Resistors

[audioguru]

Yes, but isn't 25°C room tempurature?

It is summer and my air conditioner isn't perfect. It is frequently 30 degrees C. Also, the LED is going to be mounted in something that will block cooling air flow to it.

So then it should run at 100mA?

Not if its ambient air is more than 25 degrees C.

What would you suggest running it at? And why would you run it at that?

I would probably use 80mA max because the performance isn't much less but the reliability will be greatly improved.

 

[Marks256]

But under "Absolute Maximum Ratings", it says "forward current: 1.2A"! I am so confused! GOD!! I need to get this straight.

I will say this one last time;
On the data sheet, there are two catagorys; Electrical characteristics, and Absolute maximum ratings. I am guessing the colum under Electrical Characteristics are the ratings that are ideal/recomended. I know that the Absolute maximum ratings mean the MAXIMUM. The electrical characteristics are as follows;

Radiant power output (100mA): 16mW min.
Forward Voltage: 1.2v
Forward current: 100mA It says this under Electrical Characteristics, NOT ABSOLUTE MAXIMUM RATINGS.
Viewing angle to 1/2 intensity: 45°

Now under the ABSOLUTE MAXIMUM RATINGS, it says;
Forward voltage (20mA): 1.6V
Reverse voltage: 5v
Forward current: 1.2A This is under: ABSOULTE MAXIMUM RATINGS, NOT Electrical characteristics.
Reverse current: 10uA
Wavelength: 940nm

Now, unless i am missing something, it says, under the RECOMENDED ratings, that the ideal conditions for the Infrared LED, is 1.2v@100mA. THE ABSOLUTE MAXIMUM SAYS: 1.2A(AMPS).

There is no where else that says anything other than: 100mA, and 1.2A.

I don't understand why you think 100mA is the MAX? It says the maximum is 1.2A? Now you have me so confused, ahh! I guess there is only one way to find out who is right, eh? I will test it right now, then post the outcome.

 

[Marks256]

Oh, ok. I posted my reply before i saw AUDIOGURU's. 80mA makes scence. Ok, i will try it at that. Thanks. Don't worry, i didn't try it yet. Thank you.

UPDATE: Ok, i looked for a 47ohm resistor, and couldn't find any. I will have to get a bunch of resistors, then i will try it. Thanks though!

 

[dknguyen]

My mistake, I didnt read the info carefully enough. 1.2A is the absolute maximum current rating that the diode can handle (momentary spikes, not continous), exactl as titled, while 100mA is the continous current rating (at room temperature, decreases as temperature increases obviously). YOu will find most devices have a continous current rating which will cause them to generate heat enough just as fast as it can be dissipated so the device temperature remains constant. Beyond that, more heat is generated than is dissipated so the device will continue to heat up and fry itself. So yeah, most devices have a continous rating, and a momentary/spike/instaneous/peak rating (for voltage and current).

They don't always list it as "continous ratings". Usually they separate peak ratings from regular (continous ratings). They may not label continous ratings as such, but they will always label peak ratings as absolute maximums. So you can usually pick out which is which.

You got into the habit of poring over datasheets...that's good. After a while you will sort of understand what is what exactly (uC sheets- or databooks in some cases are by far the worst).

I was typing more analogies at the end of my last post when you replied so I hope those help.

 

[audioguru]

RadioShack doesn't make anything, they just buy it and sell it. They don't have any engineers. Their non-technical people print stuff that is missing all the details and doesn't make any sense.

Look at the datasheet of an IR LED from a manufacturer. It has very detailed maximum ratings that even include the max allowable micro-seconds duration of the 1.2A pulses. The datasheet has derating curves showing the max allowable current when the ambient temp is higher than 25 degrees C.

 

[Marks256]

Yes, thank you dknguyen. Your analogies really helped. I am still alittle confused though, but not as bad. I was able to figure out(on my own), what type of resistor i needed to run an LED off of 9v, and it worked awesome! I think i know enough now to start using resistors. Once i learn alittle more about electronics (i.e. diodes, capacitors, and transistors), i will ask more about resistors(like power disipation, and the like). Thanks guys!

 

[RadioRon]

For the last time, i am 100% sure that these are the ideal/normal/recomended characteristics. Yes, i know my math was off. I didn't quite know what 100mA equals in amps. I know now that it is .1.

Is this right? I just drew it quick. Thanks.

Yes, your calculation is correct. The 47 ohm resistor will come very close to dropping about 4.8 volts with 100 mA of current flow. Exactly right.

So, what is power disipation? Is it how many watts of power the curcuit requires?

