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'[EE]: Regulator shorting?'
2003\04\25@161229 by stanton54

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I am working on a circuit that uses an Atmel AVR in a speed controller
for an R/C airplane. The AVR and R/C receiver are powered by an
LP2986IM 5 volt linear low dropout regulator.

Once the battery gets down to 6 volts it's supposed to cut off the
motor. When I tried it the motor stopped, the red light went on, and
the regulator started smoking! It draws about 400 mA which is the
short-circuit limit for the regulator. It gets very hot and I can't
leave it on more than 1 or 2 seconds. Also the output voltage rises
to 5.7 volts, which is (or close to) the input voltage (and enough to
possibly wreck the AVR). The strange part is it's fine until the voltage
drops under 6 V or so. (I raised the cutoff setting and it was fine).

Does anyone have any idea what's wrong with this thing? Right now I'm
afraid to test it too much; everything is SMT and I don't really want
to build another one.

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2003\04\25@163556 by Bob Blick

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Is it possible the circuit is turning the motor on and off repeatedly?
That could generate all sorts of mayhem, especially if you have no
provision made for catching released energy from the motor. At the very
least you should probably have a catch diode across the motor. Also you
may need some bulk capacitance across the battery voltage.

More description of the circuit or a snip of the schematic would help
further diagnosis.

Cheerful regards,

Bob

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2003\04\25@163805 by David Minkler

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Hi,

It'll be easier to help with some kind of schematic.

Dave

stanton54 wrote:
> ...
> Does anyone have any idea what's wrong with this thing?

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2003\04\25@171254 by stanton54

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I don't think that's the problem since it works fine as long as the
battery stays above ~ 6 V. I can speed up and slow down the motor and
turn it on and off and everything works fine. There is a diode going
backwards across the motor and 3 capacitors, plus a 10 uF cap right
next to the regulator. I also tried unplugging the motor as soon as it
shut off but the regulator still started getting hot.

There is another regulator on the board, an LM2940, which interestingly
seems to be OK. I don't think anything hooked up to the 2986 is causing
trouble - the AVR stays cool and the R/C receiver isn't bothering it.

I will draw a schematic as soon as I finish downloading Eagle.

Bob Blick wrote:
{Quote hidden}

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2003\04\25@181559 by stanton54

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part 0 44 bytes
his is a multi-part message in MIME format.
part 1 648 bytes content-type:text/plain; charset=us-ascii (decoded 7bit)

Not the tidiest schematic I'm afraid... Eagle and I don't get along
very well.

The AVR (not shown) is connected to the 5 volt supply and has a couple
of 0.1 uF capacitors. One ADC goes to the battery through a voltage
divider (5.6K and 3.3K resistors plus a capacitor). It also has 2 LEDs,
the PWM output to the MOSFET, and an interrupt input from the R/C
receiver.
There is also another regulator (an LM2940) but it's not connected to
anything right now except a capacitor.

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part 2 18964 bytes content-type:image/gif; name="esc.gif" (decode)


part 3 2 bytes
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2003\04\25@184750 by Ned Konz

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On Friday 25 April 2003 03:13 pm, stanton54 wrote:
> Not the tidiest schematic I'm afraid... Eagle and I don't get along
> very well.
>
> The AVR (not shown) is connected to the 5 volt supply and has a
> couple of 0.1 uF capacitors. One ADC goes to the battery through a
> voltage divider (5.6K and 3.3K resistors plus a capacitor). It also
> has 2 LEDs, the PWM output to the MOSFET, and an interrupt input
> from the R/C receiver.
> There is also another regulator (an LM2940) but it's not connected
> to anything right now except a capacitor.

Do you have a diode or snubber across the motor?

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2003\04\25@185205 by Tom Messenger

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Check your circuit for the condition noted on the data sheet regarding the
output voltage EVER getting higher than the input voltage.  If this
happens, a parasitic SCR gets turned on in the LP2986 and it will then get
very hot.  The data sheet tells how to avoid this.  So even if you don't
think that is the problem, try the schottky from Vin to Vout (cathode to
Vin) as the datasheet suggests. Make sure to put it RIGHT on the regulator
with shortest leads possible.

Good luck!
Tom M.

At 06:13 PM 4/25/03 -0400, you wrote:
{Quote hidden}

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2003\04\25@185930 by stanton54

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There is a 1N4004 and 3 capacitors, one between the terminals, and one
from each terminal to the case. I checked and the diode is still OK (I
did blow one up once; I tried to get away with using one that was too
small).

