# Why triple output in every 230v -> low voltage DC power supply? [closed]

Why do all 12v power supplies produce triple output? All power supplies that I see at Amazon www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=12v%20power%20supply%2030a or Aliexpress www.aliexpress.com/af/12v-30a-power-supply.html convert 220v AC into triple 12v

Does it mean that these are 3 isolated and independent 120w voltage sources? How do I get single 360w voltage source? The Commutation cell principle says that I cannot simply physically wire all 3 output contacts together, right?

• "Why do all 12v power supplies produce triple output?" Uhm, well, they don't? – PlasmaHH Feb 3 '17 at 10:15
• Go read the (word deleted) data sheet for the device. – Andy aka Feb 3 '17 at 10:18
• Think for a moment. You want 360 W at 12 V. How much current is that ? How much current do you think one such connector contact can handle ? I'll tell you, less than 30 A. The multiple contacts are there for high current handling. If they were separate supplies you cannot put them in parallel. But they're not, so the Commutation cell principle does not apply. – Bimpelrekkie Feb 3 '17 at 10:27
• I'm not sure what the commutation cell has to do with it. Open it up and look at the back of the PCB, you'll see they're all on the same track. – pjc50 Feb 3 '17 at 10:27
• You can connect multiple voltage sources in parallel iff they have the same voltage, but otherwise not. So the commutation cell principle is not an issue here. // Besides, as other people have mentioned, the outputs are connected = one power supply. 30 Amperes is a lot of current, too much for a single wire to conduct. – Oskar Skog Feb 3 '17 at 13:41

30 Amps is a lot of current I must tell you. It is better the manufacturer has provided 3 screw pins per polarity. It actually helps to improve the contact resistance between these screws and the connecting wires, thus reduce the heating and also more area for the heat to dissipate.

Further, think if you have 3 loads of 10 Amps each (say), you can easily connect each screw to its corresponding terminals without unnecessarily trying tie all the three terminations to one screw terminal each!!

They are not isolated by the way, they are all shorted inside within. If you still have doubts, I will suggest using a multi-meter and checking the resistance(Beep-Mode) between them.

• I'm not so sure if the issue is specifically contact resistance, but resistance as a whole. – Oskar Skog Feb 3 '17 at 15:52
• Contact resistance is vital since, it is the place of contact where the effective area of cross-section (with some areas in precise contact and some air gaps which do not help in the current flow) and plays a vital role. Often this contact area is exposed to environment and develops corrosion which further increases the contact resistance. Thus, people use Gold plated contact material often. Poor guys like me use Glue gun to cover up that spot !! as a stop gap solution... – Akash Neel Dey EEE Feb 5 '17 at 13:16

The image is self-explanatory:

• I don't think you need to circle that 30A so much, I didn't notice that you've circled the +V and -V groups. I was almost about to ask why you'd think this would help OP who already knows that 360W/12V=30A, but is asking if it is 3 separate power supplies. – Oskar Skog Feb 3 '17 at 13:48

unless otherwise specified, the three output are multi-tap connectors to single source to connect multiple devices, you can use any one of the +V/-V combination to use the full wattage

If the load need to be shared across three connectors a three core cable can be used

• I don't think those contacts are rated for 30A - they look more like 10A each. You'll want to distribute the load a bit to prevent them from overheating. – Turbo J Feb 3 '17 at 10:32
• How would that three core armoured cable be connected? – Chu Feb 3 '17 at 12:37
• @Chu hopefully it wouldn't be connected to 12vdc as that has a very mains look about it. Colour code is old 3-phase, you don't want some joker wiring 380v AC to the other end. – John U Feb 3 '17 at 15:09
• @John U, exactly, that's why I asked the question. This has been accepted by the OP as the best answer! – Chu Feb 3 '17 at 15:35
• The picture of a three-core cable is silly anyway. If you were really going to do such a thing, you'd use a six-core cable, three for each of +V and -V. – Dave Tweed Feb 3 '17 at 16:26