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From: Phil Hobbs on 9 Aug 2010 08:25
John Larkin wrote:
> On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote:
>
>> Hello, what happens when too much current is drawn from an LM317
>> configured for 3.3V output? Does the output voltage decrease?
>
> It decreases a little bit as you load it, at any current [1]. At some
> large current you hit either its current limit or its thermal limit.
> At the current limit, voltage starts to drop steeply, such as to keep
> the load current constant. If it get too hot before that happens, most
> 317s will just shut off until they cool down some.
>
> Right now, we really need a low-voltage-drop current limiter, to
> protect a circuit powered by a dc/dc converter. Something like 150 mA
> would be good. Polyfuses suck. Discrete circuits, like the things
> James was playing with, would work, but I don't have room for all the
> parts. We're considering using an adjustable LDO and running it
> wide-open, so only the current+thermal limits are in use.
>
>
> John
>
> [1] I suppose a 317 could have a slight increase in voltage with load,
> if the thermal situation were right, but most will droop in real life.

If you can afford 1.2V, there's the LT3092. Otherwise maybe a
beta-graded BJT that comes out of saturation when you hit the limit?
You could use one comparator and 12 resistors to see when any one of the
supplies comes off the peg, and use that to turn off the base bias so
you don't roast the BJT. If it had some AC hysteresis, it could even do
the retry function, or you could use a processor to do the turn off and
retry function once the BJT was doing the fast reacting.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: John Larkin on 9 Aug 2010 11:34
On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote:

>Hello, what happens when too much current is drawn from an LM317
>configured for 3.3V output? Does the output voltage decrease?

Coincidentally, I recently had reason to test a National LM317L/TO92
as a current limiter. It passes about 180 mA for a second or two, then
does this:

ftp://jjlarkin.lmi.net/LM317L_ilim.JPG

The duty cycle is sensitive to cooling, so it makes a sort of goofy
air flow sensor.

I have some voltage drop vs current data too, but I don't have it
here. I recall stuff like 1.4 volts at useful currents. I can post it
later if anyone is interested.

I like scope pictures+post-its. It's easier than fooling with USB
sticks and, in my opinion, looks better.

John


From: Grant on 9 Aug 2010 16:43
On Mon, 09 Aug 2010 08:34:47 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote:
>
>>Hello, what happens when too much current is drawn from an LM317
>>configured for 3.3V output? Does the output voltage decrease?
>
>Coincidentally, I recently had reason to test a National LM317L/TO92
>as a current limiter. It passes about 180 mA for a second or two, then
>does this:
>
>ftp://jjlarkin.lmi.net/LM317L_ilim.JPG
>
>The duty cycle is sensitive to cooling, so it makes a sort of goofy
>air flow sensor.
>
>I have some voltage drop vs current data too, but I don't have it
>here. I recall stuff like 1.4 volts at useful currents. I can post it
>later if anyone is interested.
>
>I like scope pictures+post-its. It's easier than fooling with USB
>sticks and, in my opinion, looks better.

Photos lot quicker on older scope with a floppy drive ;)

There's an app note or datasheet says they do that, nice to it's true.

Grant.
>
>John
>
From: David Eather on 9 Aug 2010 16:48
On 10/08/2010 1:34 AM, John Larkin wrote:
> On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote:
>
>> Hello, what happens when too much current is drawn from an LM317
>> configured for 3.3V output? Does the output voltage decrease?
>
> Coincidentally, I recently had reason to test a National LM317L/TO92
> as a current limiter. It passes about 180 mA for a second or two, then
> does this:
>
> ftp://jjlarkin.lmi.net/LM317L_ilim.JPG
>
> The duty cycle is sensitive to cooling, so it makes a sort of goofy
> air flow sensor.
>
> I have some voltage drop vs current data too, but I don't have it
> here. I recall stuff like 1.4 volts at useful currents. I can post it
> later if anyone is interested.
>
> I like scope pictures+post-its. It's easier than fooling with USB
> sticks and, in my opinion, looks better.
>
> John
>
>
Blessings to markp
From: John Larkin on 9 Aug 2010 20:57
On Mon, 09 Aug 2010 08:25:14 -0400, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

>John Larkin wrote:
>> On Sat, 07 Aug 2010 15:28:47 GMT, nontelo(a)dico.no (Mino) wrote:
>>
>>> Hello, what happens when too much current is drawn from an LM317
>>> configured for 3.3V output? Does the output voltage decrease?
>>
>> It decreases a little bit as you load it, at any current [1]. At some
>> large current you hit either its current limit or its thermal limit.
>> At the current limit, voltage starts to drop steeply, such as to keep
>> the load current constant. If it get too hot before that happens, most
>> 317s will just shut off until they cool down some.
>>
>> Right now, we really need a low-voltage-drop current limiter, to
>> protect a circuit powered by a dc/dc converter. Something like 150 mA
>> would be good. Polyfuses suck. Discrete circuits, like the things
>> James was playing with, would work, but I don't have room for all the
>> parts. We're considering using an adjustable LDO and running it
>> wide-open, so only the current+thermal limits are in use.
>>
>>
>> John
>>
>> [1] I suppose a 317 could have a slight increase in voltage with load,
>> if the thermal situation were right, but most will droop in real life.
>
>If you can afford 1.2V, there's the LT3092. Otherwise maybe a
>beta-graded BJT that comes out of saturation when you hit the limit?
>You could use one comparator and 12 resistors to see when any one of the
> supplies comes off the peg, and use that to turn off the base bias so
>you don't roast the BJT. If it had some AC hysteresis, it could even do
>the retry function, or you could use a processor to do the turn off and
>retry function once the BJT was doing the fast reacting.
>
>Cheers
>
>Phil Hobbs

Looks like I'll have to do this:

ftp://jjlarkin.lmi.net/DCL8.JPG

It's a lot of parts to protect a $4 dc/dc conveter, but it does all
the stuff we need and it's orthogonal, in the sense that all its
important parameters are settable independently. I could go for, say,
240 mA trip, 1 millisecond delay, and 1 second off. Or whatever.

The LM317L works fine to protect the dc/dc brick, but I can't really
tolerate the voltage drop.

The ideal way to do this would be just the mosfet and the resistor.
Have our main uP scan the currents and drive the fet gates, and do the
protections in software. But we don't have enough ADC inputs or port
pins to do 12 channels.

Really, somebody needs to make a good current limiter chip. Low drop,
resistor programmed, periodic retry, SOT23, 3-15 volts, cheap. They'd
sell zillions of them.

John


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