How much heat can a resistor handle in a 9V DC circuit?

[Jasonmatthew911]

Hey guys, the reason I'm asking this, is because I made a 555 Toy Organ circuit running on 9V power, and I noticed that the resistors going from pin 7 of 555 to my momentary spst buttons then to 9VDC power rail, the resistors start to get very hot after awhile of keeping the spst buttons pressed, while resistor is making contact with pin 7 to 9V Power rail....I even had a potentiometer let smoke out while I had the circuit breadboarded....I'm using fixed Metal Film 1/2W resistors now...I'm not sure how long these resistors will work before overheating or something happening....Normally the Toy Organ circuit operates from 6V - 12V on most the schematics I've found...I also noticed lower Ohm resistor values seemed to get a lot hotter than higher values, is this right or does the value have nothing to do with it?....I'd basically like a little help understanding what could happen to my circuit if the resistors get overheated for too long, or should I not worry about the resistors getting too hot?...Or I'd like to know how long can the 1/2W resistors  make the contact from pin 7 to Power rail before exploding?.......I'm not sure how reliable the 555 Toy organ circuit is to begin with, but just the fact that a Pot let out smoke after awhile makes me wonder about this....Also, I tried a 7805 for 5V and the resistors didn't seem to heat up anymore, but the 5V changed the frequency completely, and I had already found the right value resistors for all my notes using 9V, so I'd like to keep this running on 9V if possible....Any extra help will be appreciated, thanks.

[Jdansti]

If you could post the schematic it would help us get a better understanding of the circuit.

[iccaros]

This is an answer for......OHM's law..

V=I*R where V = voltage, I = current and R = Resistance


What is the resistor size?
measure voltage over the resistor

voltage / resistance = current

Now we take Watts law P = V*I or  P = power or watts, V = voltage and I = current

Voltage * Current = power.. 
to be safe multiply that number by 2 to get the size of resistor you need.


then voltage *

[DiscoVlad]

And then, like some unholy abomination of mathematics... we combine Ohm's Law, and Watt's Law in an orgy of algebraic depravity to create... um... Wattohm's law! 

P= V²/R, and P=I² x R

[Seljer]

Yep, all the above is true

If you combine things: power = voltage^2 / resistance

And if you turn that around: resistance = voltage^2/power


So if you assume the largest voltage difference is going to be 9 volts, the smallest 1/4watt resistor you can safely use in a circuit powered by a 9 volt power supply is  ((9 volts)^2) / (0.25 watts) = 324 ohms

Or if you've got the slightly larger 1/2watt resistors it's half that: 162ohms


If you've got a resistor smaller than that in there, it very well might be just fine, you just have to double check it's function in the circuit.

[Mike Burgundy]

And of course one would often use some kind of safety factor just to be sure it'll keep working because you're not stressing everything to the max, so maybe you'll get ((9V)^2)/0.25W*0.5(safety))=648Ohms

[Jasonmatthew911]

Ok, here is the schematic I was using...I'm not very good with the math part of electronics, so hopefully someone can explain to me if this is wrong or ok....Basically my R1 = 70K, and P8 now R8 (Fixed resistor) = 43 Ohms (Which is the lowest value and it gets very hot)...I'm using 1/2W Metal Film Resistors...

[R.G.]

You then season the mix with a little fluid mechanics and heat transfer.

The temperature of an object does not depend on how much power it dissipates. It depends on how much power it dissipates and how much surface area the heat has to get out of the object, and how fast the surface can move the heat out. A 100mW grain-of-wheat lamp filament runs at a temp of about 4000C and glows bright yellow because that 100mW has so little surface area to get heat out of. A 100W wirewound resistor dissipating only 100mW would be nearly impossible to detect a heat rise in because the heat is so easily transferred out through the big surface area.

Today's resistors come in standard physical sizes, which means they all have about the same surface area to get heat out of.  The size and other details are set up so that when there's the resistors' full power being generated inside, the outside surface only gets up to some temperature, so the temperature does not degrade the materials. The temperature is most often 100C to 200C.

So you can calculate the maximum voltage (and hence current; thank you Georg Ohm) that a resistor can stand if you know its resistance value and wattage. With a little more math or some charts, you can actually transfer that into a temperature rise.  

For a resistor rated at power P, with resistance R, the maximum current is I = SQRT(P/R) and the maximum voltage as far as power goes is V = SQRT(P*R). Each resistor value and power rating will have a different maximum current (and voltage, which amounts to the same thing).

looking at your schemo, your pots go down to zero ohms if the slider at all the way at the switch. You need to figure some way to limit that. Maybe a fixed resistor in series with each switch. The 555 doesn't work well with a zero ohm resistor there anyway.

[Jasonmatthew911]

You then season the mix with a little fluid mechanics and heat transfer.

The temperature of an object does not depend on how much power it dissipates. It depends on how much power it dissipates and how much surface area the heat has to get out of the object, and how fast the surface can move the heat out. A 100mW grain-of-wheat lamp filament runs at a temp of about 4000C and glows bright yellow because that 100mW has so little surface area to get heat out of. A 100W wirewound resistor dissipating only 100mW would be nearly impossible to detect a heat rise in because the heat is so easily transferred out through the big surface area.

Today's resistors come in standard physical sizes, which means they all have about the same surface area to get heat out of.  The size and other details are set up so that when there's the resistors' full power being generated inside, the outside surface only gets up to some temperature, so the temperature does not degrade the materials. The temperature is most often 100C to 200C.

So you can calculate the maximum voltage (and hence current; thank you Georg Ohm) that a resistor can stand if you know its resistance value and wattage. With a little more math or some charts, you can actually transfer that into a temperature rise.  

For a resistor rated at power P, with resistance R, the maximum current is I = SQRT(P/R) and the maximum voltage as far as power goes is V = SQRT(P*R). Each resistor value and power rating will have a different maximum current (and voltage, which amounts to the same thing).

looking at your schemo, your pots go down to zero ohms if the slider at all the way at the switch. You need to figure some way to limit that. Maybe a fixed resistor in series with each switch. The 555 doesn't work well with a zero ohm resistor there anyway.

The thing is that I'm not using the Pots anymore...I'm using fixed resistors, starting with 43  Ohms, so if I add a resistor I will change the tuning...I guess with 43 Ohms a lot of voltage is going through...Would it help if I have more copper to dissipate the heat in the power rail?