Ned Konz wrote:
{Quote hidden}

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2003\04\25@190351 by stanton54

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Thanks for the suggestion, I will try this. Maybe all the big
capacitors are causing the problem - the input drops down but the
5V line stays high for a bit, a little noise from the motor gets in
the wrong spot, and zzzt!

Tom Messenger wrote:
{Quote hidden}

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2003\04\25@200824 by Bob Blick

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Tom Messenger said:
> Check your circuit for the condition noted on the data sheet regarding
> the output voltage EVER getting higher than the input voltage.  If this
> happens, a parasitic SCR gets turned on in the LP2986 and it will then
> get very hot.  The data sheet tells how to avoid this.  So even if you
> don't think that is the problem, try the schottky from Vin to Vout
> (cathode to Vin) as the datasheet suggests. Make sure to put it RIGHT on
> the regulator with shortest leads possible.

That gets my vote too!

Also, the motor diode should be a fast diode if you are doing PWM... A
schottky is a good choice there, based on the voltages you are using.


-Bob

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2003\04\26@073707 by Olin Lathrop

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> Not the tidiest schematic I'm afraid... Eagle and I don't get along
> very well.

By the way, in the schematic editor you can export the schematic to an
image file.  Unfortunately this only makes a BMP file, but you can use
just about any image manipulation program to convert it to a GIF, which is
a good choice for such line art.  That way you can spare us the Eagle "+"
signs and stuff that make the schematic hard to read, not to mention avoid
wasting the bandwidth on the window borders.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\04\26@081731 by Ofer Saferman

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Hi,

I think that when you go below 6V something maybe starts to oscillate and thus causes all the problems.
Make sure you have at least 2.2 uF cap on the regulator input, as stated in the data sheet otherwise the regulator
may be unstable.

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On 25/04/2003 at 17:11 stanton54 wrote:

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~~~~~~~~~~~~~~oOo~~~~~~~~~~~~~~~
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Phone: +972-4-8642775
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2003\04\26@124403 by Ned Konz

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On Saturday 26 April 2003 04:35 am, Olin Lathrop wrote:
> > Not the tidiest schematic I'm afraid... Eagle and I don't get
> > along very well.
>
> By the way, in the schematic editor you can export the schematic to
> an image file.  Unfortunately this only makes a BMP file, but you
> can use just about any image manipulation program to convert it to
> a GIF, which is a good choice for such line art.  That way you can
> spare us the Eagle "+" signs and stuff that make the schematic hard
> to read, not to mention avoid wasting the bandwidth on the window
> borders.

Actually, at least in the version I have it makes PNG files, which are
even better than GIFs.

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2003\04\27@123456 by stanton54

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I tried adding an ordinary diode but as I expected it didn't work. The
local RadioShack doesn't have any Schottky diodes (since that isn't a
brand of cellphone ;) ) so I won't be able to get any for a while. For
now I've 'solved' the problem by adding a separate 9V battery to run
the electronic bits. Hopefully the diode will fix things and I'll be
able to ditch the extra weight.

Olin - thanks for the Eagle tip, I will try that next time.

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2003\04\27@174941 by Peter L. Peres
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>By the way, in the schematic editor you can export the schematic to an
>image file.  Unfortunately this only makes a BMP file, but you can use

I was under the impression that you can run the cam processor on the
schematic and export as whatever you like. But I have not used Eagle
enough yet. It is worth a try. I have chosen the eagle->cam[Post
Script]->ghostview->pdf way and I keep to it. Digital print shops take pdf
and once you make them understand that 'shrink to fit' and 'expand to fit'
must be ticked OFF int eh print options, and that pdf files do not damage
Macs, they will do the necessary. Registration is very good on film at A4
size.

Peter

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2003\04\27@184100 by William Chops Westfield

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   I was under the impression that you can run the cam processor on the
   schematic and export as whatever you like.

The CAM processor is a sort of object-oriented output utility for getting
data to board houses, automated CNC equipment, and so on.  It only works on
the board, and it is not the same as the image export capability (for
instance, it generally lacks color, as far as I know.)
The image export feature does different and also useful things.  It
seems to default to BMP files at several points, but apparently you
can override it to make a .PNG instead...

BillW

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