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Phil Hobbs
Guest

Fri Jan 06, 2017 3:42 pm   



Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

Phil Hobbs
Guest

Fri Jan 06, 2017 4:03 pm   



On 01/06/2017 03:25 PM, John Larkin wrote:
Quote:
On Fri, 6 Jan 2017 03:42:06 -0500, Phil Hobbs
pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

Cheers

Phil Hobbs

Why not use an opamp and a mosfet? You could reduce the voltage lost
in the shunt, and get precise current limiting.

There are beginning to be some decent current-limiter chips, but
opamps and resistors are cheap.

An LED and a bipolar make an interesting current limiter (which makes
free light!) but not well suited to high currents.

https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG


Thanks.

You want to run TECs from a current source, so one side is actually a
current conveyor (load connected to the collectors) and the other is a
voltage source (load connected to the emitters), adjusted so that the
sum of the two output voltages is constant.

The totem pole runs off the highest available supply, so getting enough
gate enhancement for the voltage-source side might be a bit of a puzzle.
One could use two of the current-source versions, I suppose.

Trying to keep the BOM cost low--the BJT thing is much faster than a
cheap op amp. I'll have a look though!

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net


Guest

Fri Jan 06, 2017 10:44 pm   



On Friday, January 6, 2017 at 3:14:13 PM UTC-5, Phil Hobbs wrote:
Quote:
Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!


I use LEDs occasionally. Some have d(Vf)/dT pretty close to a silicon
BJT's Vbe.

Q1 1ohm
O--+--------. .-------R2-----+----O
| \ ^ |
| ------ LED |
| | / |
'-----R1------+-----|>|---------'

IR LEDs in my parts bin have drops as low as 900mV. Dunno
about the t/c, though.

For maximum headroom put the load between Vcc and Q1(c).

Cheers,
James Arthur


Guest

Fri Jan 06, 2017 11:01 pm   



On Friday, January 6, 2017 at 3:44:09 PM UTC-5, dagmarg...@yahoo.com wrote:
Quote:
On Friday, January 6, 2017 at 3:14:13 PM UTC-5, Phil Hobbs wrote:
Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

I use LEDs occasionally. Some have d(Vf)/dT pretty close to a silicon
BJT's Vbe.

Q1 1ohm
O--+--------. .-------R2-----+----O
| \ ^ |
| ------ LED |
| | / |
'-----R1------+-----|>|---------'

IR LEDs in my parts bin have drops as low as 900mV. Dunno
about the t/c, though.

For maximum headroom put the load between Vcc and Q1(c).


This LED d(Vf)/dT paper from an old thread is still available...

===== quote ====On Sep 10, 3:50 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:


seen it done since forever for audio amps, I believe some claim
they are also less noisy than regular diodes, just remember to
shield the LED from light

http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf

looks like somewhere between red and UV will get close to zero tempco


-Lasse
===== /quote ====
From Lasse's reference:
"The coefficient dVf /dT is equal to
– 2.3 mV/K for the UV LED sample (λ = 375 nm),
– 5 mV/K for the blue LED sample (λ = 455 nm),
– 3.8 mV/K for the green LED sample (λ = 530 nm), and
– 1.5 mV/K for the red LED sample (λ = 605 nm). "


Plus John's measurement from the "Ring of two current source" thread...
https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG

Cheers,
James Arthur


Guest

Sat Jan 07, 2017 12:37 am   



On Friday, January 6, 2017 at 4:52:34 PM UTC-5, John Larkin wrote:
Quote:
On Fri, 6 Jan 2017 13:01:43 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

On Friday, January 6, 2017 at 3:44:09 PM UTC-5, dagmarg...@yahoo.com wrote:
On Friday, January 6, 2017 at 3:14:13 PM UTC-5, Phil Hobbs wrote:
Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round..)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

I use LEDs occasionally. Some have d(Vf)/dT pretty close to a silicon
BJT's Vbe.

Q1 1ohm
O--+--------. .-------R2-----+----O
| \ ^ |
| ------ LED |
| | / |
'-----R1------+-----|>|---------'

IR LEDs in my parts bin have drops as low as 900mV. Dunno
about the t/c, though.

For maximum headroom put the load between Vcc and Q1(c).


This LED d(Vf)/dT paper from an old thread is still available...

