When putting resistors in parallel, the values are reduced R=r1*r2/(r1+r2) BUT what happens to the current rating, does it go up or down.

John, assuming two equal value resistors the voltage across them remains the same but the current is halved.

http://physics.bu.edu/py106/notes/Circuits.html

Resistors are usually rated in watts, not amps. But they are related, Power = I^2 R

When two resisitors are put in either parallel (or series), each resistor still has the same wattage rating that it has by itself. To find the maximum wattage of the parallel combination, you need to look at each resistor individually. One resistor will max out before the other. If the wattage ratings are the same, then then smaller value resistor will control the max voltage across the two (and therefore the maximum wattage).

If they are in series, then the larger value resistor will control the maximum current through the two (and therefore the maximum wattage).

There must be a better way to explain this. Sorry.

:When putting resistors in parallel, the values are reduced R=r1*r2/(r1+r2) BUT what happens to the current rating, does it go up or down.

:When putting resistors in parallel, the values are reduced R=r1*r2/(r1+r2) BUT what happens to the current rating, does it go up or down.

One way to look at it is with a make believe circuit: Say you have a circuit with 10 volts applied. In circuit one you have just one 10 ohm resistor. That circuit would draw one amp of current and the resistor would have to dissapate 10 watts. In circuit two you also have 10 volts applied. In circuit two you have two 10 ohm resistors in parallel. Total circuit current would be 2 amps. Both resistors would draw one amp each. Each resistor would have to dissapate 10 watts each. Total circuit wattage would be 20 watts. In circuit three you have 10 volts applied. In circuit three you have two 10 ohm resistors in series. Total circuit current would be 1/2 amp. The voltage drop across each resistor would be 5 volts. The wattage each resistor would have to dissapate would be 2.5 watts. Total circuit wattage would be 5 watts. In a parallel circuit the current divides through the branches and the voltage remains the same. If those branches are equal the current though them will be equal. In a series circuit the voltage divides and the current remains the same. If the resistors are equal then the voltage drop across them will be equal.

MRO

Hi John

As others said resistors are rated in watts. Maybe this is easier.

If 2 - 1 watt equal value resistors are in parallel they can handle 2 watts.

Norm

:When putting resistors in parallel, the values are reduced R=r1*r2/(r1+r2) BUT what happens to the current rating, does it go up or down.

John, I have to apologize for that statement, the current doubles not halves! I equals E over R. If total R of circuit halves (as with adding same value resistor in parallel with another) current has to double. Even if you added a resistor in parallel that was not the same value as existing resistor the current has to increase because you have lowered total resistance and your back to I equals E over R.
Any time R goes down I has to go up. PL

John:
After you memorize ohms law for resistors in series and parallel...

Another covenient thing to remember when placing resistors in parallel is:

The total resistance of all the combined resistors will always be smaller (less) than the smallest value resistor in the group.
ie.. if you have several resistors in parallel like 10 ohms, 100 ohms and 1000 ohms... the total will be less than 10 ohms.

In all of the the responses it is mention that the two resistors are equal but what if I need a total of 82ohms 3 watts and I use (1)100ohm and (1)470ohm resistiors in parallel. How would I determine the current ratings of each.

Hi John

To find actual current you need to know voltage dropped across the resistors. Voltage divided by resistance gives current. The lowest value resistor wall carry the most current. Your 100 ohm resistor will carry most of the current, more than 2 watts of the 3 required.

If 15 volts is across the resistors there will be 2.74 watts dissipated. 2.25 watts of this will be through the 100 ohm resistor. Only .48 through the 470 ohm resistor.

Norm

:In all of the the responses it is mention that the two resistors are equal but what if I need a total of 82ohms 3 watts and I use (1)100ohm and (1)470ohm resistiors in parallel. How would I determine the current ratings of each.

Why does the smaller resistor require more wattage. All through life we think of smaller being less, now its reverse. There must be an easy way to think of this stuff.

John: With two resistors in parallel, the voltage across each is the same. Power = V^2/R. The smaller resistor's value, in the denominator, makes its power consumption higher.

If you put 1 megohm across 120V, how much power would it draw? Zilch.

Now, try putting a 1-ohm resistor across 120V. Before you reset your circuit breaker, feel how warm the resistor is. (Just kidding, don't do this!)

:Why does the smaller resistor require more wattage. All through life we think of smaller being less, now its reverse. There must be an easy way to think of this stuff.

:Why does the smaller resistor require more wattage. All through life we think of smaller being less, now its reverse. There must be an easy way to think of this stuff.

John,
Here's an oldie but goodie. Why does a large pipe pass more fluid than a smaller one? Because, it has less resistance to the flow.

will

Yes John:
Always try to think of resistors passing current similar to a hose passing water.

The SKINNY diameter hose restricts (resists) the water flow and a WIDER diameter hose has very little restriction (resistance) to the flow so more water flows through the hose with least (smaller) restriction.

If you have a single 1" diameter hose that passes 5 gallons/minute... but you need more water, you can put second 1" hose in parallel with the first ...and now the two hoses together are offerring a wider total path for the water to get through... and you will get 10 gal/min.

So... Think of resistors the same way.. the more you parallel them together the more possible combined paths allowing less overall restriction to the total current flow.

:Why does the smaller resistor require more wattage. All through life we think of smaller being less, now its reverse. There must be an easy way to think of this stuff.
::Hi John. Don't feel bad if you have a little difficulty keeping ohms law in mind. I don't usually think in formulae either. I have worked in electronic repair over fifty years. It's time for me to forget a little of that stuff don't you think. I recall first seeing the underside of a radio back in the thirties. It was some years later when I grasped the notion that there wasn't any music in all that colorful collection of things. Only voltage, current and restances, ect. I was able to repair a lot of radios by swapping tubes, wiggleing wires and loking wise. Then I got in Radar school in the military. I learned a little of what a beautiful thing ohms law is. And to think, God worked it all out before the worlds were made.
Hang in there. Look at the response your post elicited. Everyone likes to think about electronics differently. All the formulas are written down for us somwhere. I gotta go look for my slide rule.
Ivan

:When putting resistors in parallel, the values are reduced R=r1*r2/(r1+r2) BUT what happens to the current rating, does it go up or down.

John,

Resistance (R) is the opposition to current flow. Right? The more resistance in the circuit less current (Amps) can flow. The less resistance in the circuit more current (Amps)can flow. Current flowing thru a resistance produces power in the form of heat. Just get against a wirewound power resistor running at about 1/2 to 2/3 of its rating. Using a transposition(?) of Ohms law for power, P (power) = V (voltage) squared / R (resistance). Say that the voltage V is 10 volts and the resistance (R) is 10 Ohms then P = (V*V)/R = (10*10)/10 = 10 Watts. Reducing R to say 5 Ohms would be P = (10*10)/5 = 20 Watts.

Hope this may help. And you guys looking over my shoulder, if I screwed up somewhere in this, come and jump in, because I haven't scratched the rust off this part of the gray matter in many years.

Radiodoc