Electronics 101 Question .....

Let's say that we are passing a DC current through a ceramic wire-wound resistor to ground. Let's suppose we have a circuit with 6 DC volts and 4 amps going into the resistor. The resistor is grounded. Which resistor would get hottest (as in temperature), a 3 ohm resistor or a 10 ohm resistor?

So my question is, under identical conditions, would a low ohm resistor or a higher ohm resistor generate more heat? And if you can, maybe you could supply reason(s) for your answer.

Thanks in advance.

Now look at an analogy (if it can be considered that!) in politics. In politics there ARE NO RULES, so it"s no wonder that we get in such trouble (no matter what party or affiliation you happen to be!) when trying to reach a sensible political conclusion.

Which one gets hotter involves thermal transmission. That's why things have cooling fins and such. Something generating a lot of heat, but disipating it well will not get as hot as something generating a little heat, but not disipating it at all.

Which one generates more heat is a simple matter of wattage.

All cooling parameters being equal, the item generating the most heat (highest wattage) will be the hottest.

MMB, LONG ANSWER:

BEFORE we can give a correct answer we need a CORRECT question such as Volts or Amps being supplied, and, of course, if small and large resistors were in series or paralell if you wanted to use both in a single circuit for comparisons. Did you intend a fixed voltage source across the resistors or a fixed current source as you mentioned BOTH????

If you apply a voltage source across resistor(s) Ohms law tell us the current draw is E/R and the power it will disspate in the form of heat is E x I or I squared x R or E squared/R SIMPLE AS THAT.

So, at the same voltage say 10 volts across two resistors in "paralell" a smaller 5 ohm and a larger 10 ohm, the power dissipated in the form of heat in the smaller 5 ohm resistor is 100/5 = 20 watts, while in the larger 10 ohms the power is 100/10 = 10 watts THE SMALLER RESISTOR DISSIPATES THE MOST POWER IN THE PARALELL COMBINATION.

If the resistors were placed in "series" across a 10 volt source, each resistor receives the same amount of current flowing through it (10/15 = 0.666 amps) but theres a different voltage drop across each individual resistor, the sum of which equals the 10 volt source. There would be 6.666 volts dropped across the larger 10 ohm resistor but only 3.333 dropped across the smaller 5 ohm resistor. In that "series" situation the power dissipated in the smaller 5 ohm resistor would be E x I or 3.333 x 0.666 but in the larger 10 ohm resistor the power dissipated would be the same 0.666 amps x 6.666 volts SO THE LARGER RESISTOR DISSIPATES MORE POWER IN THE SERIES SITUATION.

Hope this helps explain and clarify your question, let us know, best wishes n God Bless

John T

John T

But the question was "which would generate more heat, the high ohm resistor or the low ohm resistor?".

Of course the answer is the low ohm resistor would generate more heat because it is dissipating more power.

The wattage rating of the resistor has nothing to do with the amount of heat generated. Only the amount of power flowing (or being dissipated) in the resistor determines the amount of heat generated.

A resistor rated for 100 watts will generate the same amount of heat as one rated for 10 watts provided the same amount of power is flowing in each (I squared R) or E I or E squared /R.

ALL power dissipated in ANY resistor is directly converted into heat.

A larger wattage resistor can handle more power than a lower wattage resistor but each will give off the same amount of heat with the same amount of power supplied because: ALL POWER DISSIPATED IN ANY RESISTOR IS DIRECTLY CONVERTED INTO HEAT.

Regards
Gary

I didn't mean to come across like a smart alec like my post sounds. I should have worded it differently.

I read the question differently I guess. I pretty much ignored the first part of MMB's post as general rambling on his part, as in the last paragraph he asks "So my question is..." where he asks "which resistor would generate more heat". I took that part to be his main question.

My apologies to you and others that may have read the post differently.

Best regards
Gary

Mahalo,
doogdoog

Sorry about my reply. Yes you were correct. I read the bit about the 12 watt resistor quickly and thought you were saying that it depended on wattage of the resistor like several others implied.

Best regards
Gary

"...we have a circuit with 6 DC volts and 4 amps going into the resistor..."

So which is it? 6 volts or 4 amps?

If you are applying a constant voltage to the resistor, then the smaller resistance will draw more power. (P = V²/R) (So the 3 ohm resistor draws 12 watts, while the 10 ohm resistor draws 3.6 watts)

If you are applying a constant current to the resistor, the then the larger resistor will draw more power. (P = I² x R) (In this case the 3 ohm resistor draws 48 watts while the 10 ohm resistor draws 160 watts. Note that in the real world, you won't be able to get your 4 amps because you only have 6 volts to work with. You need either more voltage or a smaller resistor to get 4 amps.)

And if you are really trying to get 4 amps at 6 volts, then you need to solve for the resistance:
R = V / I = 6/4 = 1.5 ohms. And the power consumed by the resistor is going to be V x I = 24 watts. And a 24 watt resistor is physically pretty big, maybe the size of your thumb.

You might want to back up and say what it is you're trying to do. It's always better to work with a real problem rather than a hypothetical one.

Mahalo,
doogdoog

For the two cases mentioned:
(first)
Current is 6 volts divided by 3 ohms or 2 amps
Power is 2 amps squared times 3 ohms or 12 watts
(second)
Current is 6 volts divided by 10 ohms or .6 amps
Power is .6 amps squared times 10 ohms or 3.6 watts.

How hot the resistors will get depends on what their power ratings are as Gerald mentioned and you didn"t specify any wattage ratings for the resistors.

The second equation is for dealing with power. THis equation states that power (measured in watts) = voltage x current. This can be written as P = V x I.

Using Ohm"s law in conjunction with the power equation, we can re-write it different ways such as:

P = I^2 x R or P = V^2 / R

Now looking back at the first equation (ohm"s law) you can see that if we know the voltage is 6V and the resistance is 3 ohms, then we know the current would be 2 amps. Using the power equation that says P = I^2 x R, we can calculate the power loss in the resistor as heat would be equal to 12 watts.

Using the same thought process for the 10 ohm resistor we see that the current is equal to .6 amps. Similarly the power loss in the resistor in this case would be equal to 3.6 watts.

Hope this helps.

You don't supply enough information in your question to be able to say which resistor will get the hottest. That will depend on the surface area of the resistor available for dissipating that heat.

If the two resistors have indentical physical size and identical power ratings, the resistor dissipating the most power will get the hottest, but if the 3 ohm resistor is rated a 1000 watts, it won't heat much while if the 10 ohm resistor is rated at 100 watts with 4 amps through it (and 40 volts drop) it will dissipate 160 watts and will nearly glow.

Gerald J.

A resistor will have a temperature rise proportional to the actual power disspated times the rated temperature rise divided by the rated power. So if the dissipated power is smaller than the rated power the temperature rise will be small. If the dissipated power is larger than the rated power the temperature rise will be large and it depends on the construction of the resistor whether that resistor will smoke or will survive.

Vitrious glass coated wire wound resistors on ceramic cores can take a few times rated power, but they may melt the glass and that glass dripping on adjacent objects may ignite the surroundings.

Tin oxide film resistors I've found can handle ten times rated power, but they glow and the heat radiated can ignite adjacent objects like plastic cases or fiberglass-epoxy printed circuit boards.

Old carbon composition resistors will smoke at not much above their rated power and will drift in resistance just from the passage of time, though heat and humidity will speed up the drift in resistance.

Gerald J.