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I have a small project that requires me to solder some wires to a PCB that converts 5 volts to 12 volts.

Here is a picture of the PCBs:

enter image description here

  1. On the PCB to the left, there is no label of "negative or positive". How can I tell where to solder the red (positive) and black (negative) wires. Furthermore, on the PCB to the right, it says the output is "5-16V", even though when I bought it the description said 5V to 12. I am a bit confused.

  2. What is the proper way to solder a red and black wire to the output "holes".

For instance: do I put the wire through the top of hole and solder from the bottom of PCB?

enter image description here

... or do I solder from the top?

enter image description here

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    \$\begingroup\$ Seriously? It doesn't matter how do you solder the wires. Just solder them. \$\endgroup\$ – Ale..chenski Aug 20 '17 at 18:48
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    \$\begingroup\$ @AliChen OP wants to learn, show some tolerance \$\endgroup\$ – Vladimir Cravero Aug 20 '17 at 20:17
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    \$\begingroup\$ As most people here would, and the answers describe, I would use a DVM to determine the polarity. However, based on some assumptions and your picture, I would make an educated guess that for the one on the left, the hole/pad on the left of J2 is GND and the hole/pad on the right is +Vout. However, to be sure, we'd need more information and a more detailed image, or to just test it. These appear to be USB connectors. The pin on the left appears to be directly connected to the left output pad. It's far more likely that they would do that for GND, than +Vout. \$\endgroup\$ – Makyen Aug 20 '17 at 22:06
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    \$\begingroup\$ Another time, please only put one question in your Question. Questions which contain multiple questions which are not very tightly related are considered too broad and tend to be closed. The Stack Exchange format is intended to provide a base of questions and answers which are useful to people in the future, not just the person currently asking. Questions with multiple issues tend to be confusing to people when we tell them "the answer to your question is over on this other question". Often, to solve a larger issue, it's necessary to combine the answers from multiple questions. \$\endgroup\$ – Makyen Aug 20 '17 at 22:23
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    \$\begingroup\$ Start with two of each device ... :-) \$\endgroup\$ – user134806 Aug 21 '17 at 1:15
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On the PCB to the left, there is no label of "negative or positive". How can I tell where to solder the red (positive) and black (negative) wires.

I would use a voltmeter. Or you can tell by traces on the PCB itself. The input and output sides have a common ground. So the trace that goes from the input (USB connector) straight to the output, that is ground (negative pin).

Furthermore, on the PCB to the right, it says the output is "5-16V", even though when I bought it the description said 5v to 12. I am a bit confused.

That means that the chip used is capable of supplying 5V to 16V. Usually it requires just a change of resistor value. But you are right, it is confusing.

What is the proper way to solder a red and black wire to the output "holes".

It doesn't matter as long it is soldered firmly. It just depends on how you need it, which side.

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I would use a DVM to determine the polarity on the board which does not have it marked. I would also use the DVM to measure the output voltage of the modules. Some (many?) switch-mode power supplies require some load in order to regulate correctly. With no load, the output voltage may be higher than normal.

I would probably strip 1/4 inch of insulation from the wire, poke it through the hole, and solder on the bottom of the board.

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    \$\begingroup\$ FYI, by DVM Peter means digital voltmeter. In other words, a multimeter. (In my experience most people don't have a standalone voltmeter and don't know what DVM means) \$\endgroup\$ – Clonkex Aug 20 '17 at 22:40
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Push the wire in and solder from the bottom. It'll stop you from damaging the insulation with your iron, and won't leave an exposed length of wire.

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