Hi

I have a 7805 which I am using as a Voltage regulator to drive a 7x5 dot matrix display. (There are 3 of these).

I measure roughly 275mA DC and 75mA AC going into the regulator.

What I am seeing (and smelling) is that the regulator is getting very hot.

Is this normal?
I dont know anything about heat sinks- can someone please help me?

Thank you.
a.

 

Linear regulators burn up all the over voltage as heat.

Thats why they absolutly need an heatsink to get rid of the heat.

A crude heatsink can be an thick sheet of aluminium or coper.

You probobly have one laying around.If not you can buy one.

Just screw the regulator tightly to the hetasink.If the regulator is still overheting wne the heatsink is not so hot then this is too much for it.If the whole heatsink is getting hot then you need an biger heatsink.

Why do you need to regulate voltage to an LED display anyway? You can just use an resistor.

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Il give you shocking experience.

 

Hi Someone

Thank you for your reply. I shall try and solder on a copper/ aluminium plate.

So if my regulator is say 5V and I supply it say 6V then the 1V and the associated current will be dissipated as heat?

Also if the transfomer I use before the regulator is designated as 500mA, then can the regulator supply more than 500mA? (i.e. can it convert the excess voltage to current like a transformer?)

Thank you, and
Regards
a.

 

Hello,

That is correct, the difference between the input and the output will be dissipated as heat by the regulator, that means that as higher is the difference as hotter the regulator will be (more watts it has to dissipate).

Usually you need a minimum input voltage to have a correct output voltage, I guess 2 o 3v difference so the example of 6v would not be correct if you want to get the 5v at the output.

If the secondary of the transformer can only create 500 mA you will get it as a maximum current also at the regulator, not possible to create current.

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Regards from Mallorca.
You´ll never know enough...

 

You should bolt it on, the 7805 is designed to be bolted to a heatsink.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Regarding your question about the transformer being able to deliver more than 500 ma:

The rating of the transformer suggests it is intended to deliver 500 ma under certain conditions. Those conditions might be in terms of duty cycle, temperature, voltage or other things. It does not mean that the transformer won't deliver more than 500 ma for a period of time unless there is some type of limiting device or circuitry.

As already stated, in some configurations and likely the configuration you described, you can't get more current out than you put in. What you can do, if you have enough total input power, is use a more elaborate supply to deliver the current and voltage you need. You can't do this with a 7805 alone.

An example might be: you need 5 volts at 0.5 amps - 2.5 watts. If your design were to be 80% efficient (certainly possible) you'd need 3.125 watts input power. If you had 24 vdc as a source the current requirement would be 0.13 amps.

So, in a way it's possible to do what you are thinking just not so directly.

Good luck.

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stevez

 

And the higher the input voltage to the 7805 and the higher the load current, the bigger the heat sink will have to be. In a worst-case scenario, at a full amp output with a 45 volt input, you'll be expecting the 7805 to dissipate 40 watts of heat. That's a bunch of heat in that small package, as attested to the fact that you don't run around just grabbing a 40 watt soldering iron by the tip. The point here is that you can't always use a simple clip-on heat sink to do the job, especially in heavier-duty cases.

Dean

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

when I don't have a 'proper' heatsink laying about, and don't feel like ordering one for a temporary project, I use some pennies (1 cent pieces):

3 pennies
1 machine screw that was just laying around
1 nut for said screw also just laying around
Small washers, again, just laying around

drill the pennies through the center, stack them on the screw, using washers to space the pennies a bit, then just bolt the heatsink onto the vreg

i'm sure a copper plated zinc heatsink isn't the best for efficiency, and I think it is illegal to drill holes in pennies, but I don't care about either point when I just need a quick sink. If some goonies from the Mint show up, well then one of you must have ratted me out!

 

Well it's illegal in the UK as well (but most things are!), it's called "defacing coinage of the realm" - you occasionally see coins made into jewelry, but these use either foreign coins, or discontinued UK coins.

But small coins make good washers if you drill a hole through the centre, and often cheaper than buying a washer.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

They made it illegal to deface coinage of the realm because the queen didnt like holes been drilled in her face.

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Top o, the morning te ya.