Yes, the power dissipatoin is the number of watts the circuit will use up.
The total power dissipation is the sum of the power disipated in the resistor plus the power disippated in the LED. In the resistor, it is all turned into heat, but in the LED, some is heat and some is InfraRed light energy. I believe that the conversion efficiency of an IR LED is fairly poor, so most of the energy will become heat. Anyway, the total power drawn from the battery is P=VxI = 6 x 0.1 = 0.6 watts. The resistor accounts for P=V^2/R = 4.8^2/47 = 0.49 watts, so the LED is taking 0.6-0.49=0.11 watts. So the resistor will get pretty warm while the LED will get warm too. This points out that you absolutely must use a resistor rated at at least 0.5 watts and for best reliability, perhaps use a 1 watt rated resistor.

I have another question; If i had a power supply that supplyed 2watts of power, and a curcuit that used only .25watts, would the powersupply force the two watts into the curcuit, or would it just give what the curcuit needed?

The circuit would only absorb as much as it needed. If the power supply is correctly designed, it would then only convert and deliver as much as the circuit asked for. The value of 2 watts tells you the maximum capability of the supply. Power supplies are usually constant voltage sources, so the way they work is that they provide the fixed voltage that it is designed to (or the voltage you ask for if it is a variable supply) and then hold that voltage steady while you take as much current as your circuit needs (as determined by ohm's law). So the voltage is fixed and the current is pushed into your circuit by the voltage of the power supply. When your circuit is first hooked up and turned on , the current will increase instantaneously until the voltage drop across your circuit equals the voltage available from the power supply. At that point, the power supply voltage is balanced by the voltage drop of your circuit and everything remains nice and steady at that point.

 

[RadioRon]

LEDs (and ordinary diodes, transistors etc) do not obey Ohms Law.
JimB

I was scanning through the thread and found this statement and felt I must comment. LEDs and ordinary diodes and transistors do indeed obey ohm's law and to say that they don't indicates that you don't understand ohm's law and are misleading and perhaps confusing our students on the board. Ohm's Law only states that V=I*R. It does NOT state that if you calculate R at one current then that R will remain constant at other currents. This is simply the behavior of a resistor. Ohm's law holds true at any current through a diode, for example, where you can calculate what the resistance is, AT THAT PARTICULAR CURRENT. A diode is simply one of many electronic devices where the relationship of current to voltage is not a straight line. In other words, its resistance varies as current through it varies. Same with transistors.

Off the top of my head, I can't recall any device that doesn't obey ohm's law. Can anyone suggest a device that doesn't and why?

 

[Marks256]

Thanks RadioRon. That cleared up some things. Thanks again.

 

[eblc1388]

Ohm's Law only states that V=I*R. It does NOT state that if you calculate R at one current then that R will remain constant at other currents.

I cannot agree. Ohm's law only states that the "relationship" between current and voltage across certain material remains constant, so V=I*R is not ohm's law but a direct result of applying ohm's law as ohm's law implies that for centain material, the proportional constant is called R so one can work out either the current or the voltage if the other is known, or work out R if both voltage and current is known.

This is simply the behavior of a resistor. Ohm's law holds true at any current through a diode, for example, where you can calculate what the resistance is, AT THAT PARTICULAR CURRENT. A diode is simply one of many electronic devices where the relationship of current to voltage is not a straight line. In other words, its resistance varies as current through it varies. Same with transistors.

These statements can not be true as active components does not has a constant linear relationship between current and voltage so ohm's law does not apply from the start.

Off the top of my head, I can't recall any device that doesn't obey ohm's law. Can anyone suggest a device that doesn't and why?

A common filament lamp bulb does not obey ohm's law between the range from zero to normal working current because its resistance has changed many times. But it will still obey ohm's law if it's element is immersed in liquid nitrogen and being kept at a constant temperature.

 

[RadioRon]

Ohm's law only states that the "relationship" between current and voltage across certain material remains constant, so V=I*R is not ohm's law but a direct result of applying ohm's law as ohm's law implies that for centain material, the proportional constant is called R so one can work out either the current or the voltage if the other is known, or work out R if both voltage and current is known.

Please provide text references that support the above statement.

I believe that, like JimB's statement, this is your incorrect interpretation of ohm's law. However, in case I am wrong, I will look through my text books to see what they say and provide my own references.

 

[eng1]

Ohm's law states that the ratio of the potential difference between two points of a certain conductor and the current flowing is constant. According to this law the resistance is an intrinsic property of a conductor. Actually no materials obey that law. Some materials approximate the ideal behaviour very closely, such as resistors and wires.

 

[JimB]

My understanding of Ohms Law is that the current is proportional to the voltage. So if we double the voltage, the current will double as a result.
The mathematical relationship between voltage and current (according to Mrs Ohms' little lad) can be represented by the linear relationship
y = m*x
Where y is the voltage, x is the current and m is a constant.
Comparing this with V = R * I, R the resistance is a constant.
Diodes and transistors do not exhibit constant resistance, even when the temperature remains constant, and so do not obey Ohms Law.
Bits of wire and resistors do obey Ohms law if the temperature remains constant.
Lamps do not maintain a constant temperature and so the resistance varies. If the filament temperature were to remain constant, a lamp would obey Ohms law.

