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Resistors Answered

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?


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.

is that the only way to reduce voltage? I'm pretty sure you can use a resistor instead (I have a bag of resistors)

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...

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

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?

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.

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.

thanks, the one I have has 7805 in the code, so I think it is right the exact code is LM7805C

I'd go with higher mA wallwart, for putting a bigger load on the transformer than it can handle is really bad for an extended period of time.

thats the best way if you are getting varying power supply's. if the input is fixed, a resistor works best.


10 years ago

The most basic thing to realise here is that power supplies do not put out a specific current- the amount of current flowing in a circuit is determined by the voltage and the resistance of the circuit. (The concept of "resistance" gets a bit funny for LEDs and more complicated devices but for simple electromechanical devices like lightbulbs and motors it's ok)
I nearly blew up a multimeter by trying to "measure the current of an AA battery". It doesn't work like that- voltage is a property of a power supply, current is a property of a circuit with a specific power supply.

The higher the voltage, the higher the current, but the higher the resistance, the lower the current (as in ohm's law that guyfrom7up posted). Your power supply is rated at 350ma, meaning if it tries to supply more than 350ma it is likely to overheat and possibly burn out, but it will supply the amount of current that is drawn by the circuit.

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.
Without knowing what is in your circuit I can't comment on this other than I'm not sure you can do that. If your circuit draws 350ma at 7.5 volts then it will probably draw roughly 230ma at 5 volts. Without reconfiguring the circuit you can't make the change you asked about. If you posted what your circuit actually is, then maybe I could suggest something to try.

If it involves LEDs, you have to work some things out "backwards" because LEDs don't have a simple resistance like lightbulbs do- above a certain voltage they will draw a lot of current and burn out unless a resistor is connected in series, below that voltage they don't really conduct at all (and don't light up).

I'm sorry, I wasn't very clear what I was trying to do in the first place. I am trying to make a usb power adapter inside an already existing power adapter. I have a female usb port and I have taken the adapter apart. I just wanted the power output to equal the output of a usb port so that I can plug portable devices into it. I can find an adapter that has a higher current output to use instead and I now understand more about current, but how much does voltage matter when powering a device? Would something around .2V more than 5V matter?

Ah- that makes perfect sense. USB power is 5V at up to 500mA, so the simplest way to provide that is a 5 voltage regulator on a higher-voltage power supply capable of supplying at least 500mA. It shouldn't matter if your supply can provide more than 500mA, as long as the device you are powering doesn't draw that much, which if it is designed for USB it probably won't. I say "probably" because I had some cheap USB lights that crashed my computer instantly when I plugged them in, presumably from drawing too much power from USB. The amount that voltage matters when powering a device depends on the device- you can overpower a large electric motor to double its rated voltage for a short period, but an integrated circuit would be very unhappy about this treatment.

. According to USB In A Nutshell: "Low power bus powered functions must also be designed to work down to a VBUS voltage of 4.40V and up to a maximum voltage of 5.25V measured at the upsteam plug of the device."

here's a brief intro

Volts = Resistance X Amps

You have a circuit that works at 3 volts and uses 100mA (same as 0.1 Amps). What's the resistance?

well, you set it up! 3 = Resistance x 0.1

Solving, 3/0.1= Resistance
30 = Resistance
the resistance of the circuit is 30 ohms.

You can use ohms law to calculate voltage, amps, or resistance if you know the other 2.

I really don't understand how to use the equation to limit volts or amps. What do you mean by the resistance of the circuit?

As in adding resistors, for basic electronics assuming the wire's have no resistance is ok.

now to add complexity: at high voltages, especially in higly inductive environments such as SSTCs and other resonant transformers, long wires can pick up stray inductance. adding voltage! fun fact.

Then there's the minute resistances of wires to take in to account when we're dealing with very sensitive electronics... Lets not forget jumping either... Damn switches with their tiny sparks...

that's why small caps should be in parallel with switches when necessary.

Indeed, you know someone reading these comments would just go, Yewhat to this basic stuff...

As they should! The voltages, inductances, rise times, and so on that might cause such details to be important are not likely to be relevant in most hobbyist projects.

the only project i know of that has both those problems is, ironically, my tesla coil. if the wires to the square wave generator are too long, the 555 gets confused. if the wires to the fets are too long, they pick up stray inductance and overload the fets.

Exactly... well with some of the chip based projects the jumping problem can occur... Other than that most of it can be ignored...