linear reg with current limit

or

Very pretty! I can just picture Lord Kelvin bending over it adjusting some gizmo inside.

Cheers

Phil Hobbs

Reply to
pcdhobbs
Loading thread data ...

That's the Digikey reel price for the 317L. My purchasing lady usually gets better pricing than Digikey.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
John Larkin

On a sunny day (Tue, 17 Oct 2017 06:15:56 -0700 (PDT)) it happened George Herold wrote in :

Aha!

Reply to
Jan Panteltje

How about this....

...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

The usual two-terminal BJT current limiter works better if you replace the sense transistor with a TLV431 (for NPN) or LM4041-ADJ (for PNP). I use that a lot in stuff like TEC drivers, where it's nice to have independent current limits on the four corners of the H bridge, in case a wire gets shorted to ground someplace. (Strictly linear, you understand.)

The LM385 is also PNP-like (you short FB to anode to get 1.2V, just like shorting B-C of a PNP to get Vbe).

Not as cheap as a 317L though.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

I have a 12 volt supply and only need 6. I could switch down to 9 or something, to save a little power.

I sort of did that. It's easy to drive the adj pin of an LM317 or 1117 with the opamp. It powers up low, and there's no overall loop stability issue. It would be a little trickier to get my control voltage into the fb pin of a grounded regulator. I need simplicity and low parts count/area here.

It's a shame that polyfuses are such flakey parts.

There are some cool 2-terminal LED current regulators, but not at my current.

This is interesting

formatting link

but really no improvement over the 317 for me.

Maxim and LTC have current limiter chips, but they cost 15x or so what a 317 costs, and many times the price of the ON thing. Why would anybody buy them?

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

That's nice. It would force me to think about loop compensation (or breadboard it) but it does save parts count and pcb area. I will need some microfarads of output caps, so it will probably need some compensation.

Minimum output voltage is around 2 volts, which is fine in my application.

If I swap D1 and R2, I could do local feedback at the opamp and avoid the overall closed loop.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

D1 and the OpAmp loop basically emulate a variable zener from ADJ to GND. Putting D1 in the R2 slot would shunt the internal bandgap, causing unknown grief. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

No, that doesn't work. R1 makes the voltage regulation too soft. If a single 317 is the power booster and the current limiter, it needs overall feedback. Another RC probably.

I might even have an LM317 spice model somewhere. Or I could move my body and Dremel it.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

Oh, well then what the hell, you don't need an LDO at all. What's wrong with a LM317? Built in, well defined current limit. Thermal protection on top of that. You just met all your specs with something you just mentioned!

formatting link
Fig.2

There's USB3 power switches that handle over 12V and cost less than sand.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
 Click to see the full signature
Reply to
Tim Williams

"Scratch, scratch", Can you give me a hint as to how that works? OK at 100mA I'm losing a volt across R1 so I need to pull the adj. pin down by 250 mV. Above ~125 mA I need to push voltage into the adj. pin, but the diode is in the way. Is that close to right?

BTW I hooked an LM317 to my big coil and tried to fry it. It's been happily turning itself on and off for several hours. Seems fine.

formatting link
formatting link

George H.

Reply to
George Herold

Spice is second best option: finally figured out that if I don't get a whiff of rosin every day I get grouchy :)

piglet

Reply to
piglet

Yeah, it's the conventional CCS circuit "but". Specifically, it's wired as a voltage follower while the diode is conducting, and a CCS when not.

The op-amp will try to command a higher output voltage, as the output voltage browns out (from going into CCS mode), but the diode blocks it from pulling it up, so the CCS takes over. Meanwhile, U2 saturates, which means you get full integrator windup, which is kind of crappy. If the application is okay with voltage overshoot under load step, then who cares.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
 Click to see the full signature
Reply to
Tim Williams

The 317L limits are 100 to 300 mA, which is not well enough defined for my application.

John Owen has suggested the AL5809 2-terminal current limiter, which comes in 120 and 150 mA versions. That would go upstream of a voltage regulator.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
 Click to see the full signature
Reply to
John Larkin

Standard LM317 plus dirt cheap microcontroller to monitor the current and provide fold back

Cheers

Klaus

Reply to
Klaus Kragelund

The diode plus OpAmp loop looks the same as if you placed an adjustable zener from AGJ to GND. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

Elegant, and quite useful, but (for an 'adjustable') with only a +12 power source, the op amp and diode and LM317 can only go down to about 2.2V.

The LM723 (with PNP pass transistor, figure 20) isn't as current-accurate, but with the same +12 supply spans zero to 11.5 V.

Reply to
whit3rd

Den tirsdag den 17. oktober 2017 kl. 21.47.03 UTC+2 skrev John Larkin:

but you would lose the thermal protection

Reply to
Lasse Langwadt Christensen

OK, R2 is in there for damping the loop? I've never driven an LM317 with an opamp. It's a lot cheaper than the LM395 I have used! I guess it'll go on a list of circuits to try. Maybe this week! I'm pretty much done with a project. Not a whole lot I 'have' to do. Which is nice.

George h.

Reply to
George Herold

Why not two LM317s - one configured as an accurate current source (10 Ohm resistor between ADJ and OUT, but take your output current from ADJ pin) then feeding that limited current to the IN pin of a second LM317 configured in the usual way to regulate voltage. If the load exceeds a certain value, the input to the second LM317 will collapse. The scheme is not good for very low currents because the quiescent current of the second regulator causes an inaccuracy in the current limit, but I think it should be good enough for what you want.

Reply to
Chris Jones

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.