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 Needing an unkown resistor calculation

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Cliff Jones
Site Administrator
Site Administrator

Join date : 2010-11-22

PostSubject: Needing an unkown resistor calculation   March 23rd 2018, 3:19 pm

I have a burned resistor but I don't have that value, so I have to improvise by making a parallel resistance circuit for replacement of the bad resistor.

I have had to scrounge my resistor stash to find a specific value, but ALAS no such luck!

So what do I do?

Get the next closest Higher Value and do this calculation.
If this doesn't work then I would go to the next higher value that I do have. This will be my starting point.

So I now designate a reference ID in brackets = () to numbers of each value.

Original Total Resistance for the circuit.

  = 150ohms (Rt)

The available closest higher resistor that I found in my parts bin.
= 200ohms (Ra)

Now I need to use that found value, to find what Unknown resistor value do I need to wire in parallel to come up with the original resistance value.
 =       ?   (Ru)

Calculate with this Transposed ALGEBRAIC formula: Note that the order in the original formula has changed by transposition.

From:    1     
         Rt +  Ra

 Plus to Minus

1/Rt -  1/Ra      = Ru? or 1/found or needed replacement.

 (1 / 150) - ( 1 /200 ) = Answer of 600ohms

Now let's prove my calculation formula by the reciprocal of the reciprocals for parallel resistor's method. study

1/Ra + 1/Ru? = 

200 = .0050   = Ra 

600 = 0.001667 = Ru

.0050 + 0.001667 = .006667
 .006667  =  150 total ohms for circuit  cheers

I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
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