# Electronics Few quick Electronics Q's?

Discussion in 'Modding' started by Tibby, 27 Nov 2006.

1. ### TibbyTechnologic

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Hey guys wondering if someone could quickly explain a few things to me.

First off,

How do you work out an LED power rating if it consumes 0.05A at 6v?

Would it just be 6v@0.05A?

And also how would you calculate the current from a 200W loudspeaker if the impedance is 8 Ohms?

Cheers for any help guys!

2. ### SteveyGElectromodder

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Power = V x I so, 0.05 x 6 = 0.3W

The loudspeaker one is complex because the impedance changes with frequency (it's an impedance not a resistance). You can guess an approximate current knowing that P = I{param}R so that I = 5A, but it'll vary from that figure a fair bit.

3. ### JaverhTopiary Golem

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You can also deduce quite a lot if you know the resonant frequency of the speaker.
For example if the res freq is 50Hz:
I assume the supplied impedance is the lowest impedance for that speaker. That would happen only at the res freq because as impedance is defined by
Z = ( R^2 + ( XL - XC )^2 )^½
At res freq XL = XC. This leads to Z = R = 8 ohm.
U=RI --> U=ZI and P=UI --> P=ZI^2 --> I = (P/Z)^½
Thus the current would be I = ( 200w/8ohm )^½ = 5 A

So the current is 5A as stated by SteveyG. But this is only so at resonant frequency. If the frequency changes the XL and XC kick in increasing the impedance. This in turn drops the current. So the 5A is your maximum current.

4. ### TibbyTechnologic

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Cheers guys,

Thanks for clearing things up!

Don't suppose someone could tell me what the amplitude of a Sine wave at 50Hz and 230V ac voltage taken from a household mains is?

Also what the period of the given waveform would be?

5. ### cpemmaEcky thump

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Tibby, isn't it about time you started doing your own homework? Or paying attention in class? 6. ### ch424Design Warrior

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So you're now at uni, and can't do GCSE physics?

7. ### SteveyGElectromodder

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That one gave it away that it was homework 8. ### TibbyTechnologic

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I canna do it captain!

I get soo confused, and this one kinda snuck up on me.

Normally I get one of my class mates to come over and explain it to me, but it was due at 4pm today so was desperate!

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