l7805cv too hot!

[Quack]

Hi,

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

The problem is the 7805 gets very hot! more than enough to burn you
finger on.
Sometimes even cuts out (built in high temp. cutout).

The strange thing is, when removing the 7805 and measure with a amp
meter, i am only drawing upto 150Ma.

This particular model of 7805 is rated for upto 1.5amp (heatsink
required after 1 amp). I am using a heatsink too.

My question is, apart from the input voltage being too high, OR the
drawing current being too much, what else can make a 7805 get too hot
?

The circuit is running 2 pic 16f628's @ 16mhz, and a SX28 @ 50mhz, and
a few led's.

-I have ruled out too much current by removing the 7805 and using an
external 5v source and measuring current draw. (upto 150ma).

-I have checked the input voltage is between 10 and 11 volts, well
within normal limits.

-I have carefully read the specifications on this particular 7805 in
the datasheet to confirm the above limits

-I have ruled out a faulty 7805 because i have many of them, and using
a few of them randomly present the same problem.

-I have thoroughly cleaned the board of any flux residue which i
thought could create some kind of short.

-I have carefully inspected for any other shorts (in which case the
current draw in test#1 would have been more anyway)

-I am using a heatsink, even though it should not be necesary

-I use these same 7805's in another circuit, drawing a CONSTANT 450ma,
with no heatsink, and they dont even get warm.

-Removing the PIC chips from their sockets makes no difference, in
which case all that is left is a SX chip running @ 50mhz and 3 few
leds.

-This all happens even when the circuit is disconnected from any
external connections (PC serial etc).

I noticed on the SX chip development board, their regulator (LM294OCT)
gets unexpectadly warm also - is there something about SX chips i am
missing ?
I know the I/O can be set to CMOS or TTL levels, i think mine are
configured correctly - but i dont have much experience with SX chips
(this is my first SX based design).


I am out of ideas

Any suggestions of what to try/look for ?

Thanks!,

Alex.

 

[Ian Stirling]

Hi,

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

The problem is the 7805 gets very hot! more than enough to burn you
finger on.
Sometimes even cuts out (built in high temp. cutout).

The strange thing is, when removing the 7805 and measure with a amp
meter, i am only drawing upto 150Ma.

This particular model of 7805 is rated for upto 1.5amp (heatsink
required after 1 amp). I am using a heatsink too.

Disconnect load.
Connect 30 ohm resistor in place of load.
Does it get cooler?
If so, the 7805 is oscillating, and you need to pay more attention
to bypass caps.
Measure the input current too.

 

[Spehro Pefhany]

Disconnect load.
Connect 30 ohm resistor in place of load.
Does it get cooler?
If so, the 7805 is oscillating, and you need to pay more attention
to bypass caps.
Measure the input current too.

The 7805 doesn't care one whit about output caps; in fact I've never
ever seen one oscillate, but if I was to *try* to get one to
oscillate, I'd leave out the *input* capacitor and put an inductor or
a long bit of wire in there.

The described situation (~175°C Tj) would require several watts of
dissipation in a TO220. Measuring the input current of the regulator,
as Ian suggests, should indicate that the input current is much higher
than expected. Either that or the input voltage is more like 25V than
11V.

Best regards,
Spehro Pefhany

 

[Si Ballenger]

Hi,

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

The problem is the 7805 gets very hot! more than enough to burn you
finger on.
Sometimes even cuts out (built in high temp. cutout).

The strange thing is, when removing the 7805 and measure with a amp
meter, i am only drawing upto 150Ma.

This particular model of 7805 is rated for upto 1.5amp (heatsink
required after 1 amp). I am using a heatsink too.

My question is, apart from the input voltage being too high, OR the
drawing current being too much, what else can make a 7805 get too hot
?

The circuit is running 2 pic 16f628's @ 16mhz, and a SX28 @ 50mhz, and
a few led's.

-I have ruled out too much current by removing the 7805 and using an
external 5v source and measuring current draw. (upto 150ma).

-I have checked the input voltage is between 10 and 11 volts, well
within normal limits.

-I have carefully read the specifications on this particular 7805 in
the datasheet to confirm the above limits

-I have ruled out a faulty 7805 because i have many of them, and using
a few of them randomly present the same problem.

-I have thoroughly cleaned the board of any flux residue which i
thought could create some kind of short.

-I have carefully inspected for any other shorts (in which case the
current draw in test#1 would have been more anyway)

-I am using a heatsink, even though it should not be necesary

-I use these same 7805's in another circuit, drawing a CONSTANT 450ma,
with no heatsink, and they dont even get warm.

-Removing the PIC chips from their sockets makes no difference, in
which case all that is left is a SX chip running @ 50mhz and 3 few
leds.

-This all happens even when the circuit is disconnected from any
external connections (PC serial etc).

I noticed on the SX chip development board, their regulator (LM294OCT)
gets unexpectadly warm also - is there something about SX chips i am
missing ?
I know the I/O can be set to CMOS or TTL levels, i think mine are
configured correctly - but i dont have much experience with SX chips
(this is my first SX based design).


