too many possibilities...

[server]

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

The more general issue, which I\'ve thought some about over the years,
is that there are a large number of circuits that can be made from a
given number of parts. I\'m not sure how many.

I\'ve thought it would be fun to teach a class in circuit design, where
each student gets 10 parts.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[legg]

On Sat, 10 Oct 2020 10:41:06 -0700, jlarkin@highlandsniptechnology.com
wrote:

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

The more general issue, which I\'ve thought some about over the years,
is that there are a large number of circuits that can be made from a
given number of parts. I\'m not sure how many.

I\'ve thought it would be fun to teach a class in circuit design, where
each student gets 10 parts.

If you\'ve got any resistor locations that don\'t need a tolerance, then
maybe the location that does can supply them. One reel feeder still
serves both.

RL

 

[Tim Williams]

Let the computer do it. You have your spreadsheet, well, this fully
generalizes and automates it:
http://kirr.homeunix.org/electronics/resistor-network-finder/

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

<jlarkin@highlandsniptechnology.com> wrote in message
news:fuq3ofh1bklp18olftus3nbcq058cgaijr@4ax.com...

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

The more general issue, which I\'ve thought some about over the years,
is that there are a large number of circuits that can be made from a
given number of parts. I\'m not sure how many.

I\'ve thought it would be fun to teach a class in circuit design, where
each student gets 10 parts.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[whit3rd]

On Saturday, October 10, 2020 at 10:41:22 AM UTC-7, jla...@highlandsniptechnology.com wrote:

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

The more general issue, which I\'ve thought some about over the years,
is that there are a large number of circuits that can be made from a
given number of parts. I\'m not sure how many.
....
I\'ve thought it would be fun to teach a class in circuit design, where
each student gets 10 parts.

There\'s a common solution to the too-many-to-automate and several other
related problems: split the circuit board into two, one big \'un with
the parts that (for instance) need power and dissipate heat, and
a smaller one with the precision stuff. Most PC power supplies do this,
because the required parts are not mechanically similar to each
other, thus a mechanically sturdy board with through holes works
best for the power wiring and a surface-mount daughterboard
takes on all the overhead functions.

 

[server]

On Sat, 10 Oct 2020 14:26:03 -0500, \"Tim Williams\"
<tiwill@seventransistorlabs.com> wrote:

Let the computer do it. You have your spreadsheet, well, this fully
generalizes and automates it:
http://kirr.homeunix.org/electronics/resistor-network-finder/

Tim

That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

https://www.dropbox.com/s/ajmp4kbio1uf7qn/Rugrat.jpg?raw=1





--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[John Doe]

jlarkin@highlandsniptechnology.com wrote:

\"Tim Williams\"
tiwill@seventransistorlabs.com> wrote:

Let the computer do it. You have your spreadsheet, well, this fully
generalizes and automates it:
http://kirr.homeunix.org/electronics/resistor-network-finder/

That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

https://www.dropbox.com/s/ajmp4kbio1uf7qn/Rugrat.jpg?raw=1

\"per cent\" ???

 

[Ricketty C]

On Saturday, October 10, 2020 at 1:41:22 PM UTC-4, jla...@highlandsniptechnology.com wrote:

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

The more general issue, which I\'ve thought some about over the years,
is that there are a large number of circuits that can be made from a
given number of parts. I\'m not sure how many.

I\'ve thought it would be fun to teach a class in circuit design, where
each student gets 10 parts.

For two components you don\'t need to try every component. Pick one component and search for the optimal mate. Pick the next value of the first component and you only need to start with the same value of the second component and work in one direction until the match peaks. Lather, rinse, repeat. Often only one new second component will need to be checked or two at most if the candidates are equally spaced. If you are working with the parts already in the BOM the searching may be a bit longer... or shorter is more likely. While the jumps are not even, they are also a lot wider.

With more than two components the searching is more complex, but the same method applies to trim the rather large search tree.