JimB

 

[RadioRon]

Please provide text references that support your claim.

 

[eblc1388]

Ohm's Law only states that V=I*R. It does NOT state that if you calculate R at one current then that R will remain constant at other currents. This is simply the behavior of a resistor. Ohm's law holds true at any current through a diode, for example, where you can calculate what the resistance is, AT THAT PARTICULAR CURRENT.

Please provide text references that support the above statement.

You first.

 

[dknguyen]

I cannot agree. Ohm's law only states that the "relationship" between current and voltage across certain material remains constant, so V=I*R is not ohm's law but a direct result of applying ohm's law as ohm's law implies that for centain material, the proportional constant is called R so one can work out either the current or the voltage if the other is known, or work out R if both voltage and current is known.

Uh...yeah...but at at the same time...no...that is the definition of Ohm's law. You can't say that [insert definition of Ohm's law] is not Ohm's law, but is instead [insert definition of Ohm's law].

A common filament lamp bulb does not obey ohm's law between the range from zero to normal working current because its resistance has changed many times. But it will still obey ohm's law if it's element is immersed in liquid nitrogen and being kept at a constant temperature.

Dude, it's called Ohm's law, Ohm's law, Ohm's law. However, what should be kept in mind is that R is a function of temperature. Just because something's resistance varies with temperature does not mean it does not follow ohm's law. I think what you mean to say is that Ohm's law does not take into account the fact that passing current may increase the temperature of the material, thereby increasing (usually) the resistance. It's still Ohm's law nonetheless. Ohm's law assumes that the current is what it is, and R has been chosen to represent the material at a particular temperature. This will result in one of a few things: V=IR represents an instant in time if the current does cause the material to heat up, or you work on the assumption that the current does not heat up, or you have chosen R to appropriately represent the operating temperature of the material. It's still Ohm's law. If you want to pull non-idealities and the dynamic of heating up, go ahead, but it's still Ohm's law, don't say it's not- say it's a simplification.

What you are saying is kind of like saying F=ma is not a law of Newtonian motion because it is dF= (dm/dt)(dv/dt).

 

[eblc1388]

Let's agree on the following things first. Any reference (rather than opinion) is better in this case.

1. Does Ohm's Law allows changing resistance with current?

2. Does Ohm's Law allows dropping voltage across a material with increasing current?

 

[dknguyen]

There is nothing saying that R in Ohm's law (V=IR) is a constant variable. R can be modified to be R(I,T) to account for ambient temperature and current heating effects. Usually it's not though because it's just easier to guess the operating temperature. Just stop thinking of R as a constant and voila, problems solved.

So the answer would be yes to 1 and 2.

(For 2, you would need R to be the resistance equation of one of the few materials whose resistance decreases with increasing temperature to cause the reducing voltage + increasing current).

As soon as you make R a function though, you are asking for a parametric ass kicking since you gotta know everything about the resistors being used (cross sectonal area, length, volume , resistance-temperature characteristics, specific heat capacity, heat transfer characteristics with ambient blah blah blah).

Refer to University Physics, 11th edition, Young and Freedman, pg. 949-954

 

[RadioRon]

You first.

Ok, here's my first attempt:

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“I shall now define, by the following equation, the quantity called the resistance, R, of the conductor BC:

R=V/I eq. 40.3

For some materials (for many different kinds of metallic wires, for example) and under certain conditions (at constant temperature for example) the resistance defined this way is a constant, independent of I. For other kinds of conductors (vacuum tubes, for example) the R defined in this way is not independent of I. In all cases the resistance defined by equation 40.3 is measured in ohms. Obviously, “volts divided by amperes” is equivalent to ohms. Equation 40.3 is known as Ohm’s law after Georg Simon Ohm, a German physicist (1787-1854).”

Ref: “The New College Physics, A Spiral Approach” Albert V Baez, Freeman and Company, San Francisco, 1967, pp512.
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“A German physicist named George Simon Ohm was the first to discover the relationship between current, electromotive force and resistance. The discovery is called Ohm’s law and simply expressed is - that for any circuit or part of a circuit under consideration the current in amperes is equal to the electromotive force in volts divided by the resistance in ohms. This law, mathematically expressed, is as follows:
Current = Electromotive Force/Resistance
If in the above expression we substitute the proper symbols for current, electromotive force and resistance we have the following equation:

I=V/R

This is the equation for Ohm’s Law.”

RadioRon Note: there is no mention of this Law only applying to cases where R is constant.

Ref: “Principles of Electricity applied to Telephone and Telegraph Work” A training course text, American Telephone and Telegraph Company, January, 1953 (revised from the 1938 edition), page 7.