I am out of ideas

Any suggestions of what to try/look for ?

You probably have a short, bad resistor, miswiring, or some such
in your circuit. Keep looking and sooner or later you will find
it.

 

[Mook Johnson]

You're dissipating close to one watt (6V drop * .15). Even with a heat
sink, the part will get hot if it is not well ventillated (enclosed or stuff
around it blocking air flow). Do you have the circuit enclosed? How
massive is your heatsink? You have to check the C/W rating of the heatsink,
then you have to figure out (model or measure) how hot the inside of your
enclosure gets with a certain heat load.

 

[Andrew Holme]

Hi,

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

The problem is the 7805 gets very hot! more than enough to burn you
finger on.
Sometimes even cuts out (built in high temp. cutout).
[snip]

I am out of ideas

Any suggestions of what to try/look for ?

It could be oscillating. Have you checked for RF on the power
supplies? You don't mention decoupling capacitors in your post. Are
you using any? I use tantalum 2.2uF very close to the regulator
input and 100n mono ceramic very close to the output. I also use 100n
mono ceramic for each IC. I would avoid long signal / power runs and
pay due attention to layout / grounding.

 

[Frank Bemelman]

[email protected] (Quack) wrote in message

Hi,

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

The problem is the 7805 gets very hot! more than enough to burn you
finger on.
Sometimes even cuts out (built in high temp. cutout).
[snip]

I am out of ideas

Any suggestions of what to try/look for ?

It could be oscillating. Have you checked for RF on the power
supplies? You don't mention decoupling capacitors in your post. Are
you using any? I use tantalum 2.2uF very close to the regulator
input and 100n mono ceramic very close to the output. I also use 100n
mono ceramic for each IC. I would avoid long signal / power runs and
pay due attention to layout / grounding.

Hey, tantalums on power rails are 'verboten'. We figured that out
a couple of weeks ago

100n on the input, 330n if you insist. I never had an Lx78xx oscillating
in my entire life (on a breadboard, my pcb's always have a small cap).
Is it an urban legend? A well kept secret between manufacturers of
regulators and capacitors?

 

[John Woodgate]

I read in sci.electronics.design that Si Ballenger

You probably have a short, bad resistor, miswiring, or some such in
your circuit. Keep looking and sooner or later you will find it.

What happens to a 7805 if you ground the output pin and connect the load
to the ground pin?

 

[Ian Stirling]

The 7805 doesn't care one whit about output caps; in fact I've never
ever seen one oscillate, but if I was to *try* to get one to
oscillate, I'd leave out the *input* capacitor and put an inductor or
a long bit of wire in there.

Me neither.
Maybe the OP is especially skilled though.

 

[Neil]

100n on the input, 330n if you insist. I never had an Lx78xx oscillating
in my entire life (on a breadboard, my pcb's always have a small cap).
Is it an urban legend? A well kept secret between manufacturers of
regulators and capacitors?

I had a 7812 oscillate. It was in a bit of test kit while doing an EMC test
on another unit, so at least there was the right equipment around to
diagnose where the interference was coming from.
Putting a couple of 0.1uF across input-ground and output-ground pins cured
it.
Neil

 

[CFoley1064]

Subject: l7805cv too hot!

From: [email protected] (Quack)
Date: 9/18/2004 4:11 AM Central Daylight Time
Message-id: <[email protected]>

Hi,

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

The problem is the 7805 gets very hot! more than enough to burn you
finger on.
Sometimes even cuts out (built in high temp. cutout).

The strange thing is, when removing the 7805 and measure with a amp
meter, i am only drawing upto 150Ma.

This particular model of 7805 is rated for upto 1.5amp (heatsink
required after 1 amp). I am using a heatsink too.

My question is, apart from the input voltage being too high, OR the
drawing current being too much, what else can make a 7805 get too hot
?

The circuit is running 2 pic 16f628's @ 16mhz, and a SX28 @ 50mhz, and
a few led's.

-I have ruled out too much current by removing the 7805 and using an
external 5v source and measuring current draw. (upto 150ma).

-I have checked the input voltage is between 10 and 11 volts, well
within normal limits.

-I have carefully read the specifications on this particular 7805 in
the datasheet to confirm the above limits

-I have ruled out a faulty 7805 because i have many of them, and using
a few of them randomly present the same problem.

-I have thoroughly cleaned the board of any flux residue which i
thought could create some kind of short.

-I have carefully inspected for any other shorts (in which case the
current draw in test#1 would have been more anyway)

-I am using a heatsink, even though it should not be necesary

-I use these same 7805's in another circuit, drawing a CONSTANT 450ma,
with no heatsink, and they dont even get warm.

-Removing the PIC chips from their sockets makes no difference, in
which case all that is left is a SX chip running @ 50mhz and 3 few
leds.

-This all happens even when the circuit is disconnected from any
external connections (PC serial etc).

I noticed on the SX chip development board, their regulator (LM294OCT)
gets unexpectadly warm also - is there something about SX chips i am
missing ?
I know the I/O can be set to CMOS or TTL levels, i think mine are
configured correctly - but i dont have much experience with SX chips
(this is my first SX based design).


