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Bootstrap Zener with op-amp follower?

S

Spehro Pefhany

Jan 1, 1970
0
On 23 Jan 2004 06:39:59 -0800, the renowned
Is there anything fundamentally wrong with bootstrapping a zener with
an op-amp follower?

No, nothing fundamentally wrong if you make sure it will start up. I
don't know why you are doing it that way, it makes the zener current
strongly beta dependent, which is a strange choice, especially
considering how truly crummy low-voltage zeners are.

That's a strange choice of $$$ op-amp for that application. I really
doubt your simulation results- almost 3V across the 1N5221B with small
10's of uA current? I'd guess more like <1V in that circuit.

Best regards,
Spehro Pefhany
 
A

Aubrey McIntosh

Jan 1, 1970
0
| On 23 Jan 2004 06:39:59 -0800, the renowned
| [email protected] (Aubrey McIntosh) wrote:
|
| >Is there anything fundamentally wrong with bootstrapping a zener with
| >an op-amp follower?
|
| No, nothing fundamentally wrong if you make sure it will start up. I
| don't know why you are doing it that way, it makes the zener current
| strongly beta dependent, which is a strange choice, especially
| considering how truly crummy low-voltage zeners are.
|

For a more reliable start up, I put a blocking diode back to the
op-amp. Some of the simulations reported a strange stable state
where the op-amp output was about -300V. Also I have room for the
voltage drop. I also put a 1.3M from V+ to the top of the zener to
force a positive voltage on In+ to the op-amp, and help kick start.

Does this bear any resemblance to a well designed fail-safe starter?

As AoE recommends, I'm trying to avoid beta dependency, so I am
considering the circuit under light of your comment. I did look at
the ratio of Ie(Q2) / Ib(Q2) in the simulation, and I see values
around 200. If I have read the .model correctly the beta for the
2N4401 is about 4K (Bf=4.292K). My interpretation was that I had a
lot of beta budget left over, and then I assumed I am not being beta
dependent. Did I miss the path in that reasoning?

My impression is that the only really critical component in this
circuit is R_Sense. With an op-amp, I am able to use current sense
voltage of only about 200mV for about 2uW loss. I actually felt
pretty good about that. I am stepping the input voltage over an
absurd range, and it seems pretty stable to that. Temperature
stepping shows that the load current grows at cold temperatures. I
assume that is more tolerable than higher current at high
temperatures. I don't have any sense of whether that should be
controlled or not.

I'm sort of in the frame of mind I was in 20 years ago when I had an
Argus C3 camera and no money for a flash: I want to learn how to
tame the beast, then I can move to the better substitute. (hints as
to what to use instead of a zener will gladly be squirreled away)


| >Concept circuit and simulation output online:
| >
| >http://www.jump.net/~vima/led/Active_04.pdf
| >http://www.jump.net/~vima/led/Active_04_tran.pdf
|
| That's a strange choice of $$$ op-amp for that application. I really
| doubt your simulation results- almost 3V across the 1N5221B with small
| 10's of uA current? I'd guess more like <1V in that circuit.
|
|

I made up my own .model for the 1N5221B, and now have decided not to
trust it. I have replaced this with a 1N750 with SwitcherCAD's
.model statement, and made tuning adjustments to other passives.

I don't have the "generic user supplied" op-amp under control, so I
have a lot more SwitcherCAD homework to do. The op-amp in this
circuit is only a place holder for later.

I assume that this op-amp does not need to be fast. Other criteria
that I assume make sense are 1) low power, 2) stable at DC, 3) if I
want to actually build something later -- under $2, 4) single supply.
 
J

John Larkin

Jan 1, 1970
0
(hints as
to what to use instead of a zener will gladly be squirreled away)

The little bandgap things, LM4040-series or whatever, are very stiff
over a huge current range and go down to 1.2 volts, no bootstrap
needed. Low voltage zeners suck.

John
 
S

Spehro Pefhany

Jan 1, 1970
0
The little bandgap things, LM4040-series or whatever, are very stiff
over a huge current range and go down to 1.2 volts, no bootstrap
needed. Low voltage zeners suck.

