7805 OK to get hot (do I need a heatsink)

Don, I have left your comments intact because they need repeating here once a week and even on Sci Electronics Design fairly often.

There are some people here (I wont boost their ego by calling them engineers, tho some may be.)who always want to suggest the latest and greatest design idea to somone like the current OP who just needs a little help with his next faltering step.

I have been fixing electronic things from WWII aircraft radios thru guided missiles and on to computers so I consider I have a lot of experience. 60 years and schoollboy stuff before that.

I would not attempt under any circumstance to build an SMPS from the description offered above. I am quite capable of building things but to be told and SMPS will fix the problem is laughable.

Difficult and very dangerous for a beginner.

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John G

My latest project has three SMPSs on it (DC-DC buck regulators), and each of them is about the same size as a TO-220 part. Part count ranges from 4 to 8 parts, compared to 3 for a linear regulator (including in/out capacitors). The SMPSs do not even get warm to the touch (8-16v in, 5V@1A max, 3.3/1.2v@0.5A), compared to skin-blistering temperatures of a 7805. I wonder how many newbies blindly assumed they could draw 1 amp from a 15v supply through a 7805, and either hurt themselves or damaged the parts?

With todays SMPS chips and wide range of linear regulators, "difficulty" is on par with linears, and "dangerous" is relative.

I see no reason why newbies should avoid SMPSs "just because", and in cases where a linear would generate enough heat to be dangerous or damaging, a switcher should be considered.

I bet they didn't say what the cost (in kilowatt hours) was to manufacture and ship all those power supplies was.

For many applications, wholesale, mandated replacment makes no economic or environmental sense at all. In particular, low use applications like electric razors that are recharge for an hour or two per week and wall warts associated with short life consumer products such as iPods. In those cases you don't "force" any change to existing products, you just make a requirement for new products to have compliant power supplies.

If you really wanted to save energy then you (as a government) would mandate a few standard power supply types (I am talking about wall wart and external computer/computer peripheral power supplies). You would allow exceptions for a unit price based fee, and require the power supply to be provided only as a *separate* item.

Slick! Thanks for the link. That will definitely come in handy.

Much less than the equipment they were shipped with.

Right. I bought a S5200 digital camera about five years ago. the power supply was sold seperately. I hasn't been in stock since then. I finally gave up. It is a 5 V 2.5A supply with a coaxial connecter for the camera. It was also in the $50 range.

Selling the power supply as a seperate item would stop most people from buying a product.

--
For the last time:  I am not a mad scientist, I'm just a very ticked off
scientist!!!

Okay, do the math. If you have 12 volts (you should measure it) coming in to the 7805, 5 volts going out of the 7805, and .17 amps, then the power it has to dissipate is current times voltage, i.e. 7 times .17 for 1.2 watts. And as someone else pointed out, an unregulated wall wart likely puts out more than 12 volts. Your 7805 could be dissipating something approaching a couple of watts, which is a lot. You can get rid of the problem easily by inserting a power resistor between the wall wart and the 7805. Measure the wall wart voltage and calculate the resistor value you need to result in 8 volts at the 7805 input. Then the chip will only have to dissipate 3 times .17 or about half a watt, which it can do even without a heatsink. The 7805 needs two or three volts headroom, so to caculate the power resistor, take the wall wart voltage, subtract 8, and divide by .17. Let's say your wallwart puts out 13 volts when running the device (NOT the open circuit voltage). So you need a resistor that drops 5 volts at .17 amps. That's about 30 ohms. Don't forget bypass caps for the chip. Also, the power resistor will have an effect on the regulator's ability to provide surge current, so you might want to upsize the output cap.

n

so

(starting with 15VDC regulated power)...

A heatsink, or a series resistor from +15 to the regulator input, are indicated. If it's too hot to touch, it could melt the plastic enclosure...

If you're sure full current is under, say, 0.25 A, the addition of a

27 ohm, 2W resistor will move most of the heat dissispation from the regulator IC to the power resistor. Such resistors are less expensive than good heatsinks...

Really, I didn't have to sign on.

I put some numbers in there and it started the app, did the cals, and presented me with some results and a list of parts.

Maybe you were going elsewhere?

Jamie

Yes, I got the same stuff. Then I picked a specific line item and asked for a design to be completed with THAT particular part. That's when the fun happened.

I quoted the message I got.

Jon

snipped-for-privacy@att.bizzzzzzzzzzzz:

Apart from replacing the wall wart with a nowadays inexpensive 5V switchmode wall wart and throwing away all the other crap? ;-)

Hey! No fair thinking outside the box!

What box? People are lucky to even have a plastic bag, these days.

--
For the last time:  I am not a mad scientist, I'm just a very ticked off
scientist!!!

Michael A. Terrell:

Unless they live in Italy. From today on, distribution of plastic bags is prohibited in Italy. Only paper or bio-degradable plastic (a junk made with corn) bags are allowed. As if these were biodegradable, while you can find, in the dumps, 50 years old newspapers still perfectly readable.

I was puzzled why our government would do such a stupid thing. The answer is easy: it's a law of the previous leftist government, whose application was postponed for two years, this year they had other reforms to do and did not care to defer it for another year.

They're the government. That's what they do!

I wish they'd decide they've had enough "reforms" and have passed enough laws, so just stay home for a few years. The rest of us might catch up.

snipped-for-privacy@att.bizzzzzzzzzzzz:

LOL. But leftist governments are much better at screwing up things.

In fact the reform of italian universities tries to erase 30 years of damage done by the sinisters to gain control of education.

In fact this reform has been a restoration to the previous rules.

Yes, we see that too.

At least you're trying.

Maybe we'll learn some day.

I don't think they found fragments of paper bags collected by ocean currents - see "pacific garbage patch"

a landfill is like a swamp, anything thrown in there is deprived of oxygen, cellulose will last indefinately - see "bog oak"

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?? 100% natural

Jasen Betts:

But also few plastic bags. Most of it is plastic bottles.

Since the rivers that flow into the Mediterranean bring less water than what evaporates, there is a strong incoming current, so none of our garbage can get out, it can only get in. Despite of that, and of the fact that about 100 million people live in it's drainage basin, we have nothing like it in our sea.

And probably neither you: "The size of the patch is unknown, as large items readily visible from a boat deck are uncommon. Most debris consists of small plastic particles suspended at or just below the surface, making it impossible to detect by aircraft or satellite."

It smells like ozone hole (have you noticed that, once we have been forced to change refrigerators, AC, and PCB wash, it mysteriously disappeared from headlines?) or global warming.

Exactly. The alternative, incinerators, burns plastic bags equally well. So, what's this distinction for? As usual, greens (which supported our previous administration) do things they don't understand just for the sake of doing something.

page 9, figure 2

A unattributed quote from someone claiming ignorance is evidence of what exactly?

That's because the ozone layer shows signs of recovering, good news doesn't sell papers,

Do You think there is only one alternative, or is this a straw man?

Most opposition to plasic bags isn't about landfills anyway, it's about pollution.

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?? 100% natural