# What happens when you put resistors in parallel?



## zmoz (Jun 22, 2003)

Lets just say I have two 50 ohm resistors. If I put them in parallel...what happens? Do I get 100 ohms? Or 25 ohms? 50 ohms? 323,923,281 ohms? /ubbthreads/images/graemlins/ooo.gif /ubbthreads/images/graemlins/wink.gif


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## Mednanu (Jun 22, 2003)

*Re: What happens when you put resistors in paralle*

You'll get 25 ohms at twice the wattage of the original resistors - ie - if you had two 1/4 watt 50 ohm resistors, after you put them in parallel, you'd have 'one' 25 ohm 1/2 watt resistor.


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## zmoz (Jun 22, 2003)

*Re: What happens when you put resistors in paralle*

That's what I was thinking...thanks. /ubbthreads/images/graemlins/grin.gif


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## UK Owl (Jun 22, 2003)

*Re: What happens when you put resistors in paralle*

putting resistors in parallel can also be a good way to get resistor values you do not have.

calculate the new resistance using :

1 / ( (1/r1)+(1/r2)+(1/r3)+(etc...) )

i.e.

1) take 1/r for each individual resistor
2) add the individual 1/r values
3) divide 1 by the total

note:
=====
the current through each resistor will be i = v / r
the power through each resistor will be i x v

where i is the current, r is the resistance and v the voltage across the resistor.

note:
=====
when you put resistors in series you add up the individual values to get the total resistance.

case study:
===========
suppose you had a bag of 100 ohm 0.5 watt resistors, but you wanted 100 ohms at 0.75 watts.

put two resistors in series and you have a 200 ohm resistor rated at 0.5 watts.
put another two in series to make a second 0.5 watt, 200 ohm resistor.

you now have 2 x 200 ohm 0.5 watt resistors.

put both of these 200 ohm networks in parallel and you will have a 100 ohm parallel~series resitor network rated at 1 watt. 
(although it may be a bit bulky for your mod ! )

hope this is of help to you


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## Ray_of_Light (Jun 22, 2003)

*Re: What happens when you put resistors in paralle*

There is a simplified formula if the resistors in parallel are only two.
Define the two resistors as R1 and R2. The resulting resistance R, obtained by connecting them in parallel, is:

R = (R1 x R2) / (R1 + R2)

the result is the same using the formula for n resistors indicated by UK Owl.

Regards

Anthony


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## CNC Dan (Jun 22, 2003)

*Re: What happens when you put resistors in paralle*

That math stuff is right, but for the math chalenged like me, you may want to take a diferent approach.

1. Figure out how much current each resistor will draw at some arbatrary voltage.

2. Add up the currents of all resistors that you want to put in paralell

3. Figure out what value of resistor would draw that current at that voltage.

Example: A 50 ohm resistor will draw .200 amps at 10 volts.
(10 volts/50 ohms = .200amps)

Two 50 ohm resistors in paralell will draw .400 amps.
(.200 amps X two resistors = .400 amps)

At .400 amps at 10 volts the two resistors act like a 25 ohm resistor.
(10 volts / .400 amps = 25 ohms)

It may have more steps, but for me it is easyer.


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