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Old 17th January 2008, 09:30 PM   #1
Carlp is offline Carlp  United States
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Default Plate load resistor question

OK, maybe this isn't the right forum for this question, but here goes:

I need an odd resistance for a plate load at 10Watts and need to pair resistors to make the right resistance. Can I pair two sustantially different resistances at 5 W and survive, or does the smaller resistor need to be higher rating or something?

Thanks,
Carl
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Old 17th January 2008, 09:46 PM   #2
nhuwar is offline nhuwar  United States
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Do you mean in parallel? The resistor thats smaller will pass more current. You can figure this out with ohms law. You can find the paralleled value of them with this formula r1 x r2 / r1 + r2 =total resistance. If you know the wattage being dissipated you know the voltage across them so use ohms law to find the current through each.


Hope this helps.

Nick
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Old 17th January 2008, 09:46 PM   #3
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The heat dissipated by a resistor will be the voltage it drops multiplied by the current going through it (W = VI). You should double this for a margin of safety. So, if you have a pair of resistors in parallel, you need to figure out how much current each is passing and adjust its wattage rating accordingly.
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Old 18th January 2008, 02:22 AM   #4
Carlp is offline Carlp  United States
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Thanks for the quick answers.

Hmmm, I'd forgotten how Ohm's law handles parallel resistors and it's trickier to figure than I remembered. I'm familiar with the fact that if you double the necessary resistance, the total resistance paralleled is the necessary resistance (e.g. 6.25K ohm 10W resistor can be approximated using two 5W 12.5K resistors paralleled - 12.5 x 12.5/25 = 6.25, and half of current passes through each, so 5W should do the trick). I assume if they are 1% resistors, the current flowing through each won't be much different so dissipation is close enough if the 10W has a margin of safety (which it does).

Figuring current and power dissipation is where I get confused. If the above resistance drops 180V or so (I assume that's across both since they are paralleled), each passes 14.4mA (2.6W, correct?). But if i use 9k and 20k to approximate 6.25k, current across the 20k is 9mA (for 1.62W dissipation, right?), but for the 9k it's 20mA (3.6W?). Have I got this right?

OK, in the end I remembered that I could simply use a 3k and 3.25k in series to get the same 6.25 resistance. In this case, though, they'd still need to be the full 10W or so, correct?

Thanks again for helping someone who used to know this stuff but hasn't used math in a LONG time!

Carl
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Old 18th January 2008, 02:48 AM   #5
jnb is offline jnb  Australia
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For the series style connection, each resistor will pass the same current. This will be the total voltge divided by the total resistance.

The voltage found across each resistor will be proportionate to their resistance. The larger resistor will drop the larger porton of the total voltage.

Doing the math, the larger resistance will dissipate the most power.
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Old 19th January 2008, 05:14 PM   #6
Carlp is offline Carlp  United States
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Hmmm. Thanks, jnb. It does indeed make sense. I need to start thinking like Ohm did.
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Old 20th January 2008, 05:31 AM   #7
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Quote:
Originally posted by Carlp
Hmmm. Thanks, jnb. It does indeed make sense. I need to start thinking like Ohm did.
But what about mho?
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Old 20th January 2008, 06:06 AM   #8
jnb is offline jnb  Australia
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And Larry?... Curly??
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Old 20th January 2008, 07:18 AM   #9
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Ah, you're thinking of Stooge's Law.
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Old 20th January 2008, 09:18 PM   #10
Carlp is offline Carlp  United States
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Oh, I think like a stooge all the time
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