jfet as current limiter

around:

Bullshit.

I have no doubt.

on a stabilized supply.

Even worse.

those those into account in your circuits.

Nonsense. Beta is about as uncontrollable as any parameter imaginable. Why not use 20% resistors, while you're at it. It wouldn't make the circuit any worse.

in series with the LED

You're clearly a hacker. No one in their right mind relies on beta.

close.

limiter.

On a sunny day (Thu, 11 Oct 2012 14:42:37 -0400) it happened lightning struck "bizzzzzzzzzzzz" and he screamed:

Actually it is a beautiful thing, if you know how to play it.

And the tolerance of LEDs is?

You are clearly clueless about the basics.

around:

on a stabilized supply.

those those into account in your circuits.

in series with the LED

close.

limiter.

I've done a few beta-limited current sources, as much to annoy the "never count on beta" people as any other reason. BCX70 and BCX71 come in beta graded bands of about 2:1, so in some cases that's safe.

There's too much superstition in this business: Never depend on beta. Don't create ground loops. Don't change reference planes. Don't cut slits in ground planes. Ground shields at one end only. Never use the enclosure as part of a ground plane. BNCs are only good to x hundred MHz. You need a zillion bypass caps of various values.

around:

on a stabilized supply.

those those into account in your circuits.

in series with the LED

close.

limiter.

I'll stick with that one, thanks. You really can't count on beta, particularly across lots.

It's a problem at audio. ...and can be done with audio. ;-)

They're hard remove.

I'm not tempting fate with that one, either.

For *long* audio cables, this is the norm. There's too much chance of ground currents otherwise. I ran into this at the PPoE. Ground at one end wasn't same as ground at the other and there is a diesel generator wanting to heat whatever got inbetween. :-(

Ground plane? I wouldn't do that, but bolting the ground plane to it, early and often, sure.

How much power are you wanting to put through them? A kW? No, it's probably not a good idea at a couple GHz.

Zillion is a bit much, but various values works. .1uF sprinkled liberally, and a few caps tuned so their SRF matches the internal clock works.

Your religion (above) is just as strong as it is with those you mock. There are reasons for the above but like anything, there are limits and reasons for them.

around:

is on a stabilized supply.

those those into account in your circuits.

resistor in series with the LED

close.

as limiter.

You can count on a part to meet its guaranteed min and max specs. I suspect that you usually do already. Beta is just another spec.

I do things for reasons, not superstition or tradition.

around:

source/sink

is on a stabilized supply.

those those into account in your circuits.

resistor in series with the LED

matches close.

as limiter.

One that is *rarely* guaranteed. If it is, it's basically guaranteed to be greater than one, and only under very specific circumstances. There are good reason behind the dogma of not counting on it.

Ok go for it. I can say that I've already built and use that however, I didn't use that 2.5v ref, I used a 5.1V diode instead, because it seems to be easier to get. The idea of that is to allow me to use a wide range of voltage in the loop and have the current limit cap stay with in a window..

Jamie

around:

source/sink

is on a stabilized supply.

take those those into account in your circuits.

Why

any

resistor in series with the LED

matches close.

as limiter.

Dogma. Some transistors are sold beta graded. Like BCX70J, 250 to 460 at 2 mA. That could drive an LED, or a bandgap, or current limit something.

National used to sell a power transistor that had very tight beta limits, specifically as a current limiter.

around:

source/sink

micro is on a stabilized supply.

take those those into account in your circuits.

Why

any

resistor in series with the LED

matches close.

works as limiter.

From -55°C to +125°C, beta varies 4:1 ...Jim Thompson

vlatge around:

source/sink

micro is on a stabilized supply.

take those those into account in your circuits.

Why

any

resistor in series with the LED

matches close.

works as limiter.

Vbe varies by over 250 mV over that temp range too. Ceramic caps can change 3:1 over that range. Cap ESRs can change by 1000:1. Most commercial parts aren't specified to work over that range at all.

All bipolar transistor circuits are affected by beta. If it's going to change so much that it breaks your design, do something else.

vlatge around:

source/sink

micro is on a stabilized supply.

take those those into account in your circuits.

Why

any

resistor in series with the LED

matches close.

works as limiter.

good

I didn't say it wasn't usable, just that hanging your hat on a narrow range will hang you. ...Jim Thompson

vlatge around:

source/sink

micro is on a stabilized supply.

take those those into account in your circuits.

Why

circuit any

resistor in series with the LED

matches close.

works as limiter.

good

Bad engineering usually leads to grief. Let us know how the Chinese breadboard thing works out.

vlatge around:

source/sink

micro is on a stabilized supply.

/ take those those into account in your circuits.

imaginable. Why

circuit any

resistor in series with the LED

matches close.

works as limiter.

be

good

Is there some reason you're so ignorant that you simply can't STFU?

Did you "contribution" provide any technical information? ...Jim Thompson

vlatge around:

source/sink

micro is on a stabilized supply.

/ take those those into account in your circuits.

imaginable. Why

circuit any

resistor in series with the LED

little.

matches close.

works as limiter.

be

good

I did point out that your first two power pickoffs were wrong, and that there was a pulldown dilemma that you missed. I believe you acknowledged the problems.

Post your third attempt, and we'll have fun with that one, too.

I'm just not getting that at all...looks more like a wish than anything else.

There's a classical answer, using TL431 programmable zener

(warning: bad ASCII art, use monospace font)

(+) | +-------+ | / | / R1 | +----| | |> | \ | | | | | | /---/ | / \ | / \-----+ --+-- R2 | | +-------+ | (-)

I_limit = 2.5V /R2 + (minor correction for zener bias)

which can be redone with an LM4041 for smaller Vref, using a PNP transistor... the range of 23... 30 mA should be easy to hit with 2V or so of voltage burden.

Il 12/10/2012 05:12, Jamie ha scritto:

Well, one good news and one bad news.

The good one is there is almost no difference among single items, so this issue is fixed.

The bad one is it depends too much on temperature. At room temperature with 12k + 12k the current is limited at 25 mA. But after a couple of seconds it rises up to 30 mA. Heating the bjt about 50 °C the current is over 35 mA.

I need to guarantee the current never goes above 30 mA over the whole commercial temperature range (0-70 °C).

I'm going to add a 1n4148 in series of ("of"? is it right?) the zener.

Two questions:

- do I need to add also a resistor on the emitter?

- in order to work fine I'm afraid the diode should be thermally connected to the bjt. Otherwise they share only the ambient temperature, but when the bjt heats due to his own current the diode doesn't compensate at all.

Any thoughts?

Marco

What is the maximum voltage drop you can tolerate when not in current limit?

Try it with a slight modification. Still not really good, but better.

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