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I've been working in my thesis project which consist on a PID controler to mantain a certain temperature in a case. The heater module is made of two 40W-12V 3D printer resistors and one 15W-10 Ohm wirewound resistor. In addition to these, I have an Arduino Uno, a 20x4 LCD screen, two temperature sensors and two PC coolers (2W each) which all sum up in 5W (rounding up). I'm using a 12V-10A switching power supply.

When the PID is at 100% the tension in the resistor (Rparallel= 1.5 Ohm) is supposed to be 12V, but due to the cables and transistor (IRF540N) drop, the maximum tension is 10V. So, all in all, in maximum behaviour I would be drawing 66.6 W + 5W= 71.6W.

The PID circuit is as follows: D3 is controlled by Arduino's PWM. Cable 2 conector is the conection for the resistors

The female jack that is soldered to the PCB is: 2.1 mm DC power jack

which is located near decoupling capacitors in the PCB: White * shows jack's position

The problem:

I've been doing some test to set up the PID's parameters for over a month (more than 40 hours of tests) without any problems. Suddenly last week while doing a test I smelled like something was melting and found this:

Melted input jack

Melted male conector

The power supply still works well so I wouldn't say there was an overdraw of current. My best guess would be that the PCB jack couldn't handle that much power but then, why did it resist more than 40 hours of testing? Something had to go wrong this time?

I need to know the cause of the problem so I can continue with the tests. Obviously I will replace both conectors and try to find a more robust one for the PCB but if the problem comes from the power supply or the PCB circuit itself i will face the same issue again. What would you say the problem was? Could the power supply suffered an overdraw and still be working? Is there a way to determine which component overheated first (the PCB jack or the power supply conector)? Would you recommend to solder the power supply directly to the PCB?

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  • \$\begingroup\$ That female power jack is for 12V 10A power supply? \$\endgroup\$
    – Leoman12
    Apr 24 '20 at 18:49
  • \$\begingroup\$ @Leoman12 Yes it is, I don't know the top power though \$\endgroup\$
    – Shupper
    Apr 24 '20 at 18:51
  • \$\begingroup\$ If it’s not possible to recreate your pcb and use the power connectors I mentioned then I suppose you can desolder that power connector and solder 16 or 18 gauge wire from power supply to pcb as a remedy. \$\endgroup\$
    – Leoman12
    Apr 24 '20 at 19:02
  • \$\begingroup\$ Also you can use a meanwell 12V 10A powersupply as an alternative power supply. \$\endgroup\$
    – Leoman12
    Apr 24 '20 at 19:09
  • \$\begingroup\$ Thanks for the advice! What would the meanwell power supply differ from the switching one? \$\endgroup\$
    – Shupper
    Apr 24 '20 at 19:18
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My guess is that when you used it over the past month you were using current that was under the maximum specified for that female power jack. The reason why your power supply is fine is because it can handle the power. Unfortunately it seems that the female power jack is probably not rated for 10A or anything slightly below it. If you truly plan to use high current like in 5A or up to 10A you should use some power connectors. For example MOLEX power connectors like shown below. You’d split up your positive pin into 3 wires and negative pin into 3 wires so they share the current. enter image description here

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  • \$\begingroup\$ Thanks! I'm going to try to change the conector. The problem is that my power supply conector is the 2.1 mm one, and I think i'm going to make a mess trying to change it. I was thinking of soldering the power supply directly to the PCB, do you think is a good idea? \$\endgroup\$
    – Shupper
    Apr 24 '20 at 19:01
  • \$\begingroup\$ If you can’t use a power connector or a barrel jack with enough current rating then I as a quick fix you could connect power supply to pcb. Just be sure that there is no possible way the power pins will short each other. \$\endgroup\$
    – Leoman12
    Apr 24 '20 at 19:06
  • \$\begingroup\$ I'm keen on the Molex connector but I don't understand how to use it with my actual switching power supply with barrel connector \$\endgroup\$
    – Shupper
    Apr 24 '20 at 19:25
  • \$\begingroup\$ You wouldn’t be able to use a Molex power connector with barrel connector. That’s why I recommended the meanwell power supply. Also if you want to use Molex connector you’d have to redesign your pcb. If this isn’t an option you want to use, then you can strip the wires of your power supply and solder them to your board. However the latter method is going to be more of a messy setup. \$\endgroup\$
    – Leoman12
    Apr 24 '20 at 20:32
  • \$\begingroup\$ I think I'm going to cut off the barrel connector and try to assemble the Molex one. I think i will discard the meanwell power suply, I've already bought an expensive switching one \$\endgroup\$
    – Shupper
    Apr 24 '20 at 20:34
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You converted the power jack into a melting pot and just had your 1st failure to read max current specs and heat rise calculations.

1st rule is to make a power loss budget.

  • lookup max current rating on power connectors (3A?) Use HDD MOLEX 4 pin ganged ( actual scpecs range from 2.5A min to 3.5 to 4A for best quality)
  • compute temp rise from RthJA = 62 °C/W (say Pd=0.5W with no sink)
  • determine max RdsOn . R=V²/P= 12²/100W= 1.44Ω and Pd/P * R = 0.5/100W 1.44Ω = 72 mΩ so the IRF540 is OK with Vgs> 10V

But your choice of BARREL POWER CONN is a HUGE NO-NO for 8 Amps. Try 3A max.

Use this instead. enter image description here

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  • \$\begingroup\$ There is software to compute trace size for 10 A as well called Saturn PCB among other things. Put in plated-Steel posts from a 2W power resistor wires and solder wire pairs to that. Then reinforce with sub-floor adhesive. Normally you need 18 guage traces \$\endgroup\$ Apr 24 '20 at 19:16
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Barrel power connectors like you show cannot handle as much current as you are trying to pass through it. If the center pin of the jack is the 2.1mm style they are typically rated at just 2A current. Barrel jacks with the 2.5mm style center pin are rated a bit more, sometimes as much as 3A. (This is one reason you see 2.5mm jacks used on powered USB hubs).

The voltage of the supply has little to do with the connector capabilities that we are discussing here. The connector contacts have a certain resistance and it is the I2R power level that causes the connector temperature to rise. In your case till it rose enough to melt the plastic of the connector.

When designing things you should check the data sheets of all parts that you use to assure that your use of all components is within specifications.

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  • \$\begingroup\$ Thanks for the answer! I'm going to try to find another barrel power connector. Can we then agree that the problem doesn't come from the power supply? what do you think about connecting the power supply directly to the PCB? \$\endgroup\$
    – Shupper
    Apr 24 '20 at 19:03
  • \$\begingroup\$ Salvage a Molex pair from a PC dude. @Joaquin \$\endgroup\$ Apr 24 '20 at 19:05
  • \$\begingroup\$ Direct soldered wires will probably work as long as the wires, solder joints and circuit board traces can handle the up to 10A current. Realize that even a soldered connection will have some resistance and if the wire, via and copper are too small that joint could also heat to the point of even melting the solder! \$\endgroup\$ Apr 24 '20 at 19:06
  • \$\begingroup\$ You could salvage a PC harness as @TonyStewartSunnyskyguyEE75 suggests and pigtail one off your PC board and attach the other off the power supply. This way you still have disconnectability but can avoid having to make another PCB for your project. \$\endgroup\$ Apr 24 '20 at 19:08
  • \$\begingroup\$ Hi! thanks for the advice, i've been reading a bit on Molex connector but honestly, i don't understand how to use it with my power supply connector \$\endgroup\$
    – Shupper
    Apr 24 '20 at 19:20

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