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I was looking to attach a Molex connector to the end of a USB by cutting a male Molex connector with a cable, and soldering the black and red USB wires to the black and red of the Molex cable with the attached male connector.

I was wondering if this would reduce the power draw to my device, for instance: I wanted to attach a peltier device to a USB cable for power, and I know that the peltier device has a red and black wire as does the USB. However, I am trying to avoid soldering the wires of the Peltier and the USB together as I would like to be able to de-attach them. So I wanted to take a molex connector, something like this, cut it in half and solder the male side to peltier wire and the female side to the cut usb wires. But will I lose power?

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closed as unclear what you're asking by Eugene Sh., Transistor, Wesley Lee, Trevor_G, PeterJ Jul 26 '17 at 7:40

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ How for the Electron sake can a connector reduce power usage? \$\endgroup\$ – Eugene Sh. Jul 25 '17 at 16:45
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    \$\begingroup\$ Well, the connectors (and the way you solder them) have different resistance (and other electrical properties). But in most cases these are negligible. \$\endgroup\$ – Eugene Sh. Jul 25 '17 at 16:49
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    \$\begingroup\$ Wow... "For instance... blah blah.." do you really expect folks to be able to follow that sentence... The only thing that will change the power available at the other end of the cable is if all your connections and joints significantly change the resistance of the cable. More resistance, you will have more drop along the cable and less volts at the business end. \$\endgroup\$ – Trevor_G Jul 25 '17 at 16:50
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    \$\begingroup\$ @Trevor I fixed my initial post. Thanks for helping show me how stupid and immature my beginner question was. \$\endgroup\$ – Omar Sumadi Jul 25 '17 at 17:27
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    \$\begingroup\$ Omar your question was neither stupid nor immature, just written in a way that made it really confusing. For things like this a picture or sketch goes a long way. I hope the rest of my comment helped anyway. \$\endgroup\$ – Trevor_G Jul 25 '17 at 17:34
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It's not clear without a diagram what you are actually asking, but most likely, no, it doesn't work that way.

Think about it. If a connector in series would reduce power consumption, then why do appliances draw any power when plugged into the wall as apposed to wired directly? What about using extension cords? Shouldn't those allow for free power?

Regardless of any electronics knowledge, basic high school physics says you don't get free power. Running a fan while the power source isn't putting out power violates this very basic rule of physics.

Even worse, since no connector is perfect, there will be some power dissipation in the connector. That means the ultimate load gets less power, or the power source ends up putting out more to compensate. Again, there is no free power.

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    \$\begingroup\$ LOL surprised you answered that one Olin, you must be having a better day than me.. I gave up about the second line of the second paragraph of the question. \$\endgroup\$ – Trevor_G Jul 25 '17 at 16:52
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    \$\begingroup\$ @Trevor: I figured someone had to tell this guy to go back to high school physics, and actually pay attention this time. \$\endgroup\$ – Olin Lathrop Jul 25 '17 at 16:55
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    \$\begingroup\$ You guys make me not want to ask here again \$\endgroup\$ – Omar Sumadi Jul 25 '17 at 17:11
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    \$\begingroup\$ @OmarSumadi don't think of it that way. It happens a lot with new users that ask questions here. It's an issue of asking in a way that tells us folks that don't live in your head, and can't see what you see, exactly what you are thinking. That is a skill that must be learned. It's part of our job to help you do that well. So instead of just scratching our heads and ignoring questions we give "huh" type feedback in the hope that the originator can reword the question. \$\endgroup\$ – Trevor_G Jul 25 '17 at 17:41
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    \$\begingroup\$ @Trevor: We are volunteers here, so nothing is actually part of our job. Personally, I don't feel like spending my time trying to salvage bad questions and teaching how to write good ones. Experience here has shown that neither usually works anyway. If someone can't write good questions, I just want them gone. I'd rather close confusing questions, downvote poorly written ones, and spend my time answering questions I don't have to guess at or decipher the writing of. We get lots of questions, most of them good. Discarding the bad ones and their authors is no loss. \$\endgroup\$ – Olin Lathrop Jul 25 '17 at 21:08
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I have a bit of trouble understanding this. I think it's a vocabulary issue.

Let's talk about resistance.

As you know (ask that Ohm guy) U=R*I therefore if you got 5V on the power supply and a total resistance of 1 ohm in all your cables and connectors, and you draw I=2.4A then you'll get a voltage drop of RI=2.4V and you'll only have 2.6V out of your 5V at the other end. In this case you're most likely in trouble.

Now, Molex makes a few different connectors so I'm afraid you'll have to be more specific about which one you got. Try to look up a datasheet and find its contact resistance rating (all connectors datasheets mention this).

As for the USB connector, many connectors specify 10mohm when new, and 30mohm through their lifetime, although 50mohm is acceptable for micro connectors.

So if you want to know if your connector exchange will be useful, compare USB connector contact resistance to the Molex contact resistance.

Remember you have two contacts, one for power and one for ground...

Now, add the resistance of your wire. For 2.4 amps, which is like 5x higher than the USB spec of 500mA, you might want to use a USB cable with some copper in it, something not too flimsy. Cheap cables are notorious for having the thinnest, cheapest power wires available... Sometimes you get a nice thick cable, but it's all plastic, because plastic is cheaper than copper, and the wires inside are tiny.

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  • \$\begingroup\$ Thank you for helping me out here: let me try to explain it more clearly so we all get a better picture of the question. I wanted to attach a peltier device to a USB cable for power, and I know that the peltier device has a red and black wire as does the USB. However, I am trying to avoid soldering the wires of the Peltier and the USB together as I would like to be able to de-attach them. So I wanted to take a molex connector, something like this, cut it in half and solder the male side to peltier wire and the female side to the cut usb wires. But will I lose power? \$\endgroup\$ – Omar Sumadi Jul 25 '17 at 17:23
  • \$\begingroup\$ OK, yeah this connector is OK for high current, no problem. Except where you plug your male USB into, it's gotta be a powerful enough supply... \$\endgroup\$ – peufeu Jul 25 '17 at 17:29
  • \$\begingroup\$ The USB max output is 5V 2.4A, this is going to be a very small project trying to cool some water, that's it. Do you think it's okay? \$\endgroup\$ – Omar Sumadi Jul 25 '17 at 17:43
  • \$\begingroup\$ +1 for mentioning two contacts, one for power, and one for ground. \$\endgroup\$ – Ale..chenski Jul 25 '17 at 19:48
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Using a connector in a power-hungry circuit introduces extra resistor (contact resistance) into the wire, and therefore causes some power dissipation. With fixed voltage supply this will reduce the power delivered to your load, theoretically. The actual power loss depends on construction of connector, and typically the connectors are rated by their max current. The contact rating is a tricky part, and is defined as maximum current which causes connector to overheat by 10 deg.C. Which obviously depends on housing blocking heat transfer, and wire gauge. The standard 4-pin Molex 0.062" connector has the rating of 5 A, so it is pretty good for the purpose, and losses in a long USB cable could be higher.

In sum, yes, soldered joints are are always better than any electro-mechanical connector.

SIDE NOTE: the USB max output is not 2.4A. USB standard defines MINIMUM output as 500/900mA, and maximum is not limited. On a typical PC, the absolute maximum of total USB output is limited by the system PSU, by its +5VSB standby power rail, which can be anything between 1A (older PCs) and up to PSU manufacturer limit (2.5 A recommended, and up). And, of course, each individual USB port has it's own connector rating, which is usually 1.5 A only.

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