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Resistors

Please help me! I can't seem to understand the values of resistors and how they affect current and voltage. I have a 7.5 volt, 350 milliamp circuit that I would like to limit to 5 volts, and if possible raise the current to 500 milliamps. How could I do it?

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Brennn108 years ago
To reduce the voltage you can use a 5V Voltage Regulator. A common regulator that I use, and is common among dealers is the 7805CT 5V Regulator.
jasper28 (author)  Brennn108 years ago
is that the only way to reduce voltage? I'm pretty sure you can use a resistor instead (I have a bag of resistors)
westfw jasper288 years ago
To use a resistor to lower the voltage of a power supply, you have to know exactly how much current the device will use, and the internal resistance of the power supply. What a voltage regulator does is to keep the voltage constant regardless of significant variations of these (hard to find and variable) values.

For instance, I once built a "coin flip" circuit that used a 5V logic chip and a 9V battery. To reduce the voltage to 5V, it used a 200ohm pot, "adjusted" till the circuit worked. Mathematically, it wanted the resistor to drop 4V, and the circuit used maybe 30mA, so you use ohms law:
  • V = I*R
  • 4 = 0.03*R
  • 4/.03 = R
  • 133 = R
There's no way to increase the current available from a supply using resistive elements...
jasper28 (author)  westfw8 years ago
So if my power supply is 7.5V, 350mA and I want it to be 5V, 500mA it is not possible? I am trying to build a usb wall charger for electronic devices inside a power adapter from an old pair of speakers. I know there is no way to increase the current with resistors, but I was wondering if there was any way at all to increase it.
ehhh.... you can, but you need an IC and that just adds complexity
jasper28 (author)  guyfrom7up8 years ago
What if I got an adapter with a more powerful current? Is there a way to regulate the current down to 500mA, or does it not matter that much?
westfw jasper288 years ago
It doesn't matter if the current available is too much. Actually, if you have a 7.5V 350mA unregulated wall-wart like supply, a low-dropout 5V regulator (with associated components) is probably all you need to create a pretty decent "usb-like" supply. Most devices won't draw 500mA anyway, and you might have a bit of leeway... A 9V 500mA "regulated" supply with a cheap 7805 would be more certain, simpler, and more robust, but if you have to buy such a thing, you might as well just buy the USB supply. It would be worthwhile to buy one of those 3$ meters and see just what the voltage really is, and just how much current your target device actually uses.
jasper28 (author)  westfw8 years ago
Actually, I might have a regulator from an old project. How would I test it to see if it is 5V?
. Look for the number printed on the regulator. As westfw mentioned, one of the more common ones is the 7805. The number may be something like MC78M05CT - the extra letters indicate manufacturer, materials, &c and you should be able to ignore them for this project.. Search for "7805 +pinout" to see how to hook it up.
. BTW, a 7905 is a 5V regulator, but it's for negative voltage regulation.
jasper28 (author)  NachoMahma8 years ago
thanks, the one I have has 7805 in the code, so I think it is right the exact code is LM7805C
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