Ackerman Agreement Current Limiter

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There seems to be some interest in the LED current limiter I needed for a r
ecent design.  There is another current limiter to charge the supercap that
 powers the LED and sounder.  I thought I'd paste it here for those who mig
ht be interested.  It was a post from piglet that gave me the idea.  I'm su
re it's an obvious circuit to many here, but I'm impressed with how well it
 works.  As the supercap charges from zero it holds the current rock steady
 until the limit of drop out voltage is reached at less than 0.6 V.  It's a
 hard circuit to beat for the simplicity.  It's more sensitive to the incom
ing power supply rail, but that's ok in this design.  Again, I'm just shoot
ing for something better than a resistor.  

I used a FET with a model from the vendor, so the symbol is autogenerated a
nd placed in an autogenerated directory.  You may have to tweak the symbol  
to locate the model file since it isn't on your hard drive... er, I mean my
 hard drive.  

Comments welcome.  

http://arius.com/temp/Current_Limit.zip

If you don't want to futz with the LTspice simulation there is a PDF file o
f the schematic.  

http://arius.com/temp/Current_Limit.pdf

Bonus points if you can tell me what the Ackerman agreement is from.  

--  

  Rick C.

  - Get 1,000 miles of free Supercharging
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Re: Ackerman Agreement Current Limiter
BTW, I discovered a setting in the LTspice control panel for the line width
 used in the drawings and also the waveform display.  The default 1 setting
 can be virtually impossible to see in a PDF.  Even on the display a 2 is a
 significant improvement.  I'm currently using 3.  

--  

  Rick C.

  + Get 1,000 miles of free Supercharging
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Re: Ackerman Agreement Current Limiter
On 25/05/2020 10:10 pm, Ricketty C wrote:
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I think my circuit was not so much a current regulator as a switch that  
permitted unlimited load current after the initial capacitor charge-up  
surge. But glad my contribution helped you.

Art of Electronics has no reference to Akerman circuits? :)

piglet

Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 5:44:26 AM UTC-4, piglet wrote:
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 a recent design.  There is another current limiter to charge the supercap  
that powers the LED and sounder.  I thought I'd paste it here for those who
 might be interested.  It was a post from piglet that gave me the idea.  I'
m sure it's an obvious circuit to many here, but I'm impressed with how wel
l it works.  As the supercap charges from zero it holds the current rock st
eady until the limit of drop out voltage is reached at less than 0.6 V.  It
's a hard circuit to beat for the simplicity.  It's more sensitive to the i
ncoming power supply rail, but that's ok in this design.  Again, I'm just s
hooting for something better than a resistor.
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ed and placed in an autogenerated directory.  You may have to tweak the sym
bol to locate the model file since it isn't on your hard drive... er, I mea
n my hard drive.
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le of the schematic.
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Yes, they were different circuits with different purposes, but in trying to
 understand your circuit the idea that it could be modified to produce a cu
rrent limited occurred to me.  

There is no Akerman circuit.  This was something in an (old) movie and at s
ome point I wrote it into the settings for printing PDF files.  So it shows
 up as the embedded title.  

It's funny that the "Akerman agreement" was a big deal in the movie and men
tioned more than once, but Google searches don't reveal its nature.  Maybe  
I'm not spelling it correctly.  

I've been simulating alternative components in LTspice and found that the t
ool has a large number of issues.  The most recent issue was trying to swap
 models using a copy of a previous component symbol.  I changed every acces
sible mention of the previous device and got an error saying it can't find  
"the old component name".  I went through every step and could not find the
 error.  Everything pointed to the new model file...  everything I could se
e.  

The pointer to the model file is in the symbol.  I edited the symbol by cli
cking through to it from the schematic where it is used.  That updates the  
file, but not the schematic.  I even created a small test design, but copie
d the part from the larger schematic, so the fault remained!  In the end th
e helpful guys from the LTspice group told me to check the netlist.  They c
ertainly don't make it easy to reuse symbols.  

LTspice is not a tool for the faint of heart.  

--  

  Rick C.

  -- Get 1,000 miles of free Supercharging
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Re: Ackerman Agreement Current Limiter
Ricketty C wrote:

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"My God, it's full of parts!"

Why not a depletion-mode MOSFET with a resistor?

