Heat dissipation in a circuit
Heat dissipation in a circuit
(OP)
I have an ongoing thread in the "Circuit Engineering" forum discussing potential methods to achieve a stable supply voltage. The kicker is no matter what method I come up with, I still need to dissipate 6-8W in the conversion process (using switching converters at 85-90%efficiency, linear regulators, etc.). From previous circuits I know even 1-2W is an amount of heat I can't hold in my hand without buring some flesh after a few seconds, so 6-8W is a scary thought.
My question to you guys is, how do I safely dump this kind of heat? Or better yet, is it even something I should worry about? The circuit will go into a fairly small plastic package (let's say 3" x 3" x 1"), will be used in an automobile, and will ocassionally be picked up and held for extended periods. Would it be reasonable to use a heatsink within the case and allow holes for ventilation, or am I out of my gourd thinking I can dump that much heat?
The only saving grace I can think of is a car stereo with integrated amplifier. Even today's aftermarket stereos are 25-30W rms and their efficiencies have to be less than only a switching regulator.
My question to you guys is, how do I safely dump this kind of heat? Or better yet, is it even something I should worry about? The circuit will go into a fairly small plastic package (let's say 3" x 3" x 1"), will be used in an automobile, and will ocassionally be picked up and held for extended periods. Would it be reasonable to use a heatsink within the case and allow holes for ventilation, or am I out of my gourd thinking I can dump that much heat?
The only saving grace I can think of is a car stereo with integrated amplifier. Even today's aftermarket stereos are 25-30W rms and their efficiencies have to be less than only a switching regulator.





RE: Heat dissipation in a circuit
I've got a camera in the lab that just happens to consume almost exactly 6W in a case that's a 1.5" cube. It's barely warm to the touch.
TTFN
RE: Heat dissipation in a circuit
It is not the watts alone that matters, it is the watt density. For the same watt output, increasing the surface area lowers the watt density. I do not know the specifics of what is tolerable, but I do know one application formula that I use all the time to explain a common practice on space heaters.
Lets say I want a strip heater in a box, and have 120V to power it. It I buy a 6" x 2" (12"sq., 2 sides = 24sq") strip, the smallest is rated 100W at 120V. I then have 100W over the 24sq" surface, or 4.17 W/sq". I know from experience that this is hot enough to cause at least 2nd degree burns. If I take a 6" x 2" heat strip rated at 250W, 240V (1.04A), the watt desity is 10.4 watts/sq". The surface is potentially hot enough to cause 3rd degree burns. The resistance of that heater is fixed at 231 ohms (R=E/I). If I connect the same heat strip to my 120V supply however, the wattage drops to 62 watts (P=Esq/R). So at the same 24sq" surface, the watt density drops to 2.6 watts/sq", mildly warm to the touch. So from this I know that 2.6W/sq" is a tolerable level. So with 8W to dissipate, if you can do something to increase your surface area to roughly 3sq", your in!
Hope that helps.
Quando Omni Flunkus Moritati
RE: Heat dissipation in a circuit
I'm looking at using a 12V regulator on a 12.5-14.5V supply line pulling 3A, give or take. So, I'm now left wondering how warm the plastic box will get if I use a small heatsink on the regulator. The power dissipated is about 5-6 times the resistor from the past, but the surface area of a TO-220 package combined with a small heatsink has got to be 10-20 times more.
Does this sound about right to you guys?
RE: Heat dissipation in a circuit
Quando Omni Flunkus Moritati
RE: Heat dissipation in a circuit
RE: Heat dissipation in a circuit
Does this sound about right to you guys?
RE: Heat dissipation in a circuit
Using such a thing dropping 8 W, should give you a heatsink temperature close to if not above that of boiling water !
That is, if you do not loose any energy in transporting the heat from the resistor/regulator to the heatsink.
A regulator (LM317 or sim.) in a TO-220 package (1.5 Amps max.) usually have something like 3 deg. C/W (junction to case) and requires you to work with an internal (junction) temperature below 125 deg C. (approx. 255 F)
If we say ambient temp. is around 40C (100 F)
(I understand we may be dealing with something for a car
then you need to look for a heatsink no smaller than 7 deg.C/W. And it will still be like putting your hand into boiling water !
A heatsink at 1 deg.C/W (quite big !) and an ambient temp. at 40C should give you a surface temp. arround 48C (118 F)
Closing in on the 'pain threshold'.
With an 8 W drop you would probably have to go to a TO-3 housing. (Internal thermal res. at maybe 1-2 deg.C/W)
I have seen 5A regulators in that housing.
The efficiency of a heatsink can, however, be dramatically enhanced by using a small fan to increase the flow of air over it. If air is allowed in and out of your plastic box that is !
Oops. Just came to think about it. Most positive linear regulator chips require a minimum voltage drop of 2.5 V in order to work propperly !
RE: Heat dissipation in a circuit
Thanks for the info, guys. I'll keep plugging away at ideas, and if any more questiosn pop up I'll be sure to post them.