From Newsgroup: sci.electronics.design

Hi, All,

In my other thread on the power law circuit, I mentioned the thermal
Faraday shield, which may be of interest. (Long-but-worthwhile post warning)

Simon and I filed a patent last month on a scheme to improve temperature control by a lot.

*Temperature Control is Slow*
Loops controlling the temperatures of macroscopic objects are really
slow. The slew rate is slow because the available heating or cooling
power can't move the thermal mass very fast. More fundamentally, the bandwidth is limited because thermal diffusion is exponentially slow--asymptotically you get another radian of phase shift for every 1/e
worth of rolloff. That makes the usual speedup tricks useless.

*
However, it's possible to eliminate that delay by combining the heater
and temperature sensor in a single metal element, such as a bit of
copper flex circuit. This has been done N times before, but apparently
nobody noticed one key fact: If you measure the temperature using the
heater drive current, *there's no diffusion delay at all*.

This first came up when I was doing waveguide antenna-coupled Ni-NiO-Ni
tunnel junction infrared detectors at IBM, twenty-odd years ago. Unlike photodiodes, low-barrier TJs work by actually rectifying light, so these
were basically crystal radios running at 1.6 um. The devices were about
a micron across, made by directional evaporation of gold over nickel,
with a short Ni-NiO-Ni junction at the vertex. The TJ formed a
plasmonic traveling wave structure, so that the ~30 fs RC time constant
of the Ni-NiO-Ni system didn't trash the response at 190 THz (1.6 um).

It happened by accident during testing. This plot <https://electrooptical.net/www/sed/tj2fix.png> is what a reasonably
decent device produced in response to a 30-ps pulse at 2.4 um. (I had
this very swoopy tunable laser/optical parametric oscillator system back then.)

This second plot <https://electrooptical.net/www/sed/tj3.png> shows what happened when one of my tunnel junctions shorted out during testing. I
had about 100 mV of DC bias on it, so even at 3000 ppm/K, a 2.5 mV step
is big--a good 8 degrees C. (In this plot you have to mentally subtract
the baseline.) The bolometric response looked like a step function on
this scale, because it took about 5 us to cool back down, even with 3D
heat conduction and 1-um size.

Since the DC current path was about the same as the AC, the
near-instantaneous heating of the device produced a near-instantaneous
RTD response: about 40 picoseconds.

Applying this idea to normal life, in principle your temperature
controller can have any bandwidth you want. Of course the slowness of
thermal diffusion means that at sufficiently high frequency the
temperature of the RTD decouples from the rest of the world. However,
if you tile some surface with these things, you can effectively make a
thermal version of a Faraday shield--the huge control bandwidth gives
you arbitrarily good rejection of thermal forcing, with no bulky
insulation, stirred fluid baths, or big thermal masses.

The decoupling region actually has some interesting features--as the
frequency goes up, the amount of material you have to heat goes down, so there's a region where the phase shift is 45 degrees instead of the 90
degrees you get in the low frequency (thermal mass) limit. (This is
discussed in Section 20.3 of my third edition, <https://electrooptical.net/www/beos3e/thermal3e.pdf>.)
The decoupling also means that in principle the loop bandwidth and compensation don't need to depend on what the element is stuck onto--you
get the same huge forcing rejection regardless. You don't even have to
worry about windup, despite the slew rate being slow for the bandwidth.

There are a number of control schemes for this, of which my favorite is
analog PWM. The heater is in a resistive bridge with a shunt resistor
and a reference divider. It gets turned on for a microsecond or so at
the beginning by a strobe pulse and an RS-flipflop controlling an NMOS
switch, with the . A low-noise amplifier (ADA4899-ish) driving a
comparator resets the flipflop when the instantaneous temperature error crosses zero. (The FF is a NAND type, so the heater is turned on if
both SET and RESET are active.) It's cool to watch the duty cycle
change instantly if you touch the element, and of course the effective
loop bandwidth is huge--a short transient gets nulled out in the very
next clock cycle.

The Class-H thing I talked about in the other thread is for things like
DWDM lasers and OCXOs, where you don't want a lot of EMI right in the sensitive region. (At low power, you can just use an analog loop with a
fixed supply.)

There are a whole lot of things you can do with this general scheme,
from improved thermolelectric coolers to such things as a battery
calorimeter made of metallized mylar like a chip bag.

Fun stuff--suggestions for applications and further enhancements welcome!

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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From Newsgroup: sci.electronics.design

On 25/02/2026 6:57 am, Phil Hobbs wrote:

Hi, All,

In my other thread on the power law circuit, I mentioned the thermal
Faraday shield, which may be of interest. (Long-but-worthwhile post
warning)

Simon and I filed a patent last month on a scheme to improve temperature control by a lot.

*Temperature Control is Slow*
Loops controlling the temperatures of macroscopic objects are really
slow.-a The slew rate is slow because the available heating or cooling
power can't move the thermal mass very fast.-a More fundamentally, the bandwidth is limited because thermal diffusion is exponentially slow--asymptotically you get another radian of phase shift for every 1/e worth of rolloff.-a That makes the usual speedup tricks useless.

*
However, it's possible to eliminate that delay by combining the heater
and temperature sensor in a single metal element, such as a bit of
copper flex circuit.-a This has been done N times before, but apparently nobody noticed one key fact: If you measure the temperature using the
heater drive current, *there's no diffusion delay at all*.

