PDA

View Full Version : What's the voltage tolerance of the -5v line on PSUs ? (or is it OK if it is -4.5v?)



keropi
April 7th, 2012, 01:16 AM
I have finally come around to replace my aging AT psu on my p1 machine with an ATX one (mobo has support for both).
I began testing some of the first era ATX psus that have the -5v line (since I have installed a LAPC-I that needs said voltage) and I see that 4 out of 5 that I own actually deliver -4.50v (measured on the pins of the ATX plug directly) , the one that delivers -5.02v has the "noisy transformer syndrome" (you can hear a faint bbbzzzzttt when the psu has power connected regardless if the machine is turned on or off).

Anyone knows if -4.5v is sufficient (0.5A) ? obviously I don't want to damage or reduce the life span of the LAPC-I....

Thanks in advance for any info! :cool:

modem7
April 7th, 2012, 02:49 AM
See POWER SUPPLY section of http://en.wikipedia.org/wiki/ATX

keropi
April 7th, 2012, 03:27 AM
I have re-phrased my question to be more specific, basically I want to know if -4.5v is enough for the LAPC-I.

Having looked at the card's manual I see it needs:
+5v / 550mA
-5v / 50mA

twolazy
April 7th, 2012, 04:26 AM
Yes, its within spec, but at the low end of the limit of 10%. If it was my equipment, I regulate -5v on a breakout board tapped off the 12v rail and ground with a fixed negative voltage regulator, limiting resistor and/or fuse, and if super picky a few decoupling caps. BTW, im pretty sure some floppy controllers use -5v as well, so you will have to figure out your power requirements (think only early floppy chipsets, like western digital)...



One option is, provided its the only card using negative voltage, with a fixed -5v 100ma 79L05 regulator patched in, like this ...
http://www.ebay.com/itm/Lot-10-Fixed-Negative-Voltage-Regulator-5V-100mA-79L05-/120302697317?pt=LH_DefaultDomain_2&hash=item1c02997b65

If you need more then 100ma

http://www.ebay.com/itm/LM320MP15-15-V-Fixed-Negative-Voltage-Regulator-Lot-5-LM320-/250962126339?pt=LH_DefaultDomain_0&hash=item3a6e822603
LM320 is 500ma

Or you can go a complete opposite way, tap of -12v, and use a 500ma -5v negative linear regulator such as a MC79M05.
http://www.ebay.com/itm/5V-500mA-Regulator-MC79M05CDTRK-MC79M05-DPAK-Qty-10-/400150359558?pt=LH_DefaultDomain_0&hash=item5d2ad1ee06


any more amperage your going to need something better. Should be small enough to fit on a piece of protoboard with a tiny heatsink inside your psu as well! =)

Experience from working with phantom power microphones

keropi
April 7th, 2012, 05:01 AM
@twolazy:

yes, re-creating the -5v signal is a possibility, thanks for the idea!
basically if I went that road I would use a modern ATX psu from a known manufacturer (an overkill for an old machine but more safe) and be done with it for another decade LOL

dracos
May 3rd, 2012, 01:19 PM
That's what I did with my 386, much quieter that way too.


@twolazy:

yes, re-creating the -5v signal is a possibility, thanks for the idea!
basically if I went that road I would use a modern ATX psu from a known manufacturer (an overkill for an old machine but more safe) and be done with it for another decade LOL

lucasdaytona
May 3rd, 2012, 01:24 PM
I have re-phrased my question to be more specific, basically I want to know if -4.5v is enough for the LAPC-I.

Having looked at the card's manual I see it needs:
+5v / 550mA
-5v / 50mA

It's totally ok and safe!
Now that you said LAPC-I I went into a moment of sadness, about 6 months without seeing one, I think I imported the only LAPC-I of Brazil to a friend of mine.

MikeS
May 3rd, 2012, 01:41 PM
What's the voltage tolerance of your meter?