Transistor as a current limiter

Sorry, a 2N7002 costs about 2 cents, about the same as a BCX70.

Those circuits could be made to work with a BCX70 or similar NPN.

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John Larkin         Highland Technology, Inc 

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John Larkin
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Didn't he say: "WITHOUT OPAMPS OR MOSFETS."?
Reply to
John Fields

I have some, of course. Cheapest mosfet I could find.

Perhaps. But I buy PN2222A for less than 0.4 cents each. And

3904/3906, if I keep my eyes open, I find at only slightly more. The bcx70 is something I'll look around for, too, then. But it doesn't seem quite cheap enough for me, today.

I give away 50 at a time. So the difference between .00385 and .02 is important to me.

In any case, I want to see how this is done with BJTs.

Okay. So...

Jon

Reply to
Jon Kirwan

So I just decided to take a look (besides seeing the mosfet.)

Looking at schematic A: I see a diff-amp with 0.2V input (I suppose you would tell me to use a R-divider if I were being cheap about this.) One of its collectors (let's call that PNP Q1 for now) collector is used to pull up on the base of another NPN (call it Q2 for now) that ties the mosfet gate. To get the voltage at the base of Q2 high enough that it is slightly or usefully active, the collector of Q1 must reach near to 0.65V or so. But the base is 0.2V. You see any problem here? I guess you offered as much thought as you felt I deserved.

Jon

Reply to
Jon Kirwan

The +0.2 could be a shared rail. Current ewquirement is low.

One of its collectors (let's call that PNP

No. It should work.

I guess you offered as much thought as you felt

With your attitude, you don't deserve much.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

Explain why.

Saturated BJT in the diff-amp?

Jon

Reply to
Jon Kirwan

(The base drive will have to be significant, relative to the common diff-amp emitter currents. That means a stiff divider. Gain will also be poor, I believe.)

Jon

Reply to
Jon Kirwan

I did and was summarily ignored except that he later wrote that I could replace the mosfets with BJTs if I wanted to.

I'm below 200mV of headroom required for surprisingly good current control with BJTs only. I still need to fix it so that the LEDs aren't in the middle of the circuit, but can share a common anode or cathode. That part was just because it was easier to think about at the time.

Jon

Reply to
Jon Kirwan

I can't explain control theory in a usenet post, and I can't explain transistor theory either. Get a book or something.

You can take my circuits as starting points, or you can be hostile to suggestions.

Nothing is saturated.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

Actually, this works better than I thought it would, given the constraint of no opamps but ignoring the constraint of no mosfets.

formatting link

It servoes Vsrc down to 10s of millivolts. Seems to be closed-loop AC stable, too.

formatting link

Q2 base current is nanoamps.

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John Larkin         Highland Technology, Inc 

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John Larkin

all NPN emitter marked 'e'

I_set I_LED V V | | +-----. | \ | / \| | |/ |--+----------|-----... /| |\ e e + + | | / \ \ 22R / 22R / \ | | Q1 / / +V |/ enable |/ --[1K]-| ---[1K]--| |\ |\ e e | | --+-------------------+---... scale the 22R as apropriate for your application 200mV drop is plenty scale the 1K sufficient to ensure reasonabe saturation.

Q1 can probably be replaced by the apropriate resistor (measure or calculate)

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Jasen Betts

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