LMV431 is cheaper. 1.24V and 1% tolerance. Both need 50~100uA to
operate. If you want the circuit to work down to around a single LED
voltage, you'll need an op-amp that's functional at around 2.2~2.7V,
preferably micropower, and Vos would be not be a very important
consideration (you have 160mV or whatever to work with). Maybe an
LMC7111 or something like that. BTW, you're wasting ~1mA in that
precision op-amp.

It might be more fun to try to use the LED itself as a bootstrapped
reference.

Vx o
|
|
|
+--------------+
| ---
.-. \ / LED
Ra | | ---
| | |
'-' |
| +------ Vb
Va ------+ |
.-. .-.
| | | | Rs
Rb | | | |18R
'-' '-'
| |
+--------------|
===
k = Ra/(Ra+Rb) GND

If Va = Vb, then If = (Vf/Rs)* (k/(1-k))

Best regards,
Spehro Pefhany
 
J

John Larkin

Jan 1, 1970
0
LMV431 is cheaper. 1.24V and 1% tolerance. Both need 50~100uA to
operate. If you want the circuit to work down to around a single LED
voltage, you'll need an op-amp that's functional at around 2.2~2.7V,
preferably micropower, and Vos would be not be a very important
consideration (you have 160mV or whatever to work with). Maybe an
LMC7111 or something like that. BTW, you're wasting ~1mA in that
precision op-amp.

It might be more fun to try to use the LED itself as a bootstrapped
reference.

Vx o
|
|
|
+--------------+
| ---
.-. \ / LED
Ra | | ---
| | |
'-' |
| +------ Vb
Va ------+ |
.-. .-.
| | | | Rs
Rb | | | |18R
'-' '-'
| |
+--------------|
===
k = Ra/(Ra+Rb) GND

If Va = Vb, then If = (Vf/Rs)* (k/(1-k))

Best regards,
Spehro Pefhany


Dang, Spehro, you beat me to it. I was thinking of a grounded LED with
a highside resistor, and driving the top of the resistor to 1.1 * Vled
or something like that, right from a r-r opamp. Opamp plus 4
resistors, including a startup thingie.

John
 
W

Winfield Hill

Jan 1, 1970
0
Aubrey McIntosh wrote...
As AoE recommends, I'm trying to avoid beta dependency, so I am
considering the circuit under light of your comment. I did look at
the ratio of Ie(Q2) / Ib(Q2) in the simulation, and I see values
around 200. If I have read the .model correctly the beta for the
2N4401 is about 4K (Bf=4.292K). My interpretation was that I had
a lot of beta budget left over, and then I assumed I am not being
beta dependent. Did I miss the path in that reasoning?

The 2n4401 is not a darlington transistor! Like most BJTs, it
has a beta of 50 to 200, so ideally for circuit robustness you
shouldn't require a beta of over say 25. Rather than depend on
Spice, it's wise to obtain and read data sheets and think about
their specs. http://www.onsemi.com/pub/Collateral/2N4401-D.PDF
As you know, in AoE we teach back-of-the envelope calculations,
rather than spice models, for evaluating circuits. :>)

Thanks,
- Win

whill_at_picovolt-dot-com
 
A

Arie de Muynck

Jan 1, 1970
0
Aubrey McIntosh said:
Is there anything fundamentally wrong with bootstrapping a zener with
an op-amp follower?

That circuit tries to stabilize the voltage on the emitter resistor to drive
the LED with a constant current.
This means the output voltage from the opamap will mainaly depend on the
current gain of the transistor. It is quite likely too small to drive the
zener anyway.

This is NOT a bootstrap circuit for the zener. That would have to produce a
fixed voltage at the opamp output, above the zener voltage and the drop over
D4.

Regards,
Arie de Muynck
 
F

Fred Bloggs

Jan 1, 1970
0
Aubrey McIntosh wrote:
[..snip...]

Just stop wasting time and use two opamps- one for your voltage
reference and a second to servo the output- and what purpose does that
ultra-large base resistor serve except to test the output range limits
of the opamp. What is the circuit supposed to do anyway?
 
