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### Topic: P=V*I applied on DC resistor circuits. (Read 3900 times)previous topic - next topic

#### albuino

##### Mar 06, 2013, 12:00 pm
According to Ohm's law, when a V DC voltage is applied to a R Ohms resistor, a I current will flow through the resistor.
On the other hand, I've learnt that the power that the resistor will have to dissipate could be calculated according to P=V*I. However, if I try with the following example, it seems not making sense:
V = 24 VDC, R = 12 Ohm.
So,
I = 2 A, P = 48 W.
An almost 50 W resistor... ¿¿¿??? It's too much!!! what's wrong here?

#### AWOL

#1
##### Mar 06, 2013, 12:11 pm
Quote
It's too much!!! what's wrong here?

Why do you think that?
That's a lot of current.

P=I2R (  22x12 = 48), so whichever way you do the arithmetic, 48W is the answer.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.
I speak for myself, not Arduino.

#### fungus

#2
##### Mar 06, 2013, 12:42 pm

V = 24 VDC, R = 12 Ohm.
So,
I = 2 A, P = 48 W.
An almost 50 W resistor... ¿¿¿??? It's too much!!!

No, it's correct. You need one of these: http://www.ebay.com/itm/250911320379

(Although that's going to get *very* hot at full power so you should really get 100W. Maybe 150W to be safe...)

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### Docedison

#3
##### Mar 06, 2013, 01:15 pmLast Edit: Mar 06, 2013, 01:19 pm by Docedison Reason: 1
@ fungus.. YES GREAT PART...
that's what the little "ears' are for.. Bolting to a heatsink.. That's the only way it can handle the 'rated' power
and heatsink grease is required for best results.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### JimboZA

#4
##### Mar 06, 2013, 01:55 pm
Quote
R = 12 Ohm

An almost 50 W resistor... ¿¿¿??? It's too much!!

Yeah but 12Ohms is little more than a dead short....
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

#### be80be

#5
##### Mar 06, 2013, 03:42 pm
Quote
Yeah but 12 Ohms is little more than a dead short....

Yep and could look like a glowing heater if you used the right wire, or put it in a bulb and make some light.

#### fungus

#6
##### Mar 06, 2013, 03:59 pm

Quote
Yeah but 12 Ohms is little more than a dead short....

Yep and could look like a glowing heater if you used the right wire, or put it in a bulb and make some light.

I've seen those big resistors bolted to metal plates and to heat them up (eg. in 3D printers where they want the base plate to be hot).
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### cmiyc

#7
##### Mar 06, 2013, 04:02 pm
Pick too small of a power rating and it will be a LER for a short time.

(Light Emitting Resistor)
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### JimboZA

#8
##### Mar 06, 2013, 05:54 pm
Quote
(Light Emitting Resistor)

XD ... must remember that one
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

#### jackrae

#9
##### Mar 06, 2013, 10:55 pm
Albuino

All other responders have basically stated that the dissipated power is indeed 48watts.

However, what you need to ask yourself is "Why do I not trust or believe calculations derived from very basic equations that I claim to understand"

This is not intended as a criticism but rather a suggestion that you need to enhance your experience in the application of theory before you replace it by intuition.

#### outofoptions

#10
##### Mar 07, 2013, 02:21 am

Albuino

All other responders have basically stated that the dissipated power is indeed 48watts.

However, what you need to ask yourself is "Why do I not trust or believe calculations derived from very basic equations that I claim to understand"

This is not intended as a criticism but rather a suggestion that you need to enhance your experience in the application of theory before you replace it by intuition.
Very good way to look at this.  If the math is right perhaps there may be something wrong with the circuit design that needs to be looked at.

#### MarkT

#11
##### Mar 07, 2013, 02:33 am
I use a couple of 10 ohm power resistors (25W each) bolted to a large piece of 6mm thick aluminium plate
as a dummy-load for testing things (like motor drivers, solar panels) - in parallel they give me 5 ohms, in series 20,
or just one of them gives 10.

If I put my 30V 3A power supply across one of them and crank up to full voltage the resistor gets hot
very quickly as its dissipating 30 x 3 = 90W  (I don't do this for long!).

The equations make perfect sense once you have hands-on experience like this.  Another important
realisation is that P = V x I  combines with V = I x R to yield  P = I x I x R

Power is proportional to the square of current - this means that high currents can be a real problem
to handle - 10A is a hundred times more "heat generating" than 1A for instance (which is why fuses are
very handy - they stop the wiring catching fire).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### elac

#12
##### Mar 07, 2013, 02:11 pm
Albuino why are you dropping the full 24v with the resistor? Is the resistor in series with another component in your circuit? Could you give a little detail about your project?
It's all about the skills

#### afremont

#13
##### Mar 07, 2013, 03:53 pm
This might be redundant for many, but I want to point out that Power is also proportionate to the square of the Voltage.  P=V2/R  This is something many people learn the hard way after raising the input voltage to a linear regulator.  This results in an exponential power dissipation increase just the same as increasing the current flow.  What worked coolly with a 9V battery as input, now puts the regulator in shutdown after switching to a 12V supply.
Experience, it's what you get when you were expecting something else.

#### fungus

#14
##### Mar 07, 2013, 04:15 pm

This might be redundant for many, but I want to point out that Power is also proportionate to the square of the Voltage.

ie. Volts is directly proportional to amps.

It's the law!

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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