Best way to protect a transistor from overcurrent?

You can configure the transistor with a current limiter consisting of a sense resistor from emitter to GND and two diodes in series from base to GND. You could also replace the two diodes with an LED to indicate an overload, or an optoisolator that could be used to trip a relay or alarm. The transistor would need to be able to handle the maximum current you choose with the sense resistor as well as the full voltage, which may be several watts of power, long enough for a fuse or circuit breaker to operate and remove power. There are also fold-back current limiter circuits which reduce the current to a safe continuous level until the fault is removed, but they are not as simple.

Paul

ts

Another way is to use an NSC LM395 for the 500mA transistor.

BTW, as Jim has pointed out, the LM395 die is nearly an LM317. And we have lots of respect for LM317s.

Yep. Somewhere in the _far_ past (1980) I did a full dissection of the LM395/LM317 for an Integrated Circuit Engineering (ICE) publication. If I can find it I'll scan and post.

Duh!! Ol' foggy brain... I already did...

It IS the VERY SAME die, only the metalization interconnect is different.

...Jim Thompson

m.

uits

Yeah I love that 'transistor' for output stages. Just remeber to read the data sheet and put in the correct series resistor in the base lead. (I spent a few hours pulling my hair one day. "why is this circit oscillating?")

George H.

Given it is only a slight difference in metalization, the order of magnitude difference in price is more curious. (I looked at TO-220 packages.)

Jon

A 317 (or an LM1117) makes a nice power amp stage, current and thermal limited. I put them inside opamp loops to make really good regulators.

A 317 should make a nice protected switching transistor too, if you don't mind the saturation voltage and the largish Vbe.

I tried doing all sorts of terrible things to the adjust pin of an LM1117 and couldn't damage it.

John

I've often thought (but never had the time to try) using a 317 as a cheap and dirty audio power amp. Ever tried that?

Jim

Please, sir: I don't do audio.

They only conduct one way. You could stack two, the upper as a current source, and do class A. I guess you could arrange a 317 and a 337 as a push-pull class AB thing. I have no idea what bandwidth would be like.

For full infrasonic soundstaging, you'd have to put leather bags full of gold dust on top to dampen the diagonal silicon resonances.

John

Sno-o-o-o-ort! Who was it that wrote that National audio book? Wasn't it called "Floobydust" ?:-)

...Jim Thompson

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

Jim

Excuse me all to hell. I don't do it either unless it is at the end of a long chain of devices intended to capture a microvolt of intelligence and have my customer hear something resembling a voice. And I have to do it with a minimum of components and weight that will last the lifetime of the device -- 30 years is the design bogey.

Sheesh, my limit of 10% distortion (minimum detectible distortion on spoken audio) isn't quite up there with the audiophools, y'know.

Jim

The 1973 book is the classical one. Being just a stripling I had to buy a copy at Radio Shack, which I still have. That was when I fell for the LF356--puppy love, admittedly, but we're still friends. ;)

Cheers

Phil Hobbs

I remembered this RF one from the net:

No answer here; just wanted to give you some positive feedback on finally learning to bottom-post! :-) :-) :-)

Cheers! Rich

Oops! My "Kudos to RST" post was supposed to be signed by plain ol' me, not the Dreaded Libertarian.

Sorry about that.

Thanks, Rich

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You're kidding! You _have_ a whole Radio Shack? ;-P

Cheers! Rich

Interesting! What is that reference to the 70kHz "band"? Is this a new "ham", or experimenter's band?

...Jim Thompson

Just about--you should see my parts collection. Much better quality than RS, of course. BTW that wasn't "a Radio Shack", just Radio Shack. I also still have the stripling, somewhere under a few layers of bark. ;)

Cheers

Phil Hobbs

I remember (~1958~1962) Radio Shack on Commonwealth Avenue, Boston, that was a big parts warehouse, conveyor belt delivery of your order to the front desk ;-)

...Jim Thompson