F

Fred Bloggs

Jan 1, 1970
0
As you know, in AoE we teach back-of-the envelope calculations,
rather than spice models, for evaluating circuits. :>)

LOL- How else are neophytes to be exposed to the harsh realities of 5V
opamps with 300V outputs and comparators that seek out midscale
equilibrium solutions?
 
A

Aubrey McIntosh

Jan 1, 1970
0
Fred Bloggs wrote:

| LOL- How else ...

Glad to be of help. More to follow soon.
 
A

Aubrey McIntosh

Jan 1, 1970
0
I quickly scanned this morning's postings, and I'll go digest them
soon. Meanwhile;

RESULTS
I calculated values for the bootstrapped LED with a 10 mA target
current, using the Yellow LED .model posted recently on s.e.d.

The LED grounded vs sense resistor grounded are anti-symmetric in the
bridge. I find it conceptually clearer to have the sense resistor
grounded, as the target signal is the virtual ground on the op-amp.

In order to stabilize the circuit as the control supply voltage
changes, however, I ended up switching geometries.

The problem is that the R4, the "force to start" resistor gives more
current into the op-amp at higher supply. Compensation is done with
R5, a value too big for the Mouser catalog. It seemed simpler, late
at night, to work with the grounded LED geometry to comprehend the
compensation tweaks.

Staying with 5% values, I have a choice of the drift being to higher
current or lower current. It seems philosophically better to have
the circuit shut down instead of run away, so I drifted on that side
of the line.

This approach also seems to stabilize the zener circuits, but as the
entire exercise is with my 10 Kingbright white LEDs in mind, I
thought I would just switch over.

Files at:

http://www.jump.net/~vima/led/boot_led_01_asc.pdf
http://www.jump.net/~vima/led/boot_led_01_plt.pdf
http://www.jump.net/~vima/led/led_curve_01_asc.pdf
http://www.jump.net/~vima/led/led_curve_01_plt.pdf

DISCUSSION
When I tried to step the temperature, I got the failure to converge
message. Previously I read and found which parameter to relax in
SwitcherCAD III so that the simulation can proceed in this case, but
I can't remember at the moment. If someone knows off the top of
their head, it would save me another search.

I did find the LTC.lib file with many models in it. Presumably, I
will read the spice details there, and draw some circuits with a few
op-amps in them. This should keep me out of mischief for a while.
Since I have separated the "control" and "power" supplies, I don't
really need an op amp that will work from 4 -- 32V, so there must
also be a better (low power) op-amp that springs to mind, and
available in the SwitcherCAD library.

Now that I have a clear picture of the concept, I am ready for an
actual engineering concern or two. With the deisgn goals of :
stable, as close to 10mA as possible without going over, generally
well behaved according to your long experience, and available at
Mouser, what would you choose for the values in the bridge, and why?
Carbon Film? Metallized film? Do you always choose a certain way
between KOA, Xicon, others?
 
A

Aubrey McIntosh

Jan 1, 1970
0
Winfield Hill said:
The 2n4401 is not a darlington transistor! Like most BJTs, it
has a beta of 50 to 200, so ideally for circuit robustness you
shouldn't require a beta of over say 25. Rather than depend on
Spice, it's wise to obtain and read data sheets and think about
their specs. http://www.onsemi.com/pub/Collateral/2N4401-D.PDF
As you know, in AoE we teach back-of-the envelope calculations,
rather than spice models, for evaluating circuits. :>)

Thanks. I now have the datasheet and am reading over it.

I went on a Google expedition for a SPICE model of the 2N4401.

