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Low Voltage


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6 replies to this topic

#1 swagger

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Posted 27 November 2007 - 05:12 PM

I have had this problem with my girlfriend's Compaq Presario and the Bestec 250W psu that came with it... It had 10.9 volts on the +12v rail.. I bought a new Thermaltake 450W psu and the +12v only went up to 11.20... I am now starting to wonder if it's the socket or power surge that's it's plugged into that may be the culprit of the low voltage since I am in an apartment complex and a socket or two has failed in our apartment since we have moved in only 8 months ago.

Mod Edit:Split from "Werid Noise?"~ TMacK

Edited by TMacK, 27 November 2007 - 05:34 PM.


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#2 garmanma

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Posted 27 November 2007 - 06:24 PM

10.9 is lousy, but acceptable on a cheaper power supply. 11.2 is not bad
good info on power supplies
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#3 DaChew

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Posted 27 November 2007 - 06:38 PM

I have always gone on the rule of 10% on the voltage rails

12 +/- 1.2 and I even consider that sloppy

you might have a bad software reading on the 12 volt rail

my thermaltake 450

+3.3 V 3.30 V
+5 V 5.11 V
+12 V 12.16 V


per everest

I have seen a bad or loose power cord wreck havoc(the connection coming into the power supply)

a good voltmeter would be the way to trace this problem
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#4 RLI

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Posted 02 December 2007 - 07:04 PM

I would check the outlet to verify ground and that the wiring has not been reversed. if you are using a surge protector, verify that it has not been tripped. I would then get a multimeter and manually check the voltages.

#5 dc3

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Posted 02 December 2007 - 10:52 PM

I would check the outlet to verify ground and that the wiring has not been reversed. if you are using a surge protector, verify that it has not been tripped. I would then get a multimeter and manually check the voltages.


This is AC wiring so a reversal of the line and neutral won't make any difference in what comes out of the secondary side of the transformer in the PSU, it will then be rectified to DC and broken down to the different rail voltages.

What are you using to make these voltage readings?

Have you used a voltage meter to see what your line voltage is in the circuit you are plugged into?

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#6 DaChew

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Posted 02 December 2007 - 11:00 PM

reversal of hot and common do lead to shocks when working on computers and some issues with power supplies themselves as I found out after several months when I tested one of my circuits
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#7 dc3

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Posted 02 December 2007 - 11:51 PM

reversal of hot and common do lead to shocks when working on computers and some issues with power supplies themselves as I found out after several months when I tested one of my circuits



The 110V line voltage and the neutral of a alternating current system can be attached to either of the primary taps of the transformer, and the the two secondary taps are equally reversible before the bridge rectifier. Once the voltage is converted to DC then you have a positive and negative, but until then if the only conductors you are working with are the line and neutral they can be exchanged in this application.

If you are working with anything connected to the output of the PSU then it is DC and is broken down to the different rail voltages.

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