Is 3 Watts really too much to ask?

[Cursitor Doom]

Gentlemen,

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.
Apart from advising me to permanantly give up electronics, has anyone got
any constructive suggestions to make here?
TIA.

 

[Cursitor Doom]

Sorry, forgot the link:

https://yandex.com/collections/card/5e9c89c2a12f4c16130e5aa8/

 

[Lasse Langwadt Christense]

søndag den 19. april 2020 kl. 19.41.52 UTC+2 skrev Cursitor Doom:

Gentlemen,

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.
Apart from advising me to permanantly give up electronics, has anyone got
any constructive suggestions to make here?
TIA.

buy a usb car charger

or

https://www.digikey.com/product-detail/en/recom-power/R-785.0-0.5/945-1037-ND/2256217

https://www.ebay.co.uk/itm/253057530644

 

[artie]

In article <r7i2gr$al9$5@dont-email.me>, Cursitor Doom
<cd@not4mail.com> wrote:

Gentlemen,

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.
Apart from advising me to permanantly give up electronics, has anyone got
any constructive suggestions to make here?
TIA.

With respect, if you're using a bare 78xx part in an automotive
environment, heat dissipation is the least of your worries.

You can get around the heat issue, though, by bolting the regulator to
a largish piece of metal in the vehicle. I seem to remember bolting a
7805 to a largish metal piece in the back of my jeep many years ago.
Some heat sink goo, and it tries to get warm, but it has a lot of Jeep
to heat up.

A better solution is so use a switching converter. Look on eBay for
buck converters. I like the designs from Pololu Electronics --
reliable, well built.

Please look up "alternator load dump" and ISO 16750. The automotive
electrical environment can be quite exciting.

 

[Winfield Hill]

Cursitor Doom wrote...

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.

The L7805 won't go up in smoke, because it has Thermal Overload
Protection. Also, you can do the finger sizzle test. If saliva
on your fingertip doesn't sizzle when you touch it, it's under
100C. Since Rth-JC=5C/W, the junction temp should be under 115C,
which is under the part's 125C spec limit.


--
Thanks,
- Win

 

[bitrex]

On 4/19/2020 2:20 PM, artie wrote:

In article <r7i2gr$al9$5@dont-email.me>, Cursitor Doom
cd@not4mail.com> wrote:

Gentlemen,

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.
Apart from advising me to permanantly give up electronics, has anyone got
any constructive suggestions to make here?
TIA.

With respect, if you're using a bare 78xx part in an automotive
environment, heat dissipation is the least of your worries.

You can get around the heat issue, though, by bolting the regulator to
a largish piece of metal in the vehicle. I seem to remember bolting a
7805 to a largish metal piece in the back of my jeep many years ago.
Some heat sink goo, and it tries to get warm, but it has a lot of Jeep
to heat up.

A better solution is so use a switching converter. Look on eBay for
buck converters. I like the designs from Pololu Electronics --
reliable, well built.

Please look up "alternator load dump" and ISO 16750. The automotive
electrical environment can be quite exciting.

You could use a 78xx regulator to drop the minimum voltage and help
protect the switcher! They're very rugged.

 

[Phil Hobbs]

On 2020-04-19 14:44, Winfield Hill wrote:

Cursitor Doom wrote...

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.

The L7805 won't go up in smoke, because it has Thermal Overload
Protection. Also, you can do the finger sizzle test. If saliva
on your fingertip doesn't sizzle when you touch it, it's under
100C. Since Rth-JC=5C/W, the junction temp should be under 115C,
which is under the part's 125C spec limit.

One of George's customers managed to find a way to blow one up. He
connected a large coil to the output, which drew enough current to put
the regulator into thermal limit. When it tried to turn off, the
resulting inductive kick blew the regulator up from overvoltage.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[stratus46]

On Sunday, April 19, 2020 at 10:41:52 AM UTC-7, Cursitor Doom wrote:

Gentlemen,

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.
Apart from advising me to permanantly give up electronics, has anyone got
any constructive suggestions to make here?
TIA.

Would this help?

https://www.digikey.com/product-detail/en/cui-inc/V7805-1500R/102-2184-ND/2352143

$9 and you use it like a 7800 regulator.

G²

 

[bitrex]

On 4/19/2020 2:44 PM, Winfield Hill wrote:

Cursitor Doom wrote...

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.

The L7805 won't go up in smoke, because it has Thermal Overload
Protection. Also, you can do the finger sizzle test. If saliva
on your fingertip doesn't sizzle when you touch it, it's under
100C. Since Rth-JC=5C/W, the junction temp should be under 115C,
which is under the part's 125C spec limit.

The LM78xx is hard to kill even from _intentional_ abuse.

 

[Cursitor Doom]

On Sun, 19 Apr 2020 11:46:37 -0700, stratus46 wrote:

Would this help?

https://www.digikey.com/product-detail/en/cui-inc/V7805-1500R/102-2184-
ND/2352143

$9 and you use it like a 7800 regulator.

Hmmm. I'm seeing other suggestions here for going down the switcher
route, too. It just seems totally OTT for such a simple task. I don't
doubt a buck converter or some other DC-DC down converter topology of
some sort would work best, but I have a ton of these 5v regs which I've
had for years and years and would like to use some of them up!