1. I found a link to a 1988 spice model which uses the same model as
SwitcherCAD III at
http://matrix.coe.montana.edu/sshaw/classes/2000/EE216/
http://www.qsl.net/k7qo/models.html
http://www.qsl.net/k7qo/all.mod

2. I found a download site at fairchild. They prohibit additional
disclosure of their model. Thus far, this has been enforced by
non-delivery of the promised email.
http://www.fairchildsemi.com/models/email_model_file.jsp?file=2N4401.mod

3. I found a very abbreviated, but unique model:
http://www.xcvr.com/elmer101/spice.htm reports
..MODEL 2N4401 NPN (CJE=30pF CJC=6.5pF)

4. Our own Spehro, in the very recent past. Note his Bf value !!

5. And, finally, I found something that isn't what I was seeking, but
which resonates.
In ECE6416 at GaTech, they roll their own. An academic low noise
amplifier design with specifications that the students derive certain
values, IS, BF, RB, VA, CJC, CJE, and TF, for their spice models from
their own laboratory measurements or mfr datasheets.
http://users.ece.gatech.edu/~mleach/ece6416/Labs/design.pdf

I like the idea of measuring the characteristics of a specimen, and
calculating the SPICE values that it exhibits. A sanity check, kind
of like measuring the resistance of a resistor when it is marked on
the outside, plain as the colors in the rainbow. Sounds like a
PICLIST project.
 
B

Ben Bradley

Jan 1, 1970
0
In sci.electronics.design said:
LOL- How else are neophytes to be exposed to the harsh realities of 5V
opamps with 300V outputs

Perhaps this is why Bob Pease doesn't trust simulation programs -
how else could an op-amp powered by 5V give such a dangerously high
output voltage?
Don't touch the schematic on the screen...
 
A

Aubrey McIntosh

Jan 1, 1970
0
Fred Bloggs said:
Aubrey McIntosh wrote:
[..snip...]

Just stop wasting time and use two opamps- one for your voltage
reference and a second to servo the output- and what purpose does that
ultra-large base resistor serve except to test the output range limits
of the opamp. What is the circuit supposed to do anyway?

Fair enough.

The larger problem that I am working on is this. Design, build, and
characterize a "microdistillation" probe for mass spectrometry.
Interpret the data with factor analysis, according to "On the
discrimination and quantitation of isomeric mixtures of compounds by
mass spectrometry," Dissertation, McIntosh, 1999. Using known and
unknown mixtures, report assay values. Do subtle chemistry to enable
chiral determinations. Build at least one mass spectrometer which
allows such determinations to be performed quickly.

In use, the probe will be in a high vacuum chamber, 3E-7 Torr
pressure. Sample will be on the probe, and the temperature programmed
specifically to facilitate interpretation by the Arrhenius (or
Clausius-Clayperon) equation. Multiple compounds will be mixed, and
the exact amounts are not only unknown, but the object of
determination. In replicate runs, known amounts of the compounds can
be added to the sample (Method of Standard Additions) or else run in
pure form.

Total amounts of material will be small. Typically a 1 uL deposition
of solution is made, and the solvent evaporated. The solution
deposited is typically 25 mg / 25 mL w/w concentration. This is
general purpose, so the analytes are whatever comes in the door.
However, you may wish to Google for
to get an idea of the likely
candidates.

This probe is structurally similar to a "hot wire" probe that is in
common use, with more finesse. That is, it will be a helix with a
diameter of about 0.5 -- 1.0 mm and a length of 1-2 mm. The wire
diameter is small. It can be fabricated from Wollaston wire and
etched. The dimensions should be such that a droplet of solution
should be held by the wire prior to solvent evaporation. Various
solvents, such as water, ethanol, and methylene chloride should be
accomodated.

Preliminary constraints are:

1. Go from -50C to 300C as quickly as 30 seconds.
2. Report the actual temperature of the probe.
3. Manage the temperature profile so that the value (1 / Kelvin)
changes at a linear rate.
4. Deliver sufficient power so that the analyte evaporates without
deviation from the temperature profile.
5. Log the power delivered to the probe. Log enough information so
that the power given to the sample, as distinct from heating the wire,
is recorded. For reference, read up on Differential Scanning
Calorimetry. Adapt so that the reference scan is performed on the
same hot wire, at a different time. Be able to calculate Enthalpy of
Sublimation.
6. Oxygen sensitivity contra-indicates tungsten construction
material. Rhodium and platinum are commonly used and easily
available. Nickel is also a platinum group material, and may be
preferred for the early prototypes, depending on availability and cost
of well characterized material. Platinum thermometers are likely to
be the cheapest solution.
7. In the (predominant) operation mode where cooling is not
energized, record temperature with a minumum of self heating and
regain control when servo requests heating.
8. Allow a cleaning cycle where the wire reaches incandesence. While
not necessary chemically, this is an important human interface design
aspect.
9. Respect two emergency input signals: cut power, and maintain
current power.