----------------------------------------
“One ohm is defined as the amount of resistance that allows one ampere of current to flow between two points that have a potential difference of one volt. Thus, we get Ohm’s Law, which is:

R=E/I
Where R=resistance in ohms
E= potential or EMF in volts and
I= current in amperes.”

RadioRon Note: no mention that Ohm’s Law is not obeyed when R is variable.

Ref: “The ARRL Handbook” The American Radio Relay League, Newington, CT, 2000, pp 5.2.

----------------------------------------------------------------
“The relationship between electromotive force (voltage), the flow of current (amperes), and the resistance which impedes the flow of current (ohms), is very clearly expressed in a simple but highly valuable law known as Ohm’s Law. This law states that the current in amperes is equal to the voltage in volts divided by the resistance in ohms. Expressed as an equation:
I=E/R”

RadioRon Note: no mention in this text that this law does not hold when R is a function of I

Ref: “Radio Handbook” 22nd Edition, William I. Orr, Howard W Sams Company, Indianapolis, 1981, pp 2.6-2.7

---------------------------------------------------------------------

Now, this reference explains things in more detail and I believe explains why we are arguing.


“Over a hundred years ago, Georg Simon Ohm discovered that every time he closed a switch in a circuit such as that of Figure 4-1, the current became the same constant value. He also discovered that, providing the temperature of the conductor did not change, doubling the emf doubled the current and tripling the emf tripled the current. In other words:
*For a given circuit, the ratio of the applied emf to current is a constant*
This became known as Ohm’s Law of constant proportionality. We can express Ohm’s Law in equation form as

E/I=k eq. 4-1

Where E is the applied emf in volts, I is the resulting current in amperes, and k is a numerical constant.

Carrying this discovery a step further, Ohm found that changing the conductors or the load to some other size resulted in a different value for the numerical constant. If the current is one ampere when the emf is 10 volts, the constant becomes 10/1, or simply 10; and if the current is 0.5 ampere when the emf is 10 volts, the constant becomes 10/0.5, or 20. From these results, Ohm concluded that this constant E/I ratio for given circuit is, therefore, a property of that circuit. And since, for a given emf, the value of the constant increases as the current decreases, this constant can be thought of as representing the opposition of the circuit to the flow of current. This property then should be given a name that suggests opposition, such as resistance. Therefore:
*Resistance is the opposition of an electric circuit to the flow of current through that circuit.
The Letter symbol for resistance is R*

We can substitute the symbol R for the constant k in Equation 4-1. Since resistance is an electrical property, we require a unit of measurement for it. Although it would be quite proper to express resistance in terms of volts per ampere, it was decided to honor Ohm’s original discovery by stating that:

**the ohm is the basic unit of electric resistance. The unit symbol for ohm is the Greek letter omega**

Since Equation 4-1 has now become

R=E/I or R=V/I eq. 4-2

Where R is resistance in ohms, E is the applied emf in volts, V is the voltage drop across the resistance in volts, and I is the resulting current through the resistance in amperes, the size of the ohm is automatically established.
** An electric circuit has a resistance of one ohm when an applied emf of one volt causes a current to flow at the rate of one ampere**

Few people nowadays recall the original wording of Ohm’s law of constant proportionality which allowed us to define resistance and to establish a unit of resistance. As a result, Ohm’s law is usually stated simply by the equation
R=E/I”

Ref: “Introduction to Electric Circuits” Herbert W. Jackson, Prentice-Hall Inc, Englewood Cliffs, New Jersey, 1970, pp 70 to 71.

----------------------------------------------------------
Discussion:

I find the last reference to suggest that some of you may have been taught Ohm’s Law of constant proportionality in its original form, which is now effectively obsoleted by the more modern form R=E/I. It is not appropriate to believe that the original form is the most correct, because at the time Ohm was not aware of conductors whose resistance was dependent on current such as we see in semiconductors. In addition, Jackson teaches that Ohm himself realized after further study that his so-called constant of proportionality could indeed vary depending on the materials used. This lead to the definition of resistance, not the definition of constant resistance, as one of the three variables (a variable is something that can vary) that constitutes what we now call Ohm’s Law.

I also thought it might be instructive to approach this from a logical argument point of view by asking "if a diode does not obey Ohm's Law, what law then describes the resistance of the diode when a specific emf is applied and a resulting specific value of current flows?"


Your turn.

 

[JimB]

As a counter to RadioRon I offer the following from The Art of Electronics by Horowitz and Hill, Second edition, page 44:

"Before jumpimg into some circuits with diodes, we should point out two things:
(a) A diode doesn't actually have a resistance (it doesn't obey Ohms law).
(b) If you put some diodes in a circuit, it wont have a Thevenin equivalent."

JimB