I want to power a 5V microcontroller from a car's electrical system. I have tried simply using a 78L05 directly. It seemed to work. The microcontroller packed up after a few months use, but that could, of course, have been a coincidence.
Is it OK to use a simple 78L05 to drive 5V logic from a car's 12V or
24V system?If not, is there a regulator designed for such use that is (almost) as easy to use as the 78L05?
-- RoRo
The input to the regulator needs to be protected from severe transients... as high as 400V, short duration, and _negative_ transients as well.
See...
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
The automotive electrical environment is far, far nastier than a lot of people think. It's not just a 12V battery -- it's a 12V battery plus a charging system and a whole lot of electromechanical junk that was mostly designed before people invented the 78xx regulators.
So the "12V" rail varies, and can have spikes that go negative, or to +60V.
Try these things:
On the manufacturer's sites, search for regulators with "automotive" in the name.
Google "load dump", "automotive electrical protection" and similar. Ideally you'll get a fairly recent article in the trade rags, or a white paper from some semiconductor company. They'll either be touting their own stuff, or they'll be telling you how to keep their own stuff safe.
Or, if you're doing a one-off, get a name-brand cell-phone charger with USB out, and use the 5V from that...
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
It won't handle a load dump, i.e. having the battery become disconnected when the alternator is charging it, which can lead to >50V spikes. Something like an LM2931 would do, if your power level is low. Put a
1N4001 rectifier in series with its input to protect it from somebody putting jumper cables on backwards or shorting the input.Otherwise you can put a 16V PolyZen (ZEN164V130A24LS) between the power lead and your LM7805. Although the PolyZen will give you reverse polarity protection, you may still need the rectifier in series, because you can kill 78xx parts by shorting the inputs to ground, if you have a big output cap. (I'm not sure if this happens with 5V parts, but it sure does at 12 or 15V.)
With low power regulators you have to watch out for the dissipation, because you're wasting most of the power heating the regulator.
What are you using the MCU for?
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Robert: this is the sort of white paper I was talking about. And clearly I was wrong about the 60V part!
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
I don't think your problem is the 78L05; it's what gets dumped into the
78L05. Here's an STMicro AppNote describing just how bad it can get:AN2689: Protection of automotive electronics from electrical hazards, guidelines for design and component selection
( After reading that, I'm surprised that manufacturers don't have to install lightning rods in these things. )
Enjoy...
Frank McKenney
-- Some 'advanced Thinkers' are of opinion that anyone who differs from the conventional opinion must be in the right. This is a delusion; if it were not, truth would be easier to come by than it is. There are infinite possibilities of error, and more cranks take up unfashionable errors than unfashionable truths. -- Bertrand Russell
I learned the hard way about these transients clear back in the early '60's... had an alternator regulator come back from field trials where the control chip had been melted into a sphere... best guess was the alternator had arced at 60A/400V ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Gotta use some protection diodes. I just use a 1N400X and an input blocker. You will not need one across the regulator if you do that *and* your regulator will still get enough Vin to regulate properly.
Recom make some nice switchers, as easy to use as the 7805, and designed for automotive use. IMHO you should still use a tranzorb and series diode on the input to take out some of the real nasties.
-- Regards, Adrian Jansen adrianjansen at internode dot on dot net Note reply address is invalid, convert address above to machine form.
It can be OK for 12V but you should have some series resistance preceding the overvoltage protection. Since you said 78L05 I imagine you are only drawing a few tens of milliamps. I suggest as a starting point something like a 0.5A-1A fuse or polyfuse (to prevent disasterous overcurrent) and series diode (to protect against reverse polarity events) a series resistor of 39ohm rated at 2W+ then a 5watt zener of about 22V-27V to ground and a large capacitor to ground of about 1000uF rated 35V. The capacitor absorbs transients, the zener clamps them to below the 78L05 max input rating and the series resistance protects the zener and capacitor. This will withstand only short duration overvolts but vastly better than what you had before :)
I doubt a 78L05 is wise for a 24V system. The max input volatge rating is only 30V or 35V and that is way too close.
piglet
Am Donnerstag, 7. Mai 2015 17:18:20 UTC+1 schrieb Robert Roland:
I dont know how much power you need, but NCV4264 is easy, cheap and robust for automotive applications. A diode and a 220uF on the 12V input line helps.
Phil,
Thanks for this morning's education about the PolyZen.
That will be useful around the lab.
Steve
this will drop some voltage (and get hot depending on the current). Much better something like this:
(or not?)
Bye Jack
-- Yoda of Borg am I! Assimilated shall you be! Futile resistance is, hmm?
Hmmmmm? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
Polyfuse, unipolar transzorb, capacitor will handle all cases.
The radial polyfuses are better than the surface-mount ones.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
You could also just use one of the automotive rated regulators that are already protected from load dump and spikes.
PolyZens are the bomb. The TVS heats up the polyswitch, so it switches cleanly and quickly.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Fri, 8 May 2015 13:34:10 -0400, WangoTango Gave us:
Examine how modern advanced circuits are made. They regularly use small "POL" (point of load) or POU Point of use supplies as small daughtercards on the design.
And what is "packed up"? Is that like "took a hike"?
Look at the elements they incorporate. It usually isn't a decades old
78L05.It depends how clean you want the supplied result to be..... and the efficiency as well. And the ability to withstand abuse of the type one might find on an automotive cigarette lighter socket".
Such at thing would be quite useful, if it existed.
True, at 50mA the resistor will dissipate 100mW but that is heat that would otherwise be generated in the 78L05.
The link you posted has a reverse polarity protection circuit which does nothing to protect against overvoltages.
I was trying to offer the OP a solution using universally available low-tech components that he can add to his existing 78L05 unit.
piglet
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