===== quote ====> >On Sep 10, 3:50 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Has anybody done this?

https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG


seen it done since forever for audio amps, I believe some claim
they are also less noisy than regular diodes, just remember to
shield the LED from light

http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco..pdf

looks like somewhere between red and UV will get close to zero tempco

My emitter resistor alone could explain 100 PPM or more of tempco.

Self-heating of the transistor becomes a serious error at higher
currents too.

I'm guessing that the effect of light on the LED will be way down in
the noise.





-Lasse
===== /quote ====
From Lasse's reference:
"The coefficient dVf /dT is equal to
– 2.3 mV/K for the UV LED sample (? = 375 nm),
– 5 mV/K for the blue LED sample (? = 455 nm),
– 3.8 mV/K for the green LED sample (? = 530 nm), and
– 1.5 mV/K for the red LED sample (? = 605 nm). "


Plus John's measurement from the "Ring of two current source" thread...
https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG


Looks to me like GaN LEDs' dVf/dT is too high to compensate one Vbe, and
all the others are pretty close to 'good enough,' for certain values of
'good' and 'enough.'

Cheers,
James Arthur


Guest

Sat Jan 07, 2017 12:42 am   



If I could stand a Vbe plus a volt or so, I could replace the NPN current limiters with TLV431s and the PNP ones with LM385-ADJs.

What the world needs is a fast LM10 for 20 cents. ;)

Cheers

Phil Hobbs


Guest

Sat Jan 07, 2017 1:26 am   



On Friday, January 6, 2017 at 5:42:10 PM UTC-5, pcdh...@gmail.com wrote:
Quote:
If I could stand a Vbe plus a volt or so, I could replace the NPN current limiters with TLV431s and the PNP ones with LM385-ADJs.

What the world needs is a fast LM10 for 20 cents. ;)

Cheers

Phil Hobbs


The LED-biased current sources need less than a volt of headroom. Half a
volt might do.

Cheers,
James Arthur


Guest

Sat Jan 07, 2017 2:23 am   



Quote:
I'm not totally following your scheme, but wouldn't you want to
_replace_Q2_ with the TLV431 (using drawing _above_)?  Then your
current limit is 1.2V/R2 with a _very_low_ TC.  A TLV431 is
transistor-sized... TO-92 pack, unless you're in love with SOT's.


Yup. The LM385-ADJ is the same sort of thing except that it holds the 1.2V between ADJ and K, so it works with PNPs. (To make a 1.2V reference with a TLV431 you short ADJ to K, whereas with an LM385 it's ADJ to A.)

Still costs an extra volt-and-a-bit of headroom, and another buck or so on the BOM. Might be worth it.

Cheers

Phil Hobbs


Guest

Sat Jan 07, 2017 2:23 am   



Quote:
I'm not totally following your scheme, but wouldn't you want to
_replace_Q2_ with the TLV431 (using drawing _above_)?  Then your
current limit is 1.2V/R2 with a _very_low_ TC.  A TLV431 is
transistor-sized... TO-92 pack, unless you're in love with SOT's.


Yup. The LM385-ADJ is the same sort of thing except that it holds the 1.2V between ADJ and K, so it works with PNPs. (To make a 1.2V reference with a TLV431 you short ADJ to K, whereas with an LM385 it's ADJ to A.)

Still costs an extra volt-and-a-bit of headroom, and another buck or so on the BOM. Might be worth it.

Cheers

Phil Hobbs

John Larkin
Guest

Sat Jan 07, 2017 3:25 am   



On Fri, 6 Jan 2017 03:42:06 -0500, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

Cheers

Phil Hobbs


Why not use an opamp and a mosfet? You could reduce the voltage lost
in the shunt, and get precise current limiting.

There are beginning to be some decent current-limiter chips, but
opamps and resistors are cheap.

An LED and a bipolar make an interesting current limiter (which makes
free light!) but not well suited to high currents.

https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG



--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

John Larkin
Guest

Sat Jan 07, 2017 4:52 am   



On Fri, 6 Jan 2017 13:01:43 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:

Quote:
On Friday, January 6, 2017 at 3:44:09 PM UTC-5, dagmarg...@yahoo.com wrote:
On Friday, January 6, 2017 at 3:14:13 PM UTC-5, Phil Hobbs wrote:
Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

I use LEDs occasionally. Some have d(Vf)/dT pretty close to a silicon
BJT's Vbe.

Q1 1ohm
O--+--------. .-------R2-----+----O
| \ ^ |
| ------ LED |
| | / |
'-----R1------+-----|>|---------'

IR LEDs in my parts bin have drops as low as 900mV. Dunno
about the t/c, though.