I am out of ideas

Any suggestions of what to try/look for ?

Thanks!,

Alex.

Hi, Alex. You've gotten a lot of really good advice here. Might I suggest one
other more obvious area to take a look at?

Remove the heat sink from the TO-220 device. If there's heat sink compound,
use your finger to rub it around and see if there's a piece of crud in there.
If there's no heat sink, look around for a piece of crud, anyway. Look at and
feel the mating surfaces of the heat sink and the TO-220 for burrs or other
irregularities that prevent a good mating surface. And examine the heat sink
carefully for warping that may have bent it (this is particularly common if
it's a home brew heat sink or if you drilled your own hole). At somewhere
between 1 and 2 watts dissipation, the TO-220 should get hot if it doesn't have
any heat sink. And a big ol' piece of crud, an aluminum burr or a warped heat
sink is just about the same as no heat sink at all.

When you replace the heat sink on the TO-220 package, use a little heat sink
compound (if you're using the silicone-based white stuff, you should be able to
barely see through the heat sink compound to the surface of the part).

Also, if all else fails, try replacing the 7805.

Good luck
Chris

 

[Ken Smith]

On 18 Sep 2004 12:08:31 GMT, the renowned Ian Stirling [...]
The 7805 doesn't care one whit about output caps; in fact I've never
ever seen one oscillate, but if I was to *try* to get one to
oscillate, I'd leave out the *input* capacitor and put an inductor or
a long bit of wire in there.

Leaving off the input side cap and having a long wire back to the
filter capacitor on the bridge recifier, works very well to make an LM7805
oscillate. Having a large output capacitor on an LM7805 and none on the
input makes it oscillate at some 10s of MHz. About 50MHz seems to be very
common.

 

[Pooh Bear]

I had a 7812 oscillate. It was in a bit of test kit while doing an EMC test
on another unit, so at least there was the right equipment around to
diagnose where the interference was coming from.
Putting a couple of 0.1uF across input-ground and output-ground pins cured
it.

Yes those 0.1uFs *would* fix it !

They're *meant* to be there. See any 78xx application note.

Never known any 78xx regulator to oscillate with the manufacturer's application
circuit.


Graham

 

[Leon Heller]

The 7805 doesn't care one whit about output caps; in fact I've never
ever seen one oscillate, but if I was to *try* to get one to
oscillate, I'd leave out the *input* capacitor and put an inductor or
a long bit of wire in there.

I've never had them oscillate either but a friend of mine managed it with a
7812, when he built a simple 12V power supply from a circuit I gave him,
many years ago. It didn't get hot, just didn't have any output to speak of.
It started working when I touched the chip, so I added a couple of
capacitors which cured the problem. His construction techniques were less
than satisfactory, which probably caused the problem.

Leon

 

[Si Ballenger]

I read in sci.electronics.design that Si Ballenger


What happens to a 7805 if you ground the output pin and connect the load
to the ground pin?

It is probably going to get really hot if the ground pin is also
connected to the - side of the supply power supply. This would be
a dead short. They will also get really hot if the supply power
supply polarity is reversed.

 

[Nico Coesel]

The 7805 doesn't care one whit about output caps; in fact I've never
ever seen one oscillate, but if I was to *try* to get one to
oscillate, I'd leave out the *input* capacitor and put an inductor or
a long bit of wire in there.

I've had one oscillating when I was still in school. I connected it
using long test leads.

Another thing to watch for is swapping the input and output. It will
still regulate a bit when connected wrong, but it will also get hot.

 

[Tim Shoppa]

I designed a circuit running on 10v, which uses a L7805CV (morocco
MCC1A0336)voltage regulator to give me 5v for my IC's and LED's.

Still don't recognize the manufacturer...

-I have ruled out a faulty 7805 because i have many of them, and using
a few of them randomly present the same problem.

Different makers of 7805's don't all make exactly the same thing. Lately
I've been highly suspicious of chips that have no recognizable maker. Some
brands oscillate in some circuits while others don't. (Actually most don't!,
but it's the ones that do that you remember!)

"Morocco" ? Parody of Motorola? (Who aren't even in the business anymore!)
I'm guessing it's really ST Micro based on the part number.

Tim.

 

[Quack]

Hi, Alex. You've gotten a lot of really good advice here.

Thankyou everyone!, it was a lack of input capacitors.
I have never seen that before, perhaps because this is a long way from
the originating 10v source (2 feet twisted pair cable), it was
oscillating as suggested i guess. (my scope broke down a couple of
weeks ago..)

Now that its mentioned, i cant believe i did not try that . Oh well,
lesson learned .

Thanks again!

Alex.

 

[Neil]

Yes those 0.1uFs *would* fix it !

They're *meant* to be there. See any 78xx application note.

Never known any 78xx regulator to oscillate with the manufacturer's application
circuit.
Graham

I guess the problem was that the caps were there, just that the guy who
built it mounted them about a foot away from the regulator.

 

[Pooh Bear]

I guess the problem was that the caps were there, just that the guy who
built it mounted them about a foot away from the regulator.

The application note mentions distance too ! ;-)


Graham