It\'s hard to imagine needing this sort of precision in a resistor divider when there is often not so much more precision in the rest of the circuit. What\'s the voltage reference rated at? What precision are you trying to achieve?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[server]

On Sat, 10 Oct 2020 10:41:06 -0700, jlarkin@highlandsniptechnology.com
wrote:

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

The more general issue, which I\'ve thought some about over the years,
is that there are a large number of circuits that can be made from a
given number of parts. I\'m not sure how many.

I\'ve thought it would be fun to teach a class in circuit design, where
each student gets 10 parts.

This works, with 4 resistors that are already on the BOM.

Sadly, the values were found by pure guessing and fiddling.

4.7 nF is the only reasonable cap available to use for compensation,
which further constrains the divider values.


Version 4
SHEET 1 1140 680
WIRE -64 -48 -272 -48
WIRE 32 -48 -64 -48
WIRE 288 -48 128 -48
WIRE 496 -48 288 -48
WIRE 688 -48 496 -48
WIRE 880 -48 688 -48
WIRE 992 -48 880 -48
WIRE 1056 -48 992 -48
WIRE -272 0 -272 -48
WIRE 288 0 288 -48
WIRE 688 16 688 -48
WIRE 112 48 112 0
WIRE 880 64 880 -48
WIRE 1056 64 1056 -48
WIRE 496 96 496 -48
WIRE -272 128 -272 80
WIRE 688 128 688 80
WIRE -64 144 -64 -48
WIRE 288 144 288 80
WIRE -96 160 -144 160
WIRE 48 176 -32 176
WIRE 112 176 112 128
WIRE 112 176 48 176
WIRE 880 176 880 128
WIRE -208 192 -272 192
WIRE -96 192 -208 192
WIRE 112 192 112 176
WIRE -272 224 -272 192
WIRE -64 240 -64 208
WIRE 688 272 688 208
WIRE 1056 272 1056 144
WIRE -144 288 -144 160
WIRE 112 288 112 256
WIRE 112 288 -144 288
WIRE 288 288 288 224
WIRE 288 288 112 288
WIRE 400 288 288 288
WIRE 496 288 496 176
WIRE 496 288 400 288
WIRE -272 352 -272 304
WIRE 288 352 288 288
WIRE 496 400 496 288
WIRE 288 480 288 432
WIRE 288 608 288 560
WIRE 496 608 496 480
WIRE 496 608 288 608
WIRE 288 640 288 608
FLAG 688 272 0
FLAG 1056 272 0
FLAG 288 640 0
FLAG -272 352 0
FLAG -272 128 0
FLAG -64 240 0
FLAG 880 176 0
FLAG 48 176 G
FLAG 992 -48 +20
FLAG -208 192 REF
FLAG 400 288 E
SYMBOL res 272 -16 R0
WINDOW 0 67 43 Left 2
WINDOW 3 53 76 Left 2
SYMATTR InstName R1
SYMATTR Value 6.04K
SYMBOL res 272 336 R0
WINDOW 0 61 44 Left 2
WINDOW 3 54 79 Left 2
SYMATTR InstName R2
SYMATTR Value 10K
SYMBOL nmos 32 0 R270
WINDOW 0 -7 -45 VRight 2
WINDOW 3 -43 -63 VRight 2
SYMATTR InstName M1
SYMATTR Value RSR025N05
SYMBOL res 96 32 R0
WINDOW 0 63 34 Left 2
WINDOW 3 57 68 Left 2
SYMATTR InstName R3
SYMATTR Value 49.9
SYMBOL Opamps\\\\UniversalOpamp2 -64 176 R0
WINDOW 0 30 -37 Left 2
SYMATTR InstName U1
SYMBOL cap 96 192 R0
WINDOW 0 65 14 Left 2
WINDOW 3 57 47 Left 2
SYMATTR InstName C1
SYMATTR Value 4.7n
SYMBOL cap 672 16 R0
WINDOW 0 60 17 Left 2
WINDOW 3 57 58 Left 2
SYMATTR InstName C2
SYMATTR Value 56µ
SYMBOL res 672 112 R0
WINDOW 0 59 48 Left 2
WINDOW 3 59 80 Left 2
SYMATTR InstName Resr
SYMATTR Value 30m
SYMBOL voltage -272 -16 R0
WINDOW 0 60 46 Left 2
WINDOW 3 62 81 Left 2
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL voltage -272 208 R0
WINDOW 0 50 84 Left 2
WINDOW 3 26 127 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName Vref
SYMATTR Value PULSE(3 2.9 100m 1u 1u 100m)
SYMBOL res 1040 48 R0
WINDOW 0 56 42 Left 2
WINDOW 3 52 74 Left 2
SYMATTR InstName R5
SYMATTR Value 500
SYMBOL cap 864 64 R0
WINDOW 0 62 13 Left 2
WINDOW 3 58 48 Left 2
SYMATTR InstName C3
SYMATTR Value 10µ
SYMBOL res 272 464 R0
WINDOW 0 64 43 Left 2
WINDOW 3 61 71 Left 2
SYMATTR InstName R4
SYMATTR Value 1µ
SYMBOL res 272 128 R0
WINDOW 0 62 40 Left 2
WINDOW 3 59 69 Left 2
SYMATTR InstName R6
SYMATTR Value 49.9K
SYMBOL res 480 80 R0
WINDOW 0 62 40 Left 2
WINDOW 3 63 78 Left 2
SYMATTR InstName R7
SYMATTR Value 1T
SYMBOL res 480 384 R0
WINDOW 0 72 42 Left 2
WINDOW 3 61 75 Left 2
SYMATTR InstName R8
SYMATTR Value 1Meg
TEXT 808 432 Left 2 !.tran 500m
TEXT 712 104 Left 2 ;polymer
TEXT 904 144 Left 2 ;ceramics
TEXT 784 344 Left 2 ;P505C +20 Supply
TEXT 768 384 Left 2 ;J Larkin Oct 10 2020




