Overloading an LM317

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.

Yes.

This part has a data sheet that should answer most of your basic questions.

RL

If you tune the input lead length and input bypass cap just right, you can make an AM-band transmitter with one!

Cheers

Phil Hobbs

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http://electrooptical.net

Cool. 317 type regs make nice power amps, too. I think I've seen an audio amp based on one. A 317 and a 337 could make a nice dual-function class AB push-pull amp *and* AM transmitter.

John

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

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

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.

Blessings to markp

I guess! That's all we have. Have to dump 200K per trace to floppy, then import the crap into Excel '07, which is a complete pig.

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

Sounds a lot like the controller ICs built into DC fans: If you block the blades with your finger, they'll usually stop and then pulse a bit every few seconds trying to restart...

Easy enough to do, but customers won't PAY for such smarts. ...Jim Thompson

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You could sum the voltages into one comparator and turn all of them off when there's a problem, then turn them back on one by one until you find the one with the problem. I've use daisy-chained 74HC595s driven by SPI for that sort of thing.

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

Yes, that would work. I could sum them with schottky diodes and a shared pulldown, or just use a comparator per and wire-OR the outputs.

I'll talk that over with the boys.

John

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Apparently Larkin's will? :-)

What I was really thinking was that perhaps those fan controller ICs could be pressed into service as current limiters, but I've never looked at a data sheet for one so that is wild speculation.

---Joel

Does there exist a smaller cheap lower current version of the 317? Sort of like the 78Lxx to the 78xx comparison.

Jim

LM317L is available in TO92, SO8, and a silly BGA sort of package with

0.5 mm pitch!

It current limits at about 200 mA; I posted a thermal-cycling waveform.

John

I found it. Thanks. Dumb old me thought that since the 317 is filed in the databook under the 117 that surely an L suffix would be in the same place. The 317L is only available in the 317 temperature range.

However, I only find it in the National book with the TO-92 and 8-pin SOIC packages.

It also looks like the 337L is the same device but restricted to about

100 mA.

Thanks again...

Jim

Try online. The micro-smd package is new. If I use it, it might get my lynched.

John