Power ratings

[GregH2]

Hi All,

I have some guidelines I'm hoping you can help with.

For 1/4 to 1/2 watt metal film resistors and 600mW TO92 transistors, what is the recommended maximum dissipation for actual use in a practical circuit?

I was thinking I would probably be OK if I stayed below 50% of the rating (so no more than 125mW for a 1/4 resistor and no more than 300mW for a 600mW TO92 transistor. Does that sound OK? Too conservative? Too liberal?

How hot will the devices get at these levels of dissipation (room temp, warm to touch, too hot to touch, etc)?

Your advice is appreciated!

Greg.

[Arty]

no clue on what others stick to, i tend to not stress anything more than 75% rated.

[Boofers]

With resistors I generally go with a power rating 4-5 times more than what they'll be dissipating.
This is more important as power goes up like 5W and up.
With small power resistors like 1/2W you might get away with 50%... best to put it on a breadboard and find out.

I like to go by a general rule that you should be able to hold your finger on a resistor without too much pain.
I use my less calloused fingers for this because they are more calibrated lol.

If you do have hot resistors try to keep them away from electrolytic caps.
The heat will degrade the cap over time.

As for your TO92 transistor I would go with even less heat.
Personally I would try to keep the transistor warm at most, definitely not hot.
You can get some pretty neat little TO92 clip-on heatsinks.

[Arty]

btw, does not transistors have a datasheet with thermal characteristics?

[AndrewT]

I would use peak dissipations at <50% of maximum rating.
That usually limits continuous dissipations to <25% of maximum rating.

Devices running at 25% of maximum rating will run more than warm.

Where the change in parameters with temperature will impinge on performance, I go even lower <10% for continuous dissipation.

Look up a datasheet for a To92 device, the data and graphs allow one to predict the junction temperature at any dissipation.

eg,
a 625mW To92 device with Tjmax=150degC will have Rth j-a of 125C/625mW = 0.2C/mW.
If the total heating inside a chassis brings the ambient temperature up to 40degC then Tj @ 25% dissipation (312.5mW) is ~71degC. The maximum dissipation has fallen to ~500mW, leaving your peak dissipation limit @ <250mW.

[godfrey]

Something to bear in mind: With to92s, the rated dissipation is normally dependent on some assumption about how it's mounted. There's normally something about that in the datasheet. Since much of the heat is (or should be) dissipated through the leads, max power will be lower e.g. for an air sculpture than when soldered to thick PCB tracks.

And now for something contentious: Most BJTs have much higher current gain at elevated temperatures so it seems to me that running them as hot as possible might give the best performance wrt certain parameters e.g. bias and offset currents for the input stage of a power amp.

Heh, heh - flame suit on.

[GregH2]

Great,

Thanks everyone. This helps.

Greg.

[Boofers]

Originally Posted by godfrey

And now for something contentious: Most BJTs have much higher current gain at elevated temperatures so it seems to me that running them as hot as possible might give the best performance wrt certain parameters e.g. bias and offset currents for the input stage of a power amp.

Heh, heh - flame suit on.

You got me wondering about this. I have read a lot about higher bias currents being better for performance, but temperarture?
I found this thread: Transistor and Temperature

[woody]

I have a similar question about power ratings. I just bought a J105 jfet from Siliconix
with a 360mw rating but I find that Fairchild list it as a 625mw and both are for the same to92 package. I am thinking about using verry low temp. solder and keeping the
leads verry short will help transfer some of the heat to the circuite board traces?

[digits]

Look at resistor datasheets, the info is clear...
for the 1/4W , 50% power would equate about 8C rise in temp....
You can check the other one yourself.