For maximum headroom put the load between Vcc and Q1(c).


This LED d(Vf)/dT paper from an old thread is still available...

===== quote =====
On Sep 10, 3:50 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Has anybody done this?

https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG


seen it done since forever for audio amps, I believe some claim
they are also less noisy than regular diodes, just remember to
shield the LED from light

http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf

looks like somewhere between red and UV will get close to zero tempco


My emitter resistor alone could explain 100 PPM or more of tempco.

Self-heating of the transistor becomes a serious error at higher
currents too.

I'm guessing that the effect of light on the LED will be way down in
the noise.



Quote:


-Lasse
===== /quote =====

From Lasse's reference:
"The coefficient dVf /dT is equal to
2.3 mV/K for the UV LED sample (? = 375 nm),
5 mV/K for the blue LED sample (? = 455 nm),
3.8 mV/K for the green LED sample (? = 530 nm), and
1.5 mV/K for the red LED sample (? = 605 nm). "


Plus John's measurement from the "Ring of two current source" thread...
https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG

Cheers,
James Arthur


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

Jim Thompson
Guest

Sat Jan 07, 2017 6:32 am   



On Fri, 6 Jan 2017 03:42:06 -0500, Phil Hobbs
<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

Quote:
Hi, all,

I'm designing a diode laser controller board for a customer in
Scandinavia. It has a sub-Poissonian adjustable current supply, a
resistor-linearized thermistor temperature sensor, and of course a
thermoelectric cooler driver.

The TEC requires different current limits in different quadrants. (A
TEC is actually a four-quadrant device since it generates electrical
power from the temperature difference as well as the other way round.)

With a bridged current driver, that can run into quite a few parts--you
need one current limiter per polarity, or if you want any short circuit
protection, one per leg (4 in all). So simplicity is at a bit of a premium.

The usual two-terminal, two-BJT current limiter suffers from a fairly
gross temperature coefficient, like 3000 ppm/K. Protecting a TEC isn't
a super high precision application, but something a bit better than that
would be good.

I've sometimes got round that by putting a Schottky diode in series with
the sense resistor, like this. (The base current for Q1 actually comes
from a RRO op amp via R1--it's a class B complementary bridge.)

Q1
0-*--------- -----*---->|---R2R2---*------0
| \ A | |
| ------ ------- |
| | / V Q2 |
*--R1R1-------*---- -------------*

Trouble is, Schottkies don't have the same dV/dT as BJTs--it's more like
1 mV/K at high current, vs. 2 mV/K at low current, so you don't gain
that much--about 1500 ppm/K.

A PN diode connected as above plus something like a TLV431 in series
with Q2's emitter will do a good job, probably 300-500 ppm/V, which
would be fine. However, that costs four more ICs and another 2.4V of
headroom (1.2V per side in each polarity).


I'm not totally following your scheme, but wouldn't you want to
_replace_Q2_ with the TLV431 (using drawing _above_)? Then your
current limit is 1.2V/R2 with a _very_low_ TC. A TLV431 is
transistor-sized... TO-92 pack, unless you're in love with SOT's.

Quote:

Seems like an arbitrage opportunity. ;)

My current thinking is to put the diode in series with the base of Q2
and use Q1's V_BE to provide a reasonably stable bias current, like this:

Q1 1ohm
0-*--------- --------*-------R2R2-----*------0
| \ A | |
| ------ A Schottky |
| | | |
*---R1R1------*--R3R3----* |
| | |
| ------ |
| / V Q2 |
*------- --------------*

The diode runs at much lower current, and so has a higher dV/dT, and by
choosing R2 correctly, I can adjust it to match Q2's a bit better.

One wouldn't want to bet one's firstborn child on the unit-to-unit
consistency being too wonderful, but ISTM that I ought to be able to get
a factor of maybe 6 this way.

Anybody else done this sort of thing? Better suggestions welcome!

Cheers

Phil Hobbs


...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |


Guest

Sun Jan 08, 2017 12:20 am   



Quote:
Depending on the voltages involved... and the switching speed... you
might simply make the bridge switches part of the current control.

More info about what you're attempting?


It's a fairly simple bridge driver for a thermoelectric cooler. Normally I'd put a single current limiter in series with the input, but in this case I need different current limits for positive and negative.

The most recent iteration has a class