Center tap transformer pairing with ATX PSU

Question

I'd like to use both a beefy center tap transformer (220v to 24-0-24, 1000VA) and an ATX PSU togheter, to power some lab equipment. So I isolated from case the ATX GND on DC side as the case is earth ground referenced, to make the ATX GND floating; and I'd like to connect the ATX ground to the transformer's center tap in order to have +-24v rails from the transformer and being able to use that rail togheter with the ATX rails.

Pic. 1, Simplified circuit diagram

Pic. 2, Noise filter 1

Pic. 3, Rectifier and Noise filter 2

Things I'm investigating before powering the circuit:

1) Given I don't know the ATX circuit, how can I be sure that some AC (example: 77v, a few uArms) from the center tap won't burn the ATX? Are Filter2+Filter3 enough? Or I need some diodes (example: "Moral of this story: include diode D1 even if you think that it isn't necessary." )?

2) protections: I placed glass fuses both on the primary AC input and the DC output connectors in order to have a basic overcurrent/shortcircuit protection. How can I improve that? Example: a bulb could be both a limiting resistor and a visual signal proportional to voltage/current output but I don't know how to dimension it (ie: do I need a 12v, 24v, ..., 220v bulb? Can I place it on the center tap?). Are there 10-20A resettable fuses (or PTCs) similar to the ones used on smaller circuits?

After the first powering up I'd like to further refine the filtering. So I need a way to measure and do some math for proper capacitor placing and dimensioning. I've a cheap oscilloscope but I have no idea how to track ripple and choose the right caps. Any advice?

EDIT1: added some links and corrected some typos.

EDIT2: modified question to include EMI issues raised in the comments.

EDIT3: circuit diagram fixed, with filters and higher resolution.

Answer 1

If you really need a “floating ground” then use 1nF to 4.7nF plastic cap across earth disconnect to block DC and 1nF = 1M5 ohms @ 100 Hz.

• don’t modify ATX box unless you have to, for a good reason.

  • Certs. becomes void with mods.
  • It is safe to bypass open gnd with small plastic cap. Yet connect RF to Earth.

  • ground short fault into 4n7 cap= 220V/1M5=150 uA safe. Low V, low I so would not trip fuse but might possibly permit filter current to be sensed as small tingle thru body to earth or humid air. No worries just a natural consequence of SMPS CM filter leakage current < 0.5mA in America or maybe < 2.5mA in EU. Safety limit.

  • Yet at 100 MHz, 4n7 cap is 1.5 Ohms which shunts stray noise (high Z) very well.

  • But case is no longer earth bonded, so use ATX PSU supply as is with earth-bonded ground for best RF results.
• make custom cable harness using surplus harnesses from old PC’s to make pretty. But I just used wire nuts to connect wires.

• treat ATX supply the same as you would in a PC
  • add Pwr ON jumper to MoBo plug as required.
  • Then use it for any peripheral as you wish and enhance design with variable or fixed V,I limits
• not as good as a battery + charger for heavy motor loads , where start current=V/DCR = 800 to 1200% rated current, but ok for ramped acceleration servo motors or ECU’s. (> 250us)
• Beware of surge switch effects from your in or out connection or switch.

• consider enhanced protection to load or ATX PSU;

  • add MOV, PTC, ICL’s active load, Adj. LDO & sink, fan

  • These can be ganged in a range of sizes or applied in series or parallel with predictable, expected results and switched in or bypassed or out as you desire/design in your adapter or generally use interface.

  Certs - In our country one cannot modify inside a cUL product even with a switch or regulator and keep safety certs on product. But outside the box, you can do whatever.

      - Not even for 1st generation shipments to a corporation. Some have lost their P Eng status in Canada doing so.  So  our company paid $50/ power supply for a cUL waver to circumvent cUL requirement and add our mods to a Canadian Tire battery charger in early 80’s for an SLA backup. The waiver is obtained from local  authorities (province of Mb , in our case) so any potential incident would not create a legal liability.
  

  • Or you may want a fancy PFET with 50mV shunt amplifier as a fancy current limiter with low gain feedback or high gain , you may choose to use this before external (10uF) cap near or in load or after an added bulk cap to improve transient response or reduce unwanted transient effects depending on load and application and how it is switched ON.
• some ancient or poor PSU designs needed a <10% or 10W dummy load on 5V only to function well, that was a normal practise at one time.

Anecdotal

• web buys are pot luck
  • I recently got a nice variable 3~24V 3A supply incl LED DVM , cheap, to power external ~ 16V custom fence & garden LEDs
  • it worked really well until it didn’t....
  • 3 months operating ~75% current rating in dry cool environment, no lightning storms. It was on a digital timer possibly Triac controlled, which may have caused more stress if async abrupt daily cycle to input current surge. Not verified but fault on input AC side.