PSU to bench power conversion problem

So I'm trying to convert a PSU to a bench supply unit, I found a ton of tutorials online and have followed the general guidelines of connecting the 5vsb to ground through an LED, connecting the power OK(grey) to ground through an LED and putting a switch between the green (power on) and ground, and adding a dummy load between 5v and ground. When I turn the unit on my standby LED lights up, but then when I turn on my switch to connect green to ground some weird things happen...

The power OK LED does not light up
The fan turns on for the PSU
The LED for the standby starts oscillating between dim and bright
I measure about 10V from the 12V line
When I replaced my dummy load with a lightbulb it also oscillates between bright and dim.

Have tried adding more dummy loads on the other rails but no improvement....

Any insight would be super cool

Imagine that we have no idea what PSU you are using. Imagine we don't know what inputs and outputs it has. Imagine we are folk who understand circuit schematics, not lists of one wire connected to another.

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Thank you.

//o// that is a lot of information, sorry but it does not do much other then it is not connected correctly. First I expect you do not have any electrical background based on your question. I am assuming you have a big thing a ma jig power supply model 99. When you over load it it will shut down, then when the load goes away it comes back up again. 10V from a 12V line would indicate this. Is it plugged in and turned on when there is no power light? Dummy loads are an active load that has a control loop that can interact with other control loops in the circuit such as the power supply.

You really don't need a LED on the 5V Stand By or the PWR_OK lines. I would remove them until you have things working. Both of those voltages are 5.0 Volts and you make no mention of series resistors being used with the LEDs. Just eliminate them for now. Less stuff ro be wrong.

All you need is a switch between the infamous Green wire (pin #14 on the older 20 connector and pin #16 on the newer 24 pin connectors) and any common ground Black wire. Most ATX Specification PSUs today do not even require a load and originally the consensus was a minimum 5 Watt load on the 5 Volt rail. Before that it was the 3.3 Volt rail. Using a 10 Ohm 10 Watt Ceramic resistor was popular so you can place a 10 Ohm 10 Watt resistor across your 5 Volt rail (any red wire). Now less the LEDs and only switching the Green wire to ground what happens?

Ron

Thanks Ron.

I did have series resistors with the LED’s… I know they weren’t necessary, but yeah should get things working before being fancy.

I tried what you said and just jumped green to ground, same thing as before where the fan turns on but I get an oscillating 10 to 9.8 volts on the 12v rail.

Added the resistor back to the 5v rail as I had before and no changes there.

I do have 2 thin wires sharing slots on the 24pin connector that I assume are sense wires. The orange (3.3V) one seemed standard and I have it connected to the other orange wires, but there was also a thin black wire that was sharing a slot with another ground wire. There’s 10ohms resistance between the thin black wire and ground, but I haven’t seen a ground sense wire mentioned anywhere. Not sure if that could be causing me problems.

"Sense" wires always go in pairs - one for the supply rail and one for ground. There is simply no point in having them otherwise. :roll_eyes:

This is the voluntary PSU design guide. When used sensing is done on the 3.3 volt rail and on a 20 pin connector it's pin #11 on a 24 pin connector it's pin #13. When referencing pins please use correct pin numbers. Cheap power supplies simply tie this line to another 3.3 Volt rail line, any other orange wire. This is even often done internal to the PDU. Nothing to lose by trying that, tie it to any other orange line (3.3 Volts).

All of this also assumes you have a known good PSU. What you describe is a PSU trying to start and shutting down over and over again which it will do as long as PS_ON line is held low. The idea is the psu will send a PWR_OK logic high once all the voltages are present and in tolerance. Again, all of this assumes a known good PSU.

Ron

Hi,
Did you do as @Ron_Blain suggested?

Most ATX Specification PSUs today do not even require a load and originally the consensus was a minimum 5 Watt load on the 5 Volt rail. Before that it was the 3.3 Volt rail. Using a 10 Ohm 10 Watt Ceramic resistor was popular so you can place a 10 Ohm 10 Watt resistor across your 5 Volt rail (any red wire).

Until you try it, can I suggest you do try this.
The 5V rail to work properly needs a load, in a PC the 5V supply is NEVER unloaded.
This will then make the SMPS produce enough energy to give sufficient supply to the 12V outlet.

[soapbox]
In my opinion using PC supplies as bench supplies is looking for disaster.
They have no current limit, if they did it will be a the max specs on the power supply label.
So if you are using 5V on your latest Arduino project and you have a short in the project, that short will conduct the max current the supply can deliver, that is 10 or 20 or more amps depending on the supply. Instant vaporization.
[\soapbox]

Tom.... :slight_smile:
PS do you have fuses on you DC outlets.

TomGeorge:
Hi,
Did you do as @Ron_Blain suggested?
Until you try it, can I suggest you do try this.
The 5V rail to work properly needs a load, in a PC the 5V supply is NEVER unloaded.
This will then make the SMPS produce enough energy to give sufficient supply to the 12V outlet.

[soapbox]
In my opinion using PC supplies as bench supplies is looking for disaster.
They have no current limit, if they did it will be a the max specs on the power supply label.
So if you are using 5V on your latest Arduino project and you have a short in the project, that short will conduct the max current the supply can deliver, that is 10 or 20 or more amps depending on the supply. Instant vaporization.
[\soapbox]

Tom.... :slight_smile:
PS do you have fuses on you DC outlets.

I wish these forums had a like button. :slight_smile:

Ron

Ron_Blain:
I wish these forums had a like button. :slight_smile:

Ron

It looks like this: Karma: 1412 [add]
I gave the ol' boy one for bofus. :wink:

Thanks, got it, :)…

Ron

I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.

bost1i1c1:
I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.

You joined this forum just to make this dumbass statement?