 

[Cursitor Doom]

On Sun, 19 Apr 2020 11:44:09 -0700, Winfield Hill wrote:

The L7805 won't go up in smoke, because it has Thermal Overload
Protection. Also, you can do the finger sizzle test. If saliva on
your fingertip doesn't sizzle when you touch it, it's under 100C.
Since Rth-JC=5C/W, the junction temp should be under 115C,
which is under the part's 125C spec limit.

Thanks, Win! That test could be very useful. I'm still skeptical, because
although it may well pass today, it'll be a very different story come
July/August.

 

[Pimpom]

On 4/20/2020 1:41 AM, Cursitor Doom wrote:

On Sun, 19 Apr 2020 11:46:37 -0700, stratus46 wrote:

Would this help?

https://www.digikey.com/product-detail/en/cui-inc/V7805-1500R/102-2184-
ND/2352143

$9 and you use it like a 7800 regulator.


Hmmm. I'm seeing other suggestions here for going down the switcher
route, too. It just seems totally OTT for such a simple task. I don't
doubt a buck converter or some other DC-DC down converter topology of
some sort would work best, but I have a ton of these 5v regs which I've
had for years and years and would like to use some of them up!

Can you use a bigger heatsink? That little piece you're showing
is too wimpy to dissipate 3W in an adverse environment. The
regulator may not blow up but you're likely to experience shutdowns.

 

[RheillyPhoull]

On 20/04/2020 4:11 am, Cursitor Doom wrote:

On Sun, 19 Apr 2020 11:46:37 -0700, stratus46 wrote:

Would this help?

https://www.digikey.com/product-detail/en/cui-inc/V7805-1500R/102-2184-
ND/2352143

$9 and you use it like a 7800 regulator.


Hmmm. I'm seeing other suggestions here for going down the switcher
route, too. It just seems totally OTT for such a simple task. I don't
doubt a buck converter or some other DC-DC down converter topology of
some sort would work best, but I have a ton of these 5v regs which I've
had for years and years and would like to use some of them up!

Mebbe put it in an die cast box and use that as the heat sink ?

 

[Ricky C]

On Sunday, April 19, 2020 at 1:41:52 PM UTC-4, Cursitor Doom wrote:

Gentlemen,

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.
Apart from advising me to permanantly give up electronics, has anyone got
any constructive suggestions to make here?
TIA.

Remember that a 7 watt night light glows from the heat! If I get what you are saying you are using a heat sink about an inch square. That's not likely to be big enough.

But keep in mind these things are spec'd to 85°C which is nearly hot enough to boil water. The fact that you can't touch it doesn't mean so much.. Your finger doesn't like 50 °C. The device will survive much hotter temps than 85 °C.

Get a larger heat sink. Any piece of metal will do. I was going to make my own car charger and thought about using the metal of the ash tray for the heat sink.

You can also use a dropping resistor. Try a 20 ohm power resistor dropping 6 volts at 300 mA cutting the 3 watts to more like 1.2 watts. The resistor can get a lot hotter than the regulator with no ill effect. Don't keep them close though.

Here's a couple of 8 ohm parts.

https://www.ebay.com/itm/Lot-of-2-8-Ohm-5-Watt-Wirewound-Ceramic-Power-Resistors-5W/264557630772?hash=item3d98dd1534:g:TaAAAOSwyVdd6XTG

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[mpm]

Amazon:
https://www.amazon.com/s?k=Zixtec+10+Pack+LM2596+DC-DC+Buck+Converter&ref=nb_sb_noss

You can probably find these in single quantities too.
Handy to keep a few around.

 

[Martin Brown]

On 19/04/2020 19:44, Winfield Hill wrote:

Cursitor Doom wrote...

in pursuance of what is turning out to be a jinxed project (installing a
dashcam) I'm attempting to drop 14.5V (the normal battery voltage whilst
the engine is running) to 5V for the camera. The camera draws at most
300mA so the regulator (an L7805CV) shouldn't find that task too much of
a struggle, since it only has to disspate just under 3 Watts by my
reckoning. I'd have thought this reasonably large heatsink (see 20p piece
for scale comparison) would be overkill. Unfortunately it isn't, though.
Even with a low ambient temp of about 65'F it gets a bit too hot to touch
after a few minutes, so once in the car in a couple of months with a
T_amb north of 100'F, it hasn't a hope in hell of preventing the reg from
going up in smoke.

The L7805 won't go up in smoke, because it has Thermal Overload
Protection. Also, you can do the finger sizzle test. If saliva
on your fingertip doesn't sizzle when you touch it, it's under
100C. Since Rth-JC=5C/W, the junction temp should be under 115C,
which is under the part's 125C spec limit.

It does seem to be a *very* 1970's solution to a trivial problem though.
I recall having a similar problem with powering a Robert's radio in that
era. Painting the shiny aluminium box made an enormous difference.

There are countless cheap USB 5v 1A converters certified to work in an
automotive environment which is hostile when turning the engine over.

--
Regards,
Martin Brown