# Power Dissipation Issues

Discussion in 'General Electronics Chat' started by stdio.h, Feb 19, 2015.

1. ### stdio.h Thread Starter New Member

Feb 18, 2015
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0
Hey!

I built a circuit with a 5V power supply and a 300 ohms resistor. With a multimeter I did the measurements of resistance, voltage and current across the resistor. The values was:

R = 298 ohms
E = 4.94 V
I = 16 mA

So I made the calcs to obtain the power dissipated by the resistor:

P = EI = 4.94 * .016 = 79 mW
P = E²/R = 4.94² / 298 = 81.9 mW
P = I²*R = .016² * 298 = 76.3 mW

My question is: Why there are differences between the power results? It's because of the lack of precision in measurements, the increase of resistor's resistance when powered or a combination of both?

2. ### bertus Administrator

Apr 5, 2008
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2,958
Hello,

The precision of the current is to low.
The current will be 4.94 Volts / 298 Ohms = 16.5771 mA.
When you do the calculations with this value the power will be 81.89 mWatt.

Bertus

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3. ### #12 Expert

Nov 30, 2010
17,829
9,162
Precision problems. I calculate 16.577 ma, but most meters don't go that far down in the digits. Neither do they measure closer than a tenth of an ohm.

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4. ### stdio.h Thread Starter New Member

Feb 18, 2015
3
0
So the problem is my HM-1000 multimeter.

5. ### #12 Expert

Nov 30, 2010
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9,162
You can spend big bucks for something that has 5 or 6 digits, but it usually doesn't matter unless you're servicing precision analog meters.

Most people (including me) get along with a \$10 POS except for rare occasions.

Apr 5, 2008
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7. ### stdio.h Thread Starter New Member

Feb 18, 2015
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Yes! I know. I was just justifying the lack of precision in my measurements. The HM-1000 is good enough for me.

8. ### wayneh Expert

Sep 9, 2010
13,427
4,265
Did you actually measure the 16mA current, meaning your meter was part of the circuit? That would introduce some series resistance and lower the current. If you have a second meter, you could measure the ∆V with the first meter in place.

9. ### ramancini8 Active Member

Jul 18, 2012
461
129
Also, high precision meters take longer to get accurate measurements.