Hello, I am trying to build a new PC. After connecting all the parts, I turned the switch on and... nothing. I suspect that the power supply is not working, or the circuit on the motherboard that switches the power supply on is not working. When I flip the switch on the back of the power supply on, the keyboard LED turns on for a second. However, when I press the major switch, nothing happens. No fans move, and there are no noises. I checked the major switch with an ohmmeter, and the switch seems like it works. I'm not sure if the power supply or the motherboard circuit that starts the power supply isn't working. So my question is, how can I check if the power supply is working? Thank you very much
There is a power supply spec here, so you can see the signal names.
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To start with, the power supply is split into two pieces. The +5VSB output, is available as soon as the rear switch is turned on. The +5VSB, in turn, flows down the main power cable and powers some motherboard logic.
The second half of the power supply, operates the 3.3V/5V/12V/-12V outputs. The power is "soft switched" by means of the logic level on the PS_ON# signal. The PS_ON# signal normally is pulled to +5V, by an internal resistor. To start the supply, PS_ON# needs to be grounded (zero volt level).
Inside the computer, the chain of command looks like this.
Front Motherboard Grounds Main To main ATX Power ------------ Logic runs ----- PS_ON# --------------- part of supply, Switch on +5VSB to operate Cable fans start to spin (momentary (latches when PS_ON# is contact) signal) grounded
An ATX supply can be operated on the test bench.
1) Connect a dummy load to the supply. You could use a couple old hard drives, and connect them to the Molex connectors. I have a home made load box that I use.
2) Switch on power supply at the back.
3) Connect Green to Black with a paper clip. The fans should spin.
Then, you can use a multimeter to measure the voltages if you want. I use a load box, drawing a light load (
Paul, Thanks very much for answering. I understood your explanation - now I know why the LED on the keyboard was turned on when I turned the back switch on.
I'm not sure what you meant by "Connect Green to Black". I looked at all the cables coming from the power supply, and I didn't see anything green. Only yellow, red, white, black, blue, and purple. Thanks again, I really appreciate the help.
If you check the formfactors.org PDF file, it has a pinout for the main connector, with names and wire colors.
Pin Signal Color Pin Signal Color
--- ------ ----- --- ------ -----
1 +3.3VDC Orange 13 +3.3VDC Orange [13] [+3.3 V default sense] [Brown]
2 +3.3VDC Orange 14 -12VDC Blue
3 COM Black 15 COM Black
4 +5VDC Red 16 PS_ON# Green
You already have received some good suggestions, but I shall make one more: I always connect the motherboard to the power supply before I mount it into the case. If you are able to see the POST screen on the monitor, you know you have the beginnings of a working system. If after mounting the MB into the case something fails, look for a short on the underside of the MB or a cable attached improperly. The problem you described could be a short causing an over current condition, an undersized power supply, a defective one or other possibilities. It is always best in my opinion to simplify your problem by establishing what works and adding components one by one.
Double check that the power swich is on the correct connector in the correct manner. Sometimes you can place them on the connector 90 degrees out of rotation i.e. Vertically vs horizonataly and it will not work.
That power supply tester (quick test tool) is almost useless and too expensive. But few reasons. A defective power supply can still boot a computer AND will be reported as good by a power supply tester. Second, a power supply tester does not report on other failed or good component of a power supply 'system'. Paul has discussed some parts of that 'system'
Anything a power supply tester might do is performed by the multimeter. A defective power supply may only be detected when supply is under full load from a computer. Meter will do that with a definitive answer. Power supply tester will not.
Paul has recommended jumpering the green wire to black. Another useful solution is to measure DC voltage of purple and green wires both before and when power switch is pressed. Everything in a power supply 'system' is known in less than 30 seconds by those measurements and without disconnecting things. Even the power switch is tested. No useful report is provided by that power supply tester. Disconnecting may create further failures.
Numbers also make possible useful replies from the better informed. A power supply tester means that additional assistance cannot be provided. And finally, a meter means learning how that power supply 'system' really works. Quick tester teaches nothing useful.
Try the power supply with a known working mainboard.
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You don't want to connect _all_ the parts and then test. You should first connect just enough parts for a barebones system -- mobo, one memory module, CPU, CPU cooler, graphics card (if the mobo doesn't have built-in graphics), monitor, keyboard, power switch, reset switch. Only after you prove that this will boot should you add more memory, cards, drives, etc.
A common problem that prevents turning on the computer is a short between the mobo and case, usually around one of the mounting holes or at a corner (mobos and cases can be warped enough to let them touch -- always support the mobo near the corners, even with stick-on plastic or rubber feet).
Make sure that the front panel power switch is connected to the correct pins on the mobo. Not all mobos are labelled clearly for this.
Unplug the PSU from all the computer stuff, and with it plugged into the AC power and its rear switch turned on, measure for voltage between a black wire and the purple wire. If you don't get something close to +5V, the PSU is bad. Otherwise short the green wire on the
20- or 24-pin connector to any black wire (whaddya mean you don't see a green wire??? It's between two black wires) to turn on the rest of the PSU. While the black meter lead is connected to a black wire, touch the red test lead to an orange (or brown) wire to check the
+3.3V DC, a red wire for the +5V, and a yellow wire for the +12V. Don't be surprised if each voltage is off by quite a bit because that's common when there's no load on the PSU. Also some PSUs won't even start without a load, which can be provided by a hard drive or a
10-20 watt, 5-10 ohm resistor connected beween any black and red wires (this resistor can get very hot).
PSU testers aren't very good and can indicate everything is OK even when a voltage is way off. That happened when I tried a CompUSA brand tester with a PSU whose +12V rail put out only about 10V and wouldn't even spin the hard drive.
Thanks to everyone a lot for the suggestions. In the meantime, I connected the motherboard with a known working power supply, and still nothing happened. I also checked the original power supply, as you guys suggested (connecting the green and black wires) and the fan did start spinning. I also checked the voltage. It showed 5.25 volts. I double checked the master switch, so it seems that the problem is not with the power supply, but with the motherboard. Can you guys suggest anything I can test the motherboard with before I return the motherboard? Thanks a lot
Double check the configuration of the motherboard - any jumpers and other connections and that the CPU is plugged in correctly (if this applies).
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I just followed some suggestions, by removing the mb from the case and connecting it to the powersupply and hard drive. again, no response. I also noticed that one of the pins on the motherboard said RSVD_DNU. My case comes without any wire named RSVD_DNU. I'm just curious, is that important? And what is RSVD_DNU? thanks again
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Problem solved!!!! Thanks to all!! The problem was that the "clear CMOS jumper" was in "Clear CMOS" position - it should be in "normal" position. The motherboard was A33G from PC Chips.Now my PC is working! Other parts I used:
CPU cooler - WATER COOLER XIGMATEK|AIO-S80DP
2GB RAM - MEM 2G|PQI MAD42GUOE R AMD CPU - CPU AMD|A64 X2 5000+ 2.6G AM2 2M R case - CASE ATHENATECH|A3602BB.400 BK RT DVD and HD I used from my old PC. HD had WinXP - I didn't have to reinstall any software.
One remark - water cooler is pretty big - and takes a lot of space but fits on the MB. Standard 4 pin ATX power connector must be connected to MB BEFORE installing the cooler - the cooler blocks the power connector.
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