--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[server]

On Sat, 10 Oct 2020 21:06:58 -0000 (UTC), John Doe
<always.look@message.header> wrote:

jlarkin@highlandsniptechnology.com wrote:

\"Tim Williams\"
tiwill@seventransistorlabs.com> wrote:

Let the computer do it. You have your spreadsheet, well, this fully
generalizes and automates it:
http://kirr.homeunix.org/electronics/resistor-network-finder/

That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

https://www.dropbox.com/s/ajmp4kbio1uf7qn/Rugrat.jpg?raw=1

\"per cent\" ???

That means multiply by 0.01.

Always glad to be of help to nice people.





--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Tim Williams]

<jlarkin@highlandsniptechnology.com> wrote in message
news:3v54of1ffbumr24hmb4emaron0qe42uc8j@4ax.com...

That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

Picky picky, use this one then:
http://jansson.us/resistors.html

Doesn\'t seem to be one that does both at once. You can always populate the
list with all N-level S-P combinations of your inventory.

Or, technically this does it all, it\'s just \"harder to operate\" (some code
may or may not be missing ;-) ):
https://repl.it/repls/WarpedOilyHarddrive

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

 

[Ricketty C]

On Saturday, October 10, 2020 at 9:00:38 PM UTC-4, Tim Williams wrote:

jlarkin@highlandsniptechnology.com> wrote in message
news:3v54of1ffbumr24hmb4emaron0qe42uc8j@4ax.com...
That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

Picky picky, use this one then:
http://jansson.us/resistors.html

Doesn\'t seem to be one that does both at once. You can always populate the
list with all N-level S-P combinations of your inventory.

Or, technically this does it all, it\'s just \"harder to operate\" (some code
may or may not be missing ;-) ):
https://repl.it/repls/WarpedOilyHarddrive

I created a spread sheet ages ago that uses the E24 values in a table showing all the ratios. Then finding a close fit is just a matter of scanning a diagonal. I can fill in a copy with values from the current project and do the same thing so I don\'t need to add parts to the board. This also allows me to see where the gaps are so when I do add a new part I can pick one that fills in a gap.

