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voltage question for TX650W


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I have a voltage question in regrads to the PS_ON# (pin 16) before it is grounded.

I understand that according the whitepapers I find on the ATX 2.2 standard that this should be "PS_ON# is an active low signal that turns on all of the main power rails including 3.3VDC, 5VDC,

-5VDC, 12VDC, and -12VDC power rails. When this signal is held high by the PC board or left open

circuited, outputs of the power rails should not deliver current and should be held at a zero potential with

respect to ground. Power should be delivered to the rails only if the PS_ON# signal is held at ground

potential. This signal should be held at +5VDC by a pullup resistor internal to the power supply."


However on two units that I have recieved are showing pin 16 as 2.6 volts (where as on other power supplies I have tested it is showing as ~4.96 so this leads me to belive that the problem I am having putting this into a Dell 435 MT or genieric build case is not allowing it to power on via the power button


So am I reading the above spec correctly and that my low voltage is possible casue of my power on issue or have I completly messed up hooking it up somehow so I am seeing the wrong information... ?

the 1st I RMA back to the vendor I bought it from to avoid having to pay shipping for a powersupply that did not seem to be working

I have the 2nd unit but really hate to send it back again


Thanks for any light on this issue

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We both seem to have arrived at the same conclusion. The P_ON is only 2.75V on my TX650 also and won't start the Dell 435MT motherboard. All else on the PSU checks OK. From what I've read on this and other Forums this might be the reason that Corsair can't start the Dell Motherboard. I'm also waiting for a response.
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Well capcapt23, we get no response. Technically, 2.0 volts and above is a TTL high by the ATX standard so the TX650 is within that standard at 2.6 volts. However, 5 volts is also a TTL high so the 435MT motherboard could also be within ATX standard but the two won't work together. The P_ON on the original supply draws 811 ua. when held low and the Corsair only draws 171 ua. There should be a modification that would make this supply work. I have a lot invested in the Corsair to just give up on it and buy the competitors.
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  • Corsair Employees

The definition from the ATX standard is as follows,


Remote On/Off, PS_ON

Active Low for the +3.3V, +5V,+12V and -12V DC outputs.

+5Vsb is always enabled when AC line voltage is present.

Input Low Voltage <= 0.8Vdc.

Input Low Current -1.6mA maximum.

Input High Voltage >= 2.0Vdc.

Open Circuit Voltage 5.25Vdc maximum.


i.e. the power supply should remain off if the PS_ON pin is equal to or greater than 2V and the power supply will turn on if the PS_ON pin is equal to or less than 0.8V.



The power supply shall provide an internal pull-up to TTL high which one would expect to see around 5Vdc in an “open circuit state”.

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Thanks for the reply Ram Guy. I would expect a 5Vdc open circuit on the PS_ON line but the TX650W only has 2.7Vdc open circuit on my unit and 2.6 on capcapt23's two units. After trying a little external modification to my TX650w that made it "look like" the OEM PSU to the motherboard I'm pretty sure I'm barking up the wrong tree. There must be something else different between the TX650W and the OEM PSU but I haven't found the secret yet. Lots of people are trying the same upgrade for the 435MT motherboard so it would be great if we could find the answer.
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