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Author Topic: Help with ohm's law  (Read 1451 times)
ELMER
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Posts: 18




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« on: July 05, 2012, 10:44:52 AM »

as a new member and reading my aarl book, I need some help.
I'm trying to get the math correct in how to determine the amount of non inductive resistors to build a paint can dummy load, I understand that I want to get to get 50 ohms, but after looking at the projects on this site and others I can't get the math right.
Is this correct?
30-1.5k-5watt metal oxide resistors will yield
1.5 ohms x 30 = 50 ohms
30 x5= 150 watts

shouldn't it be 33 resistors---1.5x33= 49.5 or am I missing something a factor in the equation.

I'm trying to build my first gallon can one that will handle 300, I know its' probably easier to buy one, but I enjoy making things that work, any help in helping me order the correct resistors would be appreciated.
Elmer (yes that's my name)
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WB6BYU
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Posts: 13173




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« Reply #1 on: July 05, 2012, 10:48:41 AM »

1.5k = 1500 ohms

For 30 resistors in parallel, 1500 / 30 = 50 ohms.
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KA4POL
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Posts: 1969




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« Reply #2 on: July 05, 2012, 11:08:23 AM »

The general formula for parallel circuits is:

1/Rtotal = 1/R1+1/R2+1/R3+....1/Rn (n being the last one, here 30) With all resistors being equal it is easy.

1/Rtotal = 1/1.5k+1/1.5k+...1/1.5k (all together 30 times)

1/Rtotal = 30/1.5k

Rtotal == 1.5k/30 = 50 Ohms

If you multiply it that means the same as adding the values, which means you got a series and not a parallel circuit.

As for the power you can roughly say each resistor takes 1/30 of the total current and subsequently power. So in turn the complete power to be taken is indeed 150 W.

It is good to see you homebrewing this project. It is a good starting point. Hope you'll end up with a transceiver some time. Good luck.
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N2EY
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Posts: 3877




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« Reply #3 on: July 05, 2012, 11:25:03 AM »

The resistance of identical resistors in parallel is obtained by dividing, not multiplying.

1500 ohms / 30 = 50 ohms.

However, take a look at this:

http://www.n0ss.net/dl_30w_hf-uhf.pdf

A 100 watt 50 ohm Caddock resistor is about $10.

73 de Jim, N2EY
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W0BTU
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Posts: 1651


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« Reply #4 on: July 05, 2012, 11:59:29 AM »

I know its' probably easier to buy one, but I enjoy making things that work

Yup! It's usually lots more fun using something we made ourselves, not to mention that we learn things in the process. :-)

I have a 75 ohm Heath Cantenna clone I made years ago from series/parallel carbon comp resistors, as well as a smaller UHF dummy load made from paralleled, nearly leadless resistors soldered between sheets of flashing copper. They work as well as anything you can buy, for our uses.
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ELMER
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Posts: 18




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« Reply #5 on: July 05, 2012, 04:36:41 PM »

Thank you all so much for your help. I have a 2950 and hope to have my ticket and on the air by the end of the year.
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KA4POL
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Posts: 1969




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« Reply #6 on: July 05, 2012, 10:21:10 PM »

I guess I can say we all keep our fingers crossed for your plans.

And whenever help is required, come back here.
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K8AXW
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Posts: 3777




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« Reply #7 on: July 05, 2012, 10:44:03 PM »

Quote
I guess I can say we all keep our fingers crossed for your plans.

And whenever help is required, come back here.

And I would like to add, he researched and then worked on the problem first!  Refreshing!!
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