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Old 13th July 2014, 10:08 AM   #1
s214519 is offline s214519  United States
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Default resistor?

I wasn't sure what category to put this under so I just decided to put it here. I'm a beginner so I really don't know much.

I was wondering if you change the wattage of a resistor will make a big difference or if it's just the peak. For example; if a resistor says 8 ohms 20 watts, could you use one rated at 30 watts and get the same result?

As I said before, I'm a beginner and am just trying to learn more. So please at least try not to call me dumb. :P

Thanks for any help.
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Old 13th July 2014, 10:12 AM   #2
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Yes you can always go higher in wattage, space permitting. If you go lower, there could be issues with overload and burn up of the resistor.
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Old 13th July 2014, 10:13 AM   #3
s214519 is offline s214519  United States
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That's what I thought. Thank you so much!
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Old 13th July 2014, 10:31 AM   #4
s214519 is offline s214519  United States
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I have another question.

In building a power supply, after my transformer I should have 12vac 3 amps. For my bridge rectifier, do I have to get rectifier diodes rated at 12v or can I go higher?
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Old 13th July 2014, 10:42 AM   #5
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Take a look at some datasheets for these components. Google it.

Ratings such as wattage, or voltage are MAXIMUMS, at which the component starts to suffer or even gets destroyed.

So using a 4 watt resistor where only a 100mW is ever being dissipated will do no harm, but is very wasteful of space and maybe money! But it will not get warm

For cheap-and-cheerful voltage rectification I use only 1N4007 diodes. These are 1amp max and 1,000V max. Some parts from the same family are rated at a lower voltage, but are only fractions of a penny cheaper, so it is not worth stocking more than one part.

Always search for and study datasheets of components. It will help you learn and should prompt lots of questions!

Cliff
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Old 13th July 2014, 10:52 AM   #6
SY is offline SY  United States
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Generally, diodes with higher voltage and current ratings than needed cost no more, so no need to try to see how close to the needed ratings you can go. The actual minimum will depend on the power supply topology you're using. But in this case, you can get a 100V 10A bridge (4 rectifiers in a single package) for about $1, so why not?
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Old 13th July 2014, 11:30 AM   #7
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Quote:
Originally Posted by s214519 View Post
I have another question.

In building a power supply, after my transformer I should have 12vac 3 amps. For my bridge rectifier, do I have to get rectifier diodes rated at 12v or can I go higher?
You MUST go higher. If you have a 12V AC, the rectified DC can go to 20V unloaded. Then when the AC reverses, the back-end of the diode (the anode) can go down as much as -20V. So your diode will see almost 40V across it when not conducting...

Jan
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Old 13th July 2014, 12:35 PM   #8
coresta is offline coresta  France
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Quote:
Originally Posted by s214519 View Post
I was wondering if you change the wattage of a resistor will make a big difference or if it's just the peak. For example; if a resistor says 8 ohms 20 watts, could you use one rated at 30 watts and get the same result?

Thanks for any help.
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