# Out of spec negative voltage: effect on computers?
**URL:** https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630
**Category:** Factual Questions
**Created:** [May 7, 2003, 10:53am UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630 "2003-05-07T10:53:06Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)
#### Post date: [May 7, 2003, 10:53am UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/1 "2003-05-07T10:53:06Z")
I have a relatively cheap (powmax) power supply, and the negative voltages set off alarms on monitoring software. My -12v value is often -13.4, and the -5 value is often -5.5.
The positive voltages are within acceptable range. For what it’s worth, it’s not that unstable, necesarily, as it is off. It generally stays in a relatively small range.
What does the negative voltage do? What’re the practical effect of less than ideal negative voltage?
Is it time for a new PSU?
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [May 7, 2003, 12:05pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/2 "2003-05-07T12:05:24Z")
It powers serial ports, and some older ISA sound, video, and other cards. In other words, it’s rarely used anymore.
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### Author:  [@Maxxxie](https://boards.straightdope.com/u/Maxxxie)
#### Post date: [May 7, 2003, 12:06pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/3 "2003-05-07T12:06:47Z")
**SenorBeef** , not sure about over-voltage, but I have seen under-voltages on the -12V line cause problems with serial ports. I would hazard a guess that over-voltages are not that good a thing. 1.4V out of spec on a 12V line is a 10% variance. Pretty significant variation. Get yourself a new power supply.
Max.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [May 7, 2003, 12:31pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/4 "2003-05-07T12:31:17Z")
As mentioned by others, it is primarily used for one of the power supply rails of the UART. I have also had bad experiences with a serial port not working correctly because of an out-of-spec negative voltage. But it was in the other direction. Not sure if –13.4 V will cause a problem, but I would guess not…
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### Author:  [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)
#### Post date: [May 7, 2003, 12:38pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/5 "2003-05-07T12:38:46Z")
Note that it’s not necessarily the thing being measured that’s wrong.
In this case, it’s just as likely to be the measurement device.
If you aren’t experiencing problems with your hardware, I don’t think you should worry about it.
OTOH, if you have a DVM handy, and want to check the voltages that way, go right ahead.
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### Author:  [@Popup](https://boards.straightdope.com/u/Popup)
#### Post date: [May 7, 2003, 1:25pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/6 "2003-05-07T13:25:51Z")
It is also worth noting that an unloaded power supply is very poorly reglated.
What you could do, if you want to experiment, is to connect a 100 Ohm resistor between -12 and ground, and see if the voltage becomes more stable.
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### Author:  [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)
#### Post date: [May 7, 2003, 1:40pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/7 "2003-05-07T13:40:22Z")
> [@](#):
>
> \*Originally posted by Popup \*
> \*\*It is also worth noting that an unloaded power supply is very poorly reglated. \*\*
'Fraid not, **Popup**. It’s a feedback regulated supply that we’re talking about. Its no-load voltage shouldn’t be much different from its full-load voltage.
Of course, the design of the particular thing may be completely crap, so that it’s incorrectly deriving its feedback or whatever, but in general your statement is incorrect.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [May 7, 2003, 2:21pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/8 "2003-05-07T14:21:26Z")
> [@](#):
>
> \*Originally posted by Desmostylus \*
> \*\*'Fraid not, **Popup**. It’s a feedback regulated supply that we’re talking about. Its no-load voltage shouldn’t be much different from its full-load voltage.
>
> Of course, the design of the particular thing may be completely crap, so that it’s incorrectly deriving its feedback or whatever, but in general your statement is incorrect. \*\*
**Desmostylus** : As I’m sure you’re already aware, some switched-mode power supplies have regulation problems when little or no current is pulled from them. I’m not sure if this is the case with computer supplies. (And for all I know, the –12V is supplied by a linear regulator in the main power supply, which would make this a non-issue.) At any rate, I would agree that this issue probably has nothing to do with the problem; I would assume the system is designed to properly regulate the –12V rail with the UART as its only load.
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### Author:  [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)
#### Post date: [May 7, 2003, 2:54pm UTC](https://boards.straightdope.com/t/out-of-spec-negative-voltage-effect-on-computers/173630/9 "2003-05-07T14:54:25Z")
FWIW, here’s the ATX spec:
> **[Motherboard Form Factors](https://www.intel.com/content/www/us/en/design/technologies-and-topics/motherboard-form-factors/technical-library.html?grouping=rdc%20Content%20Types&sort=title:asc)**
>
> Information to help in the design and development of related motherboard form factor compliant computer systems.
> [@](#):
>
> **4.1.4 Voltage Tolerances**
>
> Tolerance for the motherboard power rails should comply with the values listed in Table 6.
>
> **Table 6. Voltage Tolerances**
>
> ```auto
>
> **
> Voltage Rail Tolerance**
> +5VDC ± 5 %
> -5VDC (if used) ± 10 %
> +12VDC ± 5 %
> -12VDC ± 10 %
> +3.3VDC ± 4 %
> +5VSB ± 5 %
>
> ```