A board I designed some time back with 80 resistors and 70 capacitors and 15 ICs only required 35 line items in the BOM. The assembly house mentioned to me once that they were surprised they didn\'t need to change out reels for my board. I had made a significant effort to keep the BOM to a minimum by carefully selecting values that worked together. One amp stage used both positive and negative feedback so five different value resistances. I picked those values so they could be used in other circuits and only added a line item when absolutely necessary. It\'s not hard if you spend a little time on it. This has paid in lower assembly costs for the last 12 years.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

[Bill Sloman]

On Sunday, October 11, 2020 at 3:03:44 PM UTC+11, Ricketty C wrote:

On Saturday, October 10, 2020 at 9:00:38 PM UTC-4, Tim Williams wrote:
jla...@highlandsniptechnology.com> wrote in message
news:3v54of1ffbumr24hm...@4ax.com...

<snip>

> I created a spread sheet ages ago that uses the E24 values in a table showing all the ratios.

E96 resistors are readily available and not particularly expensive. If you can live with a +/-1% tolerance they can be quite cheap. Down around wha tit cost you to place them on a PCB.

The spread sheet would be bigger.

--
Bill Sloman, Sydney

 

[boB]

On Sat, 10 Oct 2020 13:30:14 -0700, jlarkin@highlandsniptechnology.com
wrote:

On Sat, 10 Oct 2020 14:26:03 -0500, \"Tim Williams\"
tiwill@seventransistorlabs.com> wrote:

Let the computer do it. You have your spreadsheet, well, this fully
generalizes and automates it:
http://kirr.homeunix.org/electronics/resistor-network-finder/

Tim

That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

https://www.dropbox.com/s/ajmp4kbio1uf7qn/Rugrat.jpg?raw=1

I like the Thevenin lower and upper limit parameters !

 

[Martin Brown]

On 10/10/2020 18:41, jlarkin@highlandsniptechnology.com wrote:

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

You only have a quite finite number of resistor values on the board
enumerate their possible values in the series and series parallel
combinations you are prepared to allow and then create a sorted list to
search for your chosen ratio at acceptable precision.

It isn\'t unlike writing a solver for the TV show \"Countdown\" where with
reasonable reliability you can hit any 3 digit target from 6 random
numbers and the various arithmetic operators. Bit more limited in this
case since you only have additive series or parallel combinations.

https://en.wikipedia.org/wiki/Countdown_(game_show)#Numbers_round

Negative resistors are very hard to come by.


--
Regards,
Martin Brown

 

[server]

On Sun, 11 Oct 2020 02:17:59 -0700, boB <boB@K7IQ.com> wrote:

On Sat, 10 Oct 2020 13:30:14 -0700, jlarkin@highlandsniptechnology.com
wrote:

On Sat, 10 Oct 2020 14:26:03 -0500, \"Tim Williams\"
tiwill@seventransistorlabs.com> wrote:

Let the computer do it. You have your spreadsheet, well, this fully
generalizes and automates it:
http://kirr.homeunix.org/electronics/resistor-network-finder/

Tim

That seems to find 2-terminal values, not dividers. It\'s really a
different problem. But it is cute.

I have a program that picks two resistors that we have in stock, to
make dividers, but I only want to use parts already on the board, and
need more complex networks than two resistors.

https://www.dropbox.com/s/ajmp4kbio1uf7qn/Rugrat.jpg?raw=1



I like the Thevenin lower and upper limit parameters !

That keeps me from using 2 ohm or 100 meg resistors.

The program reads a simple text file that our material control system
generates, so it could be used by anyone who can put their available
parts into such a file.

It does use parts that we have in stock, not the entire Exx lists. If
I was willing to buy new parts, I wouldn\'t need the program.

