Discussion in 'Homework Help' started by t-square, Jan 30, 2011.

1. ### t-square Thread Starter New Member

Jan 30, 2011
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1) a circuit current is 5mA a resister is mark with following band:brown, black, red, gold. and voltage drop of 6.5 across resistor is this resistor within it tolerance rating

2) if a 4700-ohm, 1/2-watt resistor electronic circuit board is defective. would a 4700-ohm, 1-w resistor without damaging circuit board or will higher wattage resistor generate excessive heat which could damage the other components

Any help would really be appreciated. PLEASE AND THAX

2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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What have you got so far? What value is Brn-Blk-Red with Gold Tolerance?

2) What is producing the heat? What is damaged? How would you correct the issue?

Jan 24, 2011
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The value of a brown, black, red and gold band resistor is 1K ohms with a tolerance rating of +/- 5%. This means the resistor has a tolerance value of between 950 – 1050 ohms.

Using ohms law, the value of resistance required within a circuit of 6.5V and 5mA can be ascertained.

R= V/I 6.5/0.005 = 1300 ohms

The resistor is therefore not within its tolerance.

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4. ### beenthere Retired Moderator

Apr 20, 2004
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To stuatadam1987: You came right in with a lucid solution to the problem. The OP owes you a big thank you for that.

In the future, if you would, keep in mind that this is a homework section, and we try to get the posters to make an effort to solve their problems. The time to find resistor color codes is only seconds. Ohm's law takes it from there.

To the OP: What do you think produces heat in a resistor? What does that lead you to say about replacing a 1/2 watt resistor with a 1 watt resistor of the same value?

Jan 24, 2011
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Sorry about that, I’m new to this stuff. I'll be sure to remember that for the future though.

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Please don't see that as a rap on the knuckles. It's simply that we do like to help people learn, and it is better if they do the majority of the work. If we supplied answers in every case, we would devolve into an online cheating service - not at all what we wish to be.

7. ### t-square Thread Starter New Member

Jan 30, 2011
11
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Hey stuartadam1987 thanx i multiply mind instead P=6.5x.005=.0325 which was wrong obviously

beenthere i no but im really new at this working with this

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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For the second part, what is creating the heat? The resistor, or the circuit design?

9. ### t-square Thread Starter New Member

Jan 30, 2011
11
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brown black red gold
1 0 00 +or-5% 1000KΩ ±5% or i could just write it 1K ±5%. heat produce by the resistor

10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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That's still the first question, I'm asking about the second question now.

What causes heat in a circuit? The resistor size, or the circuit?

11. ### t-square Thread Starter New Member

Jan 30, 2011
11
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well now is the size of the circuit that causing th heat. so my thought is if i adjust the resistor from 1/2 to one the problem will stop. just guessing here.

12. ### Georacer Moderator

Nov 25, 2009
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1,285
You haven't understood the question quite well. It goes like this: You have a circuit with an 4.7kΩ @0.5W resistor in it. You realize that this resistor is damaged for some reason (which doesn't concern you) and you decide to replace it. Can you use a 4.7kΩ resistor @ 1W for that purpose? Does the greater wattage prohibit you from using it?

In order to answer this question, remember that the wattage of a resistor describes how much heat it can dissipate in the environment before it burns up. It doesn't tell you how many watts the resistor generates. That quantity is defined only by the product V*I, where V is the voltage applied on the resistor and I the current through it.

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13. ### t-square Thread Starter New Member

Jan 30, 2011
11
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assuming room permits, the 4.7kΩ will work on the 1W without damaging other components. and yes the v*I would work if a number is given for it but it was so explaining because the wattage is increase would be quite sufficient don't you think

14. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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That's basically the answer. You can generally replace resistors with a higher rated component without problems, but never use a lower rated power value.

Sort-of related: Resistors hardly ever "go bad". If a resistor is failed, 99 times out of 100, there will be something else wrong as well. Probably won't be an issue in your coursework, but may be something to remember if working on things in real life.

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