Voltage dropper

LM337? TL(V)431, LM4040?

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

Zener ? VBE multiplier Resistor + current source

Hi Tim, I don't know the TLV431 but the others both use ground as the reference. So if the power supply rail changed the voltage wouldn't. I was thinking of something that always (say) 5 volts below the rail. (like my string of diodes.)

George H.

Klaus, yeah Zener is simple, but a 3 volt zener is mussy. I'll have to look up the Vbe multiplier to remind myself.

Not sure of the resistor and current source idea.

George H.

Duh, just because an appnote tells you to ground some part doesn't mean you have to. Hang it from the +V rail, let it make its own ground. :-)

Assuming you can afford the bias current, of course. (You mentioned LM317 so I'm guessing you have a few mA to spare.)

Example in the midst of assorted other stuff:

Regular zeners are fine if you don't need precision, of course.

Tim

I think the Vbe multiplier is what I want.... or at least one solution. (I'm going to have to test it.)

George H.

You could drive the ADJ pin of a 317 or 337 from a voltage divider from the supply to ground. That almost it.

The LM4041-ADJ is like an upside-down TLV431--it holds a constant voltage b etween ADJ and cathode. It's sort of like a PNP vs. The NPNish TLV431, whic h makes it suitable for negative regulators. Alternatively you could use an LM337 with its output to the positive supply.

Cheers

nics.design/XHnLmIX6dhY

There's any number of ways. Zener, rubber zener, diodes, LED, TL431, voltag e regulator, etc etc. Take your pick. Diodes have the upside that you can u se out of spec ones for jobs like this - on consumer gear anyway. Don't for get that some HV diodes drop double as they're 2 in series.

NT

Oh (head slap.) Wow! I didn't think of floating the lm317.. I was going to say I've never done that, but for lm317 (or other V_ref-R-FB) current sources the whole point is floating the reference. I'll try a '337 on the negative rail tomorrow. maybe I should do the Vbe mult. It's only power for the low power (~>100mA) A little lm337 in a to-93 would work. (I had problems once with the lm337 in a to-93, we still use 'em, but I'm forever wary.)

Side question, do you think it's safe to assume the lm317 and lm337, in whatever form factor, will be around forever? ... well my life time anyway, say 20 years.

George H.

Yeah! I'm floating a lm377 tomorrow. Then I'll want to cut down the sense R.

GH

Aren't green LEDs around 3 volts? Drive it from a current source.

On Thursday, November 16, 2017 at 6:22:57 PM UTC-5, snipped-for-privacy@gmail.com wrote :

between ADJ and cathode. It's sort of like a PNP vs. The NPNish TLV431, wh ich makes it suitable for negative regulators. Alternatively you could us e an LM337 with its output to the positive supply.

Right. Every day I still get to learn something. That's the advantage of being a bear of little brain. :^)

George H.

te:

The max current is ~100mA. GH

age regulator, etc etc. Take your pick. Diodes have the upside that you can use out of spec ones for jobs like this - on consumer gear anyway. Don't f orget that some HV diodes drop double as they're 2 in series.

I'm afraid to say I've got a circuit running off 48Vdc that's more complicated that's it needs to be. (The 'good' news is only 3-4 sold.)

George H.

Heh... they've been around for longer than I've existed.

Some things will never go away: LM317

Sure, there are tons of better alternatives for all, but none as well known, nor as common-denominator. Replace an LM317 with a new alphabet-soup equivalent, and what are the chances the new one will be around any longer?

Predicting the future being what it is, of course. But, hey, more than several near-Earth asteroids have swung past us throughout the LM317's lifetime, if that means anything (it doesn't).

Tim

It's OK if you have the minimum headroom and the minimum output current (5 mA minimum, 10 mA better).

The parts with the most uses, are the ones that stay around longest. Tone decoders died, but not 3-terminal regulators. I kinda liked the four-terminal (in, ground, out, sense) regulators, before those sailed into the sunset. Unlike LM337 and its ilk, they didn't have minimum output currents.

What's your point? You can't figure out how to use an LED with a transistor?

On Thursday, November 16, 2017 at 6:22:57 PM UTC-5, snipped-for-privacy@gmail.com wrote :

between ADJ and cathode. It's sort of like a PNP vs. The NPNish TLV431, wh ich makes it suitable for negative regulators. Alternatively you could us e an LM337 with its output to the positive supply.

Hi Phil, well I thought I understood the lm337 idea, but in the light of da y I'm not getting it. I was first thinking it looked like the V_be multiplier. Resistors from V_in and V_out to the adjust pin. But that doesn't work... well always gives a ~1.5 V drop. I've gotta pull the adjust pin down more.

The Vbe multiplier works fine though. I'll have to look at the tlv431.

George H.