JA , tells us how hot the junction gets when 1 the regulator is dissipating a given amount of power and 2 the regulator is sitting in open air, at a given ambient temperature. In fact, we can only tolerate 1. This, however, is for the case of the TO radiating to ambient air— almost a worst-case situation.
If we can add a heat sink or otherwise cool the regulator, we can do much better. The opposite end of the spectrum is given by the other thermal specification: the thermal resistance junction-case, R thJC. This is the relevant number if you can quickly remove heat from the package, for example if you have a very good heat sink hooked up to the outside of the TO package. This represents the absolute minimum heating that you can expect under ideal conditions.
Depending on the engineering requirements, you can start from this point to build a full power budget, to account for the thermal conductivity of every element of your system, from the regulator itself, to the thermal interface pad between it and the heat sink, to the thermal coupling of the heat sink to the ambient air. You can then verify the couplings and relative temperatures of each component with a spot-reading non-contact infrared thermometer. For the present situation, one might consider moving to a surface mount regulator that offers better power handling capability by using the circuit board as a heat sink or it may be worth looking into adding a power resistor or zener diode before the regulator to drop most of the voltage outside the regulator package, easing the load on it.
The principles that we have gone over are quite general, and can be used to help understand power consumption in most types of passive elements and even most types of integrated circuits. There are real limitations, however, and one could spend a lifetime learning the nuances of power consumption, particularly at lower currents or high frequencies where small losses that we have neglected become important.
Good article, but… 1 its hard to compare sizes of the resistor when the photos have different resolutions 2 any guidelines on how much we can trim the leads, as they are important for power dissipation?
As well, I am careful not to put temperature sensitive components right by it. The resistor can take the heat, but you can be sure the PC board will cook eventually, and diodes will leak electrically and transistor biases will drift, etc… Under the PC board, I trim the leads like any other components. Dropping from 9V say a PP3 battery to 5V for a microcontroller is a pretty common situation and so a great example.
I would be very useful if you could indicate what sort of common heat-sink might typically sovle the over-power issue and allow a safe 4W heat dissipation.
Obviously it would only be a guide but somewhere between bolting it to an old key and taking the heat-sink off a Pentium-4 there must be a "this ought to do it" level that can be judged from experience. Interested to know what you would pick if faced with this situation and a box of mixed heat-sinks. Heat sinks sold for that purpose have ratings. If you know the power dissipation, you can look up all the thermal resistances and calculate the temperature rise from ambient all the way back to the die.
You have theta from junction to case, theta from case to heatsink, and the heatsink has a theta to ambient. All those thetas times the power will give you the temperature rise above ambient.
Then you figure your max ambient and you can calculate the junction temp. The math is all tractable, and you can rearrange to solve for whichever variable is of interest. For improvised heatsinks, I think you would really have to have a lot of experience.
But one thing that might help is to just imagine an incandescent lightbulb. A 25 watt bulb is too hot to touch. But if you ran a Watt bulb at 4 Watts, it would probably just be warm.
So if you need to dissipate 4 Watts, you probably need a heat sink with the area of a good old-fashioned light bulb or so. Hope that helps.
Another way to get rid of heat is to use a large copper pad on the PCB. Some very small SMT devices have thermal pads on the bottom that have very low thermal resistance to the die. If you design the PCB right, you can pull an enormous amount of heat out of these tiny packages. Active 1 year, 4 months ago. Viewed 36k times. And so evidently the peak power dissipated would be the peak voltage divided by the resistance.
Improve this question. Then you can't get confused, since there is nothing complicated to do. Add a comment. Active Oldest Votes. Improve this answer. Alfred Centauri John Rennie John Rennie k gold badges silver badges bronze badges.
Alfred Centauri Alfred Centauri Harish Chandra Rajpoot 2, 13 13 gold badges 20 20 silver badges 37 37 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. For a resistor, so the average power dissipated is. A comparison of p t and is shown in Figure d. To make look like its dc counterpart, we use the rms values of the current and the voltage.
By definition, these are. With we obtain. This equation further emphasizes why the rms value is chosen in discussion rather than peak values. Alternating voltages and currents are usually described in terms of their rms values. For example, the V from a household outlet is an rms value. The amplitude of this source is Because most ac meters are calibrated in terms of rms values, a typical ac voltmeter placed across a household outlet will read V.
For a capacitor and an inductor, respectively. Since we find from Figure that the average power dissipated by either of these elements is Capacitors and inductors absorb energy from the circuit during one half-cycle and then discharge it back to the circuit during the other half-cycle.
This behavior is illustrated in the plots of Figure , b and c , which show p t oscillating sinusoidally about zero. The phase angle for an ac generator may have any value. If the generator produces power; if it absorbs power.
In terms of rms values, the average power of an ac generator is written as. Power Output of a Generator An ac generator whose emf is given by. Strategy The rms voltage is the amplitude of the voltage times. The impedance of the circuit involves the resistance and the reactances of the capacitor and the inductor.
The average power is calculated by Figure , or more specifically, the last part of the equation, because we have the impedance of the circuit Z , the rms voltage , and the resistance R. Significance If the resistance is much larger than the reactance of the capacitor or inductor, the average power is a dc circuit equation of where V replaces the rms voltage. Check Your Understanding An ac voltmeter attached across the terminals of a Hz ac generator reads 7.
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