The current area of effort is to move from linear-temperature-in-time
control without power logging, to linear in "reciprical Kelvins"
control with power logging and energy integration.

My choices are to learn enough electronics, or to hire a EE postdoc
and teach him vacuum systems and chemistry. So I am working through
various stable voltage controlled power supply concepts. You may
choose to view the LEDs as a red herring, a double-dipping project, or
a way to make the project seem interesting to the readership of s.e.d.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Aubrey McIntosh <[email protected]
a.austin.tx.us> wrote (in <[email protected]
om>) about 'Bootstrap Zener with op-amp follower?', on Tue, 27 Jan 2004:
My choices are to learn enough electronics, or to hire a EE postdoc and
teach him vacuum systems and chemistry.
You could make Bill an offer he can't refuse. To me, this sounds like a
tailor-made project for him, but I could be wildly wrong.
 
F

Fred Bloggs

Jan 1, 1970
0
Aubrey said:
Fred Bloggs said:
Aubrey McIntosh wrote:
[..snip...]

Just stop wasting time and use two opamps- one for your voltage
reference and a second to servo the output- and what purpose does that
ultra-large base resistor serve except to test the output range limits
of the opamp. What is the circuit supposed to do anyway?


Fair enough.

The larger problem that I am working on is this. Design, build, and
characterize a "microdistillation" probe for mass spectrometry.
Interpret the data with factor analysis, according to "On the
discrimination and quantitation of isomeric mixtures of compounds by
mass spectrometry," Dissertation, McIntosh, 1999. Using known and
unknown mixtures, report assay values. Do subtle chemistry to enable
chiral determinations. Build at least one mass spectrometer which
allows such determinations to be performed quickly.

In use, the probe will be in a high vacuum chamber, 3E-7 Torr
pressure. Sample will be on the probe, and the temperature programmed
specifically to facilitate interpretation by the Arrhenius (or
Clausius-Clayperon) equation. Multiple compounds will be mixed, and
the exact amounts are not only unknown, but the object of
determination. In replicate runs, known amounts of the compounds can
be added to the sample (Method of Standard Additions) or else run in
pure form.

Total amounts of material will be small. Typically a 1 uL deposition
of solution is made, and the solvent evaporated. The solution
deposited is typically 25 mg / 25 mL w/w concentration. This is
general purpose, so the analytes are whatever comes in the door.
However, you may wish to Google for
to get an idea of the likely
candidates.

This probe is structurally similar to a "hot wire" probe that is in
common use, with more finesse. That is, it will be a helix with a
diameter of about 0.5 -- 1.0 mm and a length of 1-2 mm. The wire
diameter is small. It can be fabricated from Wollaston wire and
etched. The dimensions should be such that a droplet of solution
should be held by the wire prior to solvent evaporation. Various
solvents, such as water, ethanol, and methylene chloride should be
accomodated.

Preliminary constraints are:

1. Go from -50C to 300C as quickly as 30 seconds.
2. Report the actual temperature of the probe.
3. Manage the temperature profile so that the value (1 / Kelvin)
changes at a linear rate.
4. Deliver sufficient power so that the analyte evaporates without
deviation from the temperature profile.
5. Log the power delivered to the probe. Log enough information so
that the power given to the sample, as distinct from heating the wire,
is recorded. For reference, read up on Differential Scanning
Calorimetry. Adapt so that the reference scan is performed on the
same hot wire, at a different time. Be able to calculate Enthalpy of
Sublimation.
6. Oxygen sensitivity contra-indicates tungsten construction
material. Rhodium and platinum are commonly used and easily
available. Nickel is also a platinum group material, and may be
preferred for the early prototypes, depending on availability and cost
of well characterized material. Platinum thermometers are likely to
be the cheapest solution.
7. In the (predominant) operation mode where cooling is not
energized, record temperature with a minumum of self heating and
regain control when servo requests heating.
8. Allow a cleaning cycle where the wire reaches incandesence. While
not necessary chemically, this is an important human interface design
aspect.
9. Respect two emergency input signals: cut power, and maintain
current power.