PowerBasic.

I was able to solve my original problem with three resistors and a lot
of fiddling and no addition to the BOM.


Version 4
SHEET 1 1140 680
WIRE -64 -48 -336 -48
WIRE 288 -48 112 -48
WIRE 528 -48 288 -48
WIRE 720 -48 528 -48
WIRE 832 -48 720 -48
WIRE 896 -48 832 -48
WIRE -336 0 -336 -48
WIRE 288 0 288 -48
WIRE 528 16 528 -48
WIRE 720 64 720 -48
WIRE 896 64 896 -48
WIRE -336 128 -336 80
WIRE 528 128 528 80
WIRE -64 144 -64 -48
WIRE 288 144 288 80
WIRE -96 160 -144 160
WIRE 48 176 -32 176
WIRE 112 176 112 -48
WIRE 112 176 48 176
WIRE 720 176 720 128
WIRE -208 192 -336 192
WIRE -96 192 -208 192
WIRE 112 192 112 176
WIRE -64 240 -64 208
WIRE -336 256 -336 192
WIRE 528 272 528 208
WIRE 896 272 896 144
WIRE -144 288 -144 160
WIRE 112 288 112 256
WIRE 112 288 -144 288
WIRE 288 288 288 224
WIRE 288 288 112 288
WIRE 288 352 288 288
WIRE -336 384 -336 336
WIRE 288 480 288 432
FLAG 528 272 0
FLAG 896 272 0
FLAG 288 480 0
FLAG -336 384 0
FLAG -336 128 0
FLAG -64 240 0
FLAG 720 176 0
FLAG 48 176 G
FLAG 832 -48 +20
FLAG -208 192 REF
SYMBOL res 272 -16 R0
WINDOW 0 67 43 Left 2
WINDOW 3 53 76 Left 2
SYMATTR InstName R1
SYMATTR Value 23.7K
SYMBOL res 272 336 R0
WINDOW 0 61 44 Left 2
WINDOW 3 54 79 Left 2
SYMATTR InstName R2
SYMATTR Value 4.99K
SYMBOL Opamps\\\\UniversalOpamp2 -64 176 R0
WINDOW 0 30 -37 Left 2
SYMATTR InstName U1
SYMATTR Value2 Avol=1Meg GBW=1Meg Slew=1Meg
SYMATTR SpiceLine ilimit=1 rail=0 Vos=0 phimargin=45
SYMBOL cap 96 192 R0
WINDOW 0 65 14 Left 2
WINDOW 3 57 47 Left 2
SYMATTR InstName C1
SYMATTR Value 4.7n
SYMBOL cap 512 16 R0
WINDOW 0 60 17 Left 2
WINDOW 3 57 58 Left 2
SYMATTR InstName C2
SYMATTR Value 56µ
SYMBOL res 512 112 R0
WINDOW 0 59 48 Left 2
WINDOW 3 59 80 Left 2
SYMATTR InstName Resr
SYMATTR Value 30m
SYMBOL voltage -336 -16 R0
WINDOW 0 60 46 Left 2
WINDOW 3 62 81 Left 2
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL voltage -336 240 R0
WINDOW 0 52 87 Left 2
WINDOW 3 16 126 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName Vref
SYMATTR Value PULSE(3 3.1 100m 1u 1u 100m)
SYMBOL res 880 48 R0
WINDOW 0 56 42 Left 2
WINDOW 3 52 74 Left 2
SYMATTR InstName R5
SYMATTR Value 500
SYMBOL cap 704 64 R0
WINDOW 0 62 13 Left 2
WINDOW 3 58 48 Left 2
SYMATTR InstName C3
SYMATTR Value 10µ
SYMBOL res 272 128 R0
WINDOW 0 62 40 Left 2
WINDOW 3 59 69 Left 2
SYMATTR InstName R6
SYMATTR Value 4.99K
TEXT 616 432 Left 2 !.tran 500m
TEXT 552 104 Left 2 ;polymer
TEXT 744 144 Left 2 ;ceramics
TEXT 584 344 Left 2 ;P505C +20 Supply
TEXT 568 392 Left 2 ;J Larkin Oct 10 2020
TEXT 752 -16 Left 2 ;20.25
TEXT -48 104 Left 2 ;TCA0372