    Best regards, Piotr

Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 2:33:50 PM UTC-4, Piotr Wyderski wrote:
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I don't think he can afford the ~1 V drop of the d-fet.  
(I hear you responding that the above thing has a Vbe drop..
which I guess he can afford.)  I'm not sure about the  
Schottky diode.  Is that to reduce temperature effects?  

George H.  

Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 3:24:39 PM UTC-4, George Herold wrote:
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file of the schematic.
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The diode is to prevent back current.  By putting in the Vsense loop its vo
ltage drop is not added to the Vbe which is less than 0.6 volts.  At that p
oint the cap is pretty well charged and the current can taper off without p
roblem.  I don't know that 1 volt would be enough for the depletion mode de
vice, but I see no reason to add another part to the BOM just to save using
 one transistor and a two resistors.  Every part in this circuit would eith
er remain or still be used elsewhere in the circuit.  The real killer is th
e lack of p-channel depletion mode devices.  

--  

  Rick C.

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Re: Ackerman Agreement Current Limiter
Ricketty C wrote:

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You didn't specify that constraint in your initial message, but said  
"It's a hard circuit to beat for the simplicity."

It is actually not hard at all, "my" circuit is 3x simpler. I am not  
counting the diode, as you may want it to prevent the current flowing  
backwards.

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Why would you need one? This constant current diode is a floating  
2-terminal device. Feel free to use it on either rail. It would work  
equally well with a P nad N DMOS, as well as with a P and N JFET, which  
are easily available.

    Best regards, Piotr


Re: Ackerman Agreement Current Limiter
George Herold wrote:

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Vbe + the Schottky and the sim say that Vin=5.25V, which gives 2.75V of  
headroom. This is a lot.

    Best regards, Piotr

Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 5:05:47 PM UTC-4, Piotr Wyderski wrote:
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The charging is to get as much voltage on the supercap as possible.  2 volt
s is the minimum level the rest of the circuit will operate when power is l
ost and the supercap discharges.  

So the headroom goes down to zero as the supercap charges.  The issue is at
 what point does the charging drop off and turn into a trickle?  As I've sa
id, the present design charges at full rate up to within 0.57 volts of the  
input power.  That's pretty good without adding opamps and such.  Also, the
 1 volt drop of the depletion mode FET doesn't include the diode.  Can that
 be inserted in the Vsense leg with the resistor like I am doing?  I suppos
e so.  

Where do you get the 1 volt number?  Are there parts that will work at lowe
r voltages at 200 mA current?  I'm sure there is a relation between current
 and voltage.  A part that passes 10 mA at 1 volt would pass more current a
t a lower voltage, right?  

I've dug around and have not found a part that would work.  Thanks for the  
input.  

--  

  Rick C.

  --- Get 1,000 miles of free Supercharging
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Re: Ackerman Agreement Current Limiter
Ricketty C wrote:

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With a depletion mode device there is no such point, as there is none  
with a regular resistor. You will be gradually reaching your R_DS_ON and  
the device will become more and more ohmic.

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This is the typical level of V_GS, i.e. drop on the series resistor  
where you get very decent regulation. If you want to rely solely on the  
saturation current of the device, which I very often do, you don't need  
the resistor.

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Sure, but you may not like the price. However, if you want to go to the  
extreme just to see what's possible, check the UJ3N065025K3S: 25mOhms.

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At lower voltages it is just equivalent to a resistor.

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How about DN2450N8-G? 10Ohm max. Or IXTY1R6N50D2, 2.3Ohm max.

    Best regards, Piotr

Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 7:17:16 PM UTC-4, Piotr Wyderski wrote:
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rickle?  
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 passes 10 mA at 1 volt would pass more current at a lower voltage, right?
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the input.

The IXTY1R6N50D2 might do the job.  The IXTT16N10D2 would be better, but I  
am going to need a power switch to control a 12 volt line to a motor at 5.5
 amps and I can use the PMPB13UP for that.. well, the PMPB20XPE actually wh
ich has replaced the PMPB13UP in the last few days just because of this req
uirement.  I can't see having two, so similar parts on the BOM.  

One of the requirements is for the components to be "readily available" any
where.  This is vague and some aspects of the design (not this board) are d
eviating from that pretty oddly.  They will need a plastic membrane to cove
r the front panel, display, etc.  That is going to be custom.  Can that be  
made everywhere?  I don't know.  