The current area of effort is to move from linear-temperature-in-time
control without power logging, to linear in "reciprical Kelvins"
control with power logging and energy integration.

My choices are to learn enough electronics, or to hire a EE postdoc
and teach him vacuum systems and chemistry. So I am working through
various stable voltage controlled power supply concepts. You may
choose to view the LEDs as a red herring, a double-dipping project, or
a way to make the project seem interesting to the readership of s.e.d.

The basic hot-wire anemometer circuit looks like so. Step the rheostat
in accordance with your 1/Kelvin time profile and digitize Vout:

Please view in a fixed-width font such as Courier.


Vout
+-----------+-----------e c--Vreg R1 x Rpot
| | \ / Rprobe= ---------
| | --- R3
/ / |
R1 R3 |
/ / | Vout
\ \ | Iprobe= -----------
| | | R1 + Rprobe
| | |\ |
+-----------|--------|-\ |
| | | >-+ Rpot
| +--------|+/ Vprobe= Vout x ---------
| | |/ Rpot + R3
/ /
Rprobe \<+
/ / | Pprobe= Vprobe x Iprobe
\ \ |
| +-+ Rpot<-digital pot
| | rheostat mode
| |
--- ---
/// ///
 
J

John Larkin

Jan 1, 1970
0
My choices are to learn enough electronics, or to hire a EE postdoc
and teach him vacuum systems and chemistry. So I am working through
various stable voltage controlled power supply concepts. You may
choose to view the LEDs as a red herring, a double-dipping project, or
a way to make the project seem interesting to the readership of s.e.d.


Why a postdoc? He'd probably be a researcher in some specific
discipline. A real circuit designer would probably be better at, well,
designing circuits.

The real problem sounds more interesting; there are way too many LED
questions here already.

John
 
J

James Arthur

Jan 1, 1970
0
LMV431 is cheaper. 1.24V and 1% tolerance. Both need 50~100uA to
operate.

A 1.2V TL431 -- neat!
If you want the circuit to work down to around a single LED
voltage, you'll need an op-amp that's functional at around 2.2~2.7V,
preferably micropower, and Vos would be not be a very important
consideration (you have 160mV or whatever to work with). Maybe an
LMC7111 or something like that.

BTW, you're wasting ~1mA in that
precision op-amp.

If the LED's getting 10mA then a) this is not a micropower application & b)
1mA lost is merely 10%. Is there a low-voltage requirement? I couldn't find
it.

It might be more fun to try to use the LED itself as a bootstrapped
reference.

Vx o
|
|
|
+--------------+
| ---
.-. \ / LED
Ra | | ---
| | |
'-' |
| +------ Vb
Va ------+ |
.-. .-.
| | | | Rs
Rb | | | |18R
'-' '-'
| |
+--------------|
===
k = Ra/(Ra+Rb) GND

If Va = Vb, then If = (Vf/Rs)* (k/(1-k))


Vf(LED), however, is a rather soft reference, not to mention
its drift with temp.

The object appears to be:
o to drive an LED
o with a constant 10mA
o from a single supply
o with "low" power consumption
o for less than $2

Why not go straight at it?

+5V
-+-
|
|
.-.
| | R1
| | 180 | i=11mA
'-' V
|
.---------+ <--- Va=Vf+1.24v=3.2v
| |
| --- LED
| \ / ~> Vf = 2.0v
LMV | ---
431 | |
--- |
/ \<-------+
--- |
| |
| .-.
| | | Rs
| | | 124
| '-'
| |
| |
| |
--- ---


A white LED with Vf=3.6 could be accommodated with
a higher supply voltage and appropriate R1, or at +5v
by using an LM10 for the shunt regulator.

OTOH, if the aim is to get constant *luminous flux* from
the LED, temperature compensation is essential; closed loop
control of temp. (or flux) preferred. (An LED heater + servo loop
stabilizing Vf(LED) should suffice.)


James Arthur
 
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