small question about Ohm's law

[TronicBrain]

Hi
I have a small question about Ohm law

Suppose I have 4 identical resistors
50 Ohms 1 Watt
And I want to connect them in order to achieve 30 Ohme resistor with 4 Watts

Is this scheme correct?

 

[Nigel Goodwin]

I can't be bothered to work out the resistance, but the total wattage won't be 4W, it will be less than two watts.

 

[ecerfoglio]

Yes and no

The resulting resistance will be 30 hm: as you calculated:

R1 in paralel with R3 = 25 hm:,
25hm: in series with R2 = 75 hm:,
75 hm: in paralel with R4 = 30 hm:.​

BUT those resistors don't share the power evenly.

You must calculate the power dissipated by each one:

R4 will dissipate V^2/50hm: = 1 W,
so V should be less or equal than the square root of (1 W * 50hm = 7.07 V.

With a 7.07 V supply, the 75 hm: branch will have a current of 94 mA, which gives:
0.444 W in R2 ( = 94 mA^2 * 50hm: ), and
0.111 W each in R1 and R3 (with half the current, 94/2 mA^2 * 50hm: )

So, as you must limit the power dissipated in every resistor to 1W,
the "new" 30hm: resistor will be able to dissipate less than 2 W (in fact, just 1.666 W):

1 W + 0.444 W + 2 x 0.111 W = 1.666 W, or

7.07 V ^2 / 30 hm: = 50/30 W = 1.666 W

 

[Nigel Goodwin]

Thanks for the 1.66W calculation, like I said - less than 2W - but I couldn't be bothered to do the sums, other than the 'instant' approximation of under 2W

 

[audioguru]

R4 will dissipate V^2/50hm: = 1 W

No.
R4 will dissipate 2W and since it is rated for a max of only 1W then it will smoke and burn.

 

[rmn_tech]

I can't be bothered to work out the resistance, but the total wattage won't be 4W, it will be less than two watts.

I'm sorry But I thought the idea of this forum was to help people. But if you can't be bothered sorry to have disturbed you.

Why answer if you're not bothered ?

 

[audioguru]

It is not a technical question.
It is arithmatic for 10 year old kids.

Most of us can't be bothered to do simple arithmatic.

 

[Leftyretro]

It is not a technical question.
It is arithmatic for 10 year old kids.

Most of us can't be bothered to do simple arithmatic.

Well I think RMN TECH response was that it takes more effort to post about not being bothered to answer then to not respond with a post at all. One chooses to respond to a post or not.

Lets be honest, it was a grouchy response. We all have bad days at times but doesn't hurt to try and be civilized, especially to newer posters.

Lefty

 

[audioguru]

I think the little kids couldn't be bothered to ask their teacher about the simple arithmatic. So they asked us.

 

[Leftyretro]

I think the little kids couldn't be bothered to ask their teacher about the simple arithmatic. So they asked us.

Probably correct, but some here don't mind responding to their requests for help.

Lefty

 

[Tomble]

I'm sorry But I thought the idea of this forum was to help people. But if you can't be bothered sorry to have disturbed you.

Why answer if you're not bothered ?

If you look at his post again, he said that he couldn't be bothered to work out the resistance, but he answered very very clearly that it would not be able to take 4W. So he did in fact answer, he simply didn't answer ALL of it. And why should he work out half of the answer if he already knew and said that the other half was "it won't work"?

Now can we stop all the damn stupid poo-flinging, everybody?

 

[Nigel Goodwin]

I'm sorry But I thought the idea of this forum was to help people. But if you can't be bothered sorry to have disturbed you.

Why answer if you're not bothered ?

I answered one major part of the question, with didn't need any sums doing - as suggested we're not here to do primary school arithmetic for people.

 

[arunb]

'TronicBrain' you should study some basic electronics books, as the question is quite a simple one.

Internet is a great idea.. but I think a book will go a long way in solving some of the fundamentals of electronics.

 

[HiTech]

I can understand both Nigel's initial response as well as TronicBrain's question. To a knowledgable technician, it's quickly obvious that the wattage is under 2 watts. However, TronicBrain apparently is trying to understand Ohm's Law as it applies to his diagram despite the elementary math needed to solve it. I can certainly add 2 + 2 x 2, but if I don't know how to apply it to a specific situation, it's meaningless.

 

[audioguru]

it's quickly obvious that the wattage is under 2 watts.

No.
R4 is burning up and is trying to dissipate 2.0W. The remaining resistors dissipate 1.33W so the total dissipation is 3.33W, not less than 2W.

 

[ecerfoglio]

No.
R4 will dissipate 2W and since it is rated for a max of only 1W then it will smoke and burn.

As I said in my post, in order to limit R4 to 1W you must limit the voltage to 7,07V.

With that voltage R4 dissipates 1W and the other 3 resistors less than that, so it is the limit of the circuit.

Of course, with more than 7.07V R4 would overheat, smoke and burn.

 

[Nigel Goodwin]

No.
R4 is burning up and is trying to dissipate 2.0W. The remaining resistors dissipate 1.33W so the total dissipation is 3.33W, not less than 2W.

Like me, HiTech was quoting the maximum dissipation of the circuit - NOT how much it would dissipate as connected.

 

[audioguru]

Yes you are correct.
Using 1W resistors, the voltage must be limited to 7.07V then R4 dissipates its max of 1W and the remaining resistors dissipate 1.666W.
Then the total dissipation is 1.666W which certainly is less than 2W.

 

[TronicBrain]

Hi
Sorry because of my late reply

Thank you all guys for your response

I did some calculations and now I understand what I need

Once the tow branches have different values 50 Ohm & 75 Ohm

So there must be a difference in the watt capacity in order to have 4W 30 Ohm resisor

I did the calculations in order to have the new wattage for each resistor
So I found that

Pr4=2.4 Watt
Pr2=1 Watt
Pr1=Pr3= 0.26 Watt

Thanks again

 

[luisgerman]

I can't be bothered to work out the resistance, but the total wattage won't be 4W, it will be less than two watts.

Your stupidity goes beyond all my expectations
(Bothered by such a simple task)