that voltage (e.g. 5V +/-10%) and implement
a voltage monitor or reset chip that detects this and releases reset only then.
The state of the circuit should be such that nothing unexpected happens when the
voltage is outside this range (either way and no matter in which direction it is
changing and how slowly). Imho this is failry basic common sense for any circuit
or system that must start and stop unsupervised.

Peter P.


2006\11\14@074554 by Howard Winter

face
flavicon
picon face
Mohit,

On Tue, 14 Nov 2006 16:36:12 +0530, Mohit M. (Lists) wrote:

> > and three DVD writers
> Just curious... what do you do with three of these?
>
> :-)
> Mohit.

The project is to create three identical DVDs (Data DVDs, not video) at the same time from files on a memory card.  

And it's all very legal indeed!  :-)

Cheers,


Howard Winter
St.Albans, England


2006\11\14@080407 by Tony Smith

picon face
> > You need to put a load on the 12V rail, it won't start up
> otherwise.  
> >
> > A lot of people put a 10W resistor across it so it draws a
> half amp or so.
> > A light bulb works too.  Occasionally I'll just plug in a old
> hard drive.
>
> Sorry, I didn't make it clear that the rest of the PC is still
> there and connected to the PSU, so there is plenty of load on
> all the power lines.


Ah, that's a bit different.  I though you just had a PSU lying about on the
bench.

This won't help you much, but I tried this with a PSU, and got zero volts on
both the 5 & 12v rails.  The 5v standby is only on the 20 pin plug, it's the
purple wire.

I tried just plugging it in, then powered it up, and back off without a
load.  I tried it again with a load (hard drive) and got the same results.

Maybe you have 2 duds?  Mine was just you usual no-name generic.

Tony

(sometimes they'll power up without a load, but don't regulate properly
without a load.)

2006\11\14@083726 by Howard Winter

face
flavicon
picon face
Alan,

On Tue, 14 Nov 2006 11:32:45 -0000, Alan B. Pearce wrote:

> >> and three DVD writers
> >Just curious... what do you do with three of these?
>
> he has to generate an income somehow ...

I wish!  This is the first paying project I've had for some time, and I'm so far spectacularly failing to deliver anything :-(

And Yes, I know what you're implying!  ;-)  In fact it's a project for the Police...

Cheers,


Howard Winter
St.Albans, England


2006\11\14@112109 by gacrowell

flavicon
face
Worked with an Intel server system that did the same thing but kept up
about 3V.  Became a problem when the keyboards would lock up.  Intel's
solution? Beat up the keyboard vendor to change their controller reset
threshold.  Never mind that some customer is later going to change
keyboards and have problems.

Gary Crowell

> {Original Message removed}

2006\11\14@140135 by Steve Baldwin

flavicon
face

> So I put a meter on the connector and found that when the PSU has
> mains connected, but is turned off, there was 1.8V on the 5V line, so
> powering the PC down and up does not power the device I mentioned down
> properly.  From cold it seems to work if you connect the power lead
> and then press the power-button within a couple of seconds - more than
> that and it locks up the device.

This is purely conjecture, but it would fit with your symptoms. If you have a
device that is connected to the standby power supply and doesn't behave
properly, it may be trying to power up the rest of the system through the
protection diodes of an IO pin.
It's a pretty common mistake in embedded systems, so it wouldn't be a
surprise to find it in a PC peripheral card.
I would try removing as much as possible from the motherboard and see if
the supply drops to 0V. If it does, add in cards until the problem returns.


Steve.



==========================================
Steve Baldwin                          Electronic Product Design
TLA Microsystems Ltd             Microcontroller Specialists
PO Box 15-680, New Lynn                http://www.tla.co.nz
Auckland, New Zealand                     ph  +64 9 820-2221
email: stevespamKILLspamtla.co.nz                      fax +64 9 820-1929
=========================================


More... (looser matching)
- Last day of these posts
- In 2006 , 2007 only
- Today
- New search...