# COMMUNITY : FORUMS : TECH

## 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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Brennn109 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)  Brennn109 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)
9 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)  westfw9 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.
9 years ago
ehhh.... you can, but you need an IC and that just adds complexity
jasper28 (author)  guyfrom7up9 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?
9 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)  westfw9 years ago
Actually, I might have a regulator from an old project. How would I test it to see if it is 5V?
9 years ago
. 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)  NachoMahma9 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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