Thanks for the insights into depletion mode devices.  I've learned a few th
ings.  

--  

  Rick C.

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Re: Ackerman Agreement Current Limiter
On Thu, 28 May 2020 12:24:34 -0700 (PDT), George Herold

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The little LND150 Supertex parts get ohmic, around 1K, at low
voltages. The LED will get a bit dimmer at low supply voltage. That's
not lethal.

I recently invented a cute LED current limiter, where the voltage drop
across the LED is the voltage reference for its own current. 4 parts,
8 cents, nearly zero headroom.

--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 6:39:01 PM UTC-4, John Larkin wrote:
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You'd scribble it on the side here, but there's  
not enough room in the margins :^)

Say, can you give us a hint?  I bet 'we' can figure it out.

George H.  
(who will have to go look at the I-V curve of the lnd150 again.)



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Re: Ackerman Agreement Current Limiter
On Thu, 28 May 2020 17:44:16 -0700 (PDT), George Herold

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I'll draw it up and post it, but the thing I described devolves to a
negative resistor.



--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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Re: Ackerman Agreement Current Limiter
On Thursday, May 28, 2020 at 9:04:47 PM UTC-4, snipped-for-privacy@highlandsniptechnology.com wrote:
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Huh.. OK.. I've got a vague memory of a capacitor feedback circuit that
looked like a negative resistance.  Does this gizmo oscillate?  
(It's like 'What's my line', Soupy Sales has the next question. :^)

George H.  
  
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Re: Ackerman Agreement Current Limiter
On Thu, 28 May 2020 19:36:12 -0700 (PDT), George Herold

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This is it:

https://www.dropbox.com/s/awl6qmpax8ym8kl/CCLED_May_2020.JPG?raw=1

It only needs three parts, but negative resistors are perverse, so it
might not start up. So it needs R2A or R2B or something to kick it
off.

It only loses millivolts.





--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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Re: Ackerman Agreement Current Limiter
On Friday, May 29, 2020 at 1:07:15 AM UTC-4, jla...@highlandsniptechnology.
com wrote:
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logy.com wrote:
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e:
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 a PDF file of the schematic.
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rop
s,

Yes, it is very low drop out and the current will be fairly constant.  But  
it misses on one important mark.  It doesn't set the current very well.  Th
e current is  

Id = hfe(Q2) * (Vd - Vbe(Q1)) / R1

While Vd and Vbe will be fairly predictable, hfe(Q2) has a range of nearly  
2:1.  Design the circuit for 1 mA through the LED and it can vary almost fr
om 0.75 mA to 1.5 mA without regard to any other conditions.  

The issue is not so much that the current sense resistor needs to be out of
 the diode current path, but that it needs to be compared to a lower voltag
e than Vbe.  A Schottky diode can be added between the LED cathode and the  
emitter of Q1.  A small current sense resistor can be added between the cat
hode and the base.  This will offset the  Vbe voltage so the sense resistor
 will only see the difference ~0.25V.  Then not only will the current not v
ary with the applied voltage, it can be set by the value of the current sen
se resistor.  That would be an improvement over the circuit I'm currently u
sing only requiring a diode be added.  Still, I don't think it is worth it.
  The two transistor + 2 resistor circuit I'm using only starts to lower th
e current when the applied voltage gets down to 2.2 volts.  

--  

  Rick C.

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Re: Ackerman Agreement Current Limiter
On 29/05/2020 6:07 am, snipped-for-privacy@highlandsniptechnology.com wrote:
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Damned ingenious Sir! Violates classical teaching about relying on beta  
variation but modern more tightly specified devices mean you could get  
away with it sometimes.

piglet

Re: Ackerman Agreement Current Limiter
wrote:

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Yes, breaking old-fart holy rules is worth doing for its own sake, if
just to hear them squeal. Nobody will ship it back for a refund if an
LED is a little brighter or dimmer than they expected.

There are a few more precise opamp versions of the self-reference LED
thing, but they are boring.

Beta limiting can be good, like for instance when you have a 3.3 volt
supply and want to stuff some current into a 3 volt bandgap.

I do often put an LED in series with a depletion fet running at Idss.
It stays on at constant brightness until the supply caps are almost
totally discharged, then winks out.


--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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