This first came up when I was doing waveguide antenna-coupled Ni-NiO-Ni tunnel junction infrared detectors at IBM, twenty-odd years ago.-a Unlike photodiodes, low-barrier TJs work by actually rectifying light, so these were basically crystal radios running at 1.6 um. The devices were about
a micron across, made by directional evaporation of gold over nickel,
with a short Ni-NiO-Ni junction at the vertex.-a The TJ formed a
plasmonic traveling wave structure, so that the ~30 fs RC time constant
of the Ni-NiO-Ni system didn't trash the response at 190 THz (1.6 um).

It happened by accident during testing.-a This plot <https://electrooptical.net/www/sed/tj2fix.png> is what a reasonably
decent device produced in response to a 30-ps pulse at 2.4 um.-a (I had
this very swoopy tunable laser/optical parametric oscillator system back then.)

This second plot <https://electrooptical.net/www/sed/tj3.png> shows what happened when one of my tunnel junctions shorted out during testing.-a-a I had about 100 mV of DC bias on it, so even at 3000 ppm/K, a 2.5 mV step
is big--a good 8 degrees C.-a (In this plot you have to mentally subtract the baseline.)-a The bolometric response looked like a step function on
this scale, because it took about 5 us to cool back down, even with 3D
heat conduction and 1-um size.

Since the DC current path was about the same as the AC, the near-instantaneous heating of the device produced a near-instantaneous
RTD response: about 40 picoseconds.

Applying this idea to normal life, in principle your temperature
controller can have any bandwidth you want.-a Of course the slowness of thermal diffusion means that at sufficiently high frequency the
temperature of the RTD decouples from the rest of the world.-a However,
if you tile some surface with these things, you can effectively make a thermal version of a Faraday shield--the huge control bandwidth gives
you arbitrarily good rejection of thermal forcing, with no bulky
insulation, stirred fluid baths, or big thermal masses.

The decoupling region actually has some interesting features--as the frequency goes up, the amount of material you have to heat goes down, so there's a region where the phase shift is 45 degrees instead of the 90 degrees you get in the low frequency (thermal mass) limit.-a (This is discussed in Section 20.3 of my third edition, <https://electrooptical.net/www/beos3e/thermal3e.pdf>.)
The decoupling also means that in principle the loop bandwidth and compensation don't need to depend on what the element is stuck onto--you
get the same huge forcing rejection regardless.-a You don't even have to worry about windup, despite the slew rate being slow for the bandwidth.

There are a number of control schemes for this, of which my favorite is analog PWM.-a The heater is in a resistive bridge with a shunt resistor
and a reference divider.-a It gets turned on for a microsecond or so at
the beginning by a strobe pulse and an RS-flipflop controlling an NMOS switch, with the .-a A low-noise amplifier (ADA4899-ish) driving a comparator resets the flipflop when the instantaneous temperature error crosses zero.-a (The FF is a NAND type, so the heater is turned on if
both SET and RESET are active.)-a It's cool to watch the duty cycle
change instantly if you touch the element, and of course the effective
loop bandwidth is huge--a short transient gets nulled out in the very
next clock cycle.

The Class-H thing I talked about in the other thread is for things like
DWDM lasers and OCXOs, where you don't want a lot of EMI right in the sensitive region.-a (At low power, you can just use an analog loop with a fixed supply.)

There are a whole lot of things you can do with this general scheme,
from improved thermolelectric coolers to such things as a battery calorimeter made of metallized mylar like a chip bag.

Fun stuff--suggestions for applications and further enhancements welcome!

Nice work. It sounds extremely cute. The idea of embedding the volume
whose temperature you want to control
inside a set of temperature controlled shields isn't new. The paper I
recall had six separate temperature controller on the six faces of a
cube with a better seventh for the core of the cube. Using the sensing
current as the heating current really is cute and does strike me as patentable, but there's always the risk that somebody did patent it
before there was a market that actually needed it.
--
Bill Sloman, Sydney
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From Newsgroup: sci.electronics.design

On 2/25/26 05:53, Bill Sloman wrote:

Using the sensing
current as the heating current really is cute and does strike me as patentable, but-a there's always the risk that somebody did patent it
before there was a market that actually needed it.

not quite the same but similar; a hot wire mass airflow meter, the
current through the wire to keep a resistance bridge balanced is the output




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From Newsgroup: sci.electronics.design

On 2026-02-25 13:34, Lasse Langwadt wrote:

On 2/25/26 05:53, Bill Sloman wrote:

Using the sensing current as the heating current really is cute and
does strike me as patentable, but-a there's always the risk that
somebody did patent it before there was a market that actually needed it.

not quite the same but similar; a hot wire mass airflow meter, the
current through the wire to keep a resistance bridge balanced is the output

You're quite right that using the heater as the sensor has been done N
times. A PTC thermistor heater is another example, and folk have even
done it with nitinol shape memory actuators.

I think that the ability to achieve tens of kilohertz bandwidth in a macroscopic temperature control loop is quite novel, though, and it
enables a lot of things.

You don't usually see thermal forcing that fast, of course, but most
loops run with bandwidths way below 1 hertz, meaning that this gizmo can
get you 80 or 100 dB more forcing rejection at all frequencies, whereas
you had little or none before, apart from adding insulation and thermal
mass. With a bandwidth of 0.1 Hz (pretty good going for most things), a
cold draft from somebody opening the lab door will get through
essentially unattenuated, whereas with the TF you'd never see it, even
in a highly sensitive setup.

It does it with a dumb loop with completely vanilla frequency
compensation and almost no additional apparatus.

For instance, you can make a thermoelectric cooler slew much faster, as
well as giving it a huge bandwidth and very high forcing rejection, and
then use that as an actuator in an outer loop that controls the
temperature of more distant parts of the system.

We're working on a few demos. One is sort of a "thermal theremin"
producing a tone that varies widely when you touch it or breathe on it,
but becomes rock-steady when the TFS is turned on.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

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