--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[server]

On Sun, 11 Oct 2020 15:57:27 +0100, Martin Brown
<\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 10/10/2020 18:41, jlarkin@highlandsniptechnology.com wrote:


I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

You only have a quite finite number of resistor values on the board
enumerate their possible values in the series and series parallel
combinations you are prepared to allow and then create a sorted list to
search for your chosen ratio at acceptable precision.

It isn\'t unlike writing a solver for the TV show \"Countdown\" where with
reasonable reliability you can hit any 3 digit target from 6 random
numbers and the various arithmetic operators. Bit more limited in this
case since you only have additive series or parallel combinations.

https://en.wikipedia.org/wiki/Countdown_(game_show)#Numbers_round

Negative resistors are very hard to come by.

Actually, once one gets close to the target voltage, there becomes the
possibility of adding a tweak resistor to the positive feedback path
of the opamp.

As a college EE project, I built a 2-terminal -1K resistor, plugged
the -1K value into the usual circuit equations, and demonstrated that
they worked. Rising exponentials, voltage dividers with gain, negative
Q\'s, things like that.



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[Bill Sloman]

On Monday, October 12, 2020 at 1:57:37 AM UTC+11, Martin Brown wrote:

On 10/10/2020 18:41, jla...@highlandsniptechnology.com wrote:


I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.
You only have a quite finite number of resistor values on the board
enumerate their possible values in the series and series parallel
combinations you are prepared to allow and then create a sorted list to
search for your chosen ratio at acceptable precision.

It isn\'t unlike writing a solver for the TV show \"Countdown\" where with
reasonable reliability you can hit any 3 digit target from 6 random
numbers and the various arithmetic operators. Bit more limited in this
case since you only have additive series or parallel combinations.

https://en.wikipedia.org/wiki/Countdown_(game_show)#Numbers_round

Negative resistors are very hard to come by.

Negative resistance is fairly easy to set up - over a finite frequency range. Philips had a motor control circuit that cancelled out the series resistance of the windings in a DC motor to that iwould keep it on spinning at the same speed even if the load increased - but only up to a point.

--
Bill Sloman, Sydney

 

[Chris Jones]

On 11/10/2020 04:41, jlarkin@highlandsniptechnology.com wrote:

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

No, if you write it in C you should be able to do pretty much all
possible combinations of (many) more than 4 resistors, in a time that is
negligible for a human. Computers are very fast nowadays.

1. Make a list, of all the resistor values given.
2. Make another list with each thing on the first list in parallel with
each thing on the first list.
3. Make a third list with each thing on the first list in series with
each thing on the first list.
4. Put everything on any of the lists you have so far, into the first list.
5. If you haven\'t used enough resistors yet, go to step 2 again.
6. Now you have a big list. Go through it and pick the thing closest to
what you want. In your case, try all combinations of two things on that
list, as upper and lower resistors in a divider. Try them all and pick
the best one, by whatever criteria make it best for you.

You could do it with some recursive program I guess. I just stored them
all in RAM because I had a lot of resistors to approximate. I also made
it figure out what the standard deviation of the combined resistor was,
because some topologies give better averaging than others.

There are lots of ways to be clever and do it quicker, but there is no need.

 

[John Doe]

Chris Jones wrote:

jlarkin wrote:

I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

No, if you write it in C you should be able to do pretty much all
possible combinations of (many) more than 4 resistors, in a time that is
negligible for a human. Computers are very fast nowadays.

Yes they are. Including NVMe (pronounced \"envy me\") drives. Get yourself a
modern motherboard with at least two such drive slots and handle gigabyte
size files with ease.





--

1. Make a list, of all the resistor values given.
2. Make another list with each thing on the first list in parallel with
each thing on the first list.
3. Make a third list with each thing on the first list in series with
each thing on the first list.
4. Put everything on any of the lists you have so far, into the first
list.
5. If you haven\'t used enough resistors yet, go to step 2 again.
6. Now you have a big list. Go through it and pick the thing closest to
what you want. In your case, try all combinations of two things on that
list, as upper and lower resistors in a divider. Try them all and pick
the best one, by whatever criteria make it best for you.

You could do it with some recursive program I guess. I just stored them
all in RAM because I had a lot of resistors to approximate. I also made
it figure out what the standard deviation of the combined resistor was,
because some topologies give better averaging than others.

There are lots of ways to be clever and do it quicker, but there is no
need.

 

[server]

On Tue, 13 Oct 2020 00:12:58 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

On 11/10/2020 04:41, jlarkin@highlandsniptechnology.com wrote:


I\'m tweaking a big schematic to get to my layout guy first thing
Monday.

I\'ve got a few places where I need a specific resistor ratio. One is
for a home-made LDO to make a lot of clean 3.3 volt power. Another is
to make a very accurate +20 volt supply. Another is to scale a DAC
output into a varicap. Our pick-and-place will use almost all its
feeders on this board, so I prefer to use only resistors that are
already on the BOM.

A little futzing with a calculator suggests that I don\'t have values
available, given two resistors per divider and parts already on the
BOM. The BOM has already been brutally minimized.

I can tweak a divider with a third resistor. In series with the upper
or lower resistor, or in parallel with either. I could even use four
resistors. We also have several values of quad resistor pack on the
board, and a quad pack has a lot of options.

So the general problem is that there are zillions of possibilities,
surely some good ones, but no good way to find them. Spicing would at
least save a lot of calculator use, but still has no methodical
approach.

I could write a program that brute-force tries all possibilities of
values in some series-parallel circuit, say with 8 places where 4
resistors might go. It might run in minutes or years. Ugly.

No, if you write it in C you should be able to do pretty much all
possible combinations of (many) more than 4 resistors, in a time that is
negligible for a human. Computers are very fast nowadays.

1. Make a list, of all the resistor values given.
2. Make another list with each thing on the first list in parallel with
each thing on the first list.
3. Make a third list with each thing on the first list in series with
each thing on the first list.
4. Put everything on any of the lists you have so far, into the first list.
5. If you haven\'t used enough resistors yet, go to step 2 again.
6. Now you have a big list. Go through it and pick the thing closest to
what you want. In your case, try all combinations of two things on that
list, as upper and lower resistors in a divider. Try them all and pick
the best one, by whatever criteria make it best for you.

You could do it with some recursive program I guess. I just stored them
all in RAM because I had a lot of resistors to approximate. I also made
it figure out what the standard deviation of the combined resistor was,
because some topologies give better averaging than others.

There are lots of ways to be clever and do it quicker, but there is no need.

I think that there are 8 different dividers that you can make from
four resistors

https://www.dropbox.com/s/m56k4bp21z4fb6z/Rpak_Dividers_1.jpg?raw=1

Now I have 19 different resistor values on my BOM. That makes
something like 8^19 possibilities. I also have 5 quad r-packs, each of
which can have 15 possible arrangements to make an equivalent
resistor.

So we have 8^34 possibilities. Many more if you are flexible about how
you use the r-pack sections. The program would run for millenia. It
might take millenia just to write.

And this is a simple problem. My main point is that very many circuits
can be made from a few parts.

It\'s a 32-page schematic, and this is just one dumb voltage regulator.

Somehow our brains deal with this, and we get things to work.

It would be cool to write a program that found the best divider that
can be made from two 1K quad r-packs. Even that would be a serious
chore. How many dividers can you make from 8 resistors?

Next, allow two different r-pack values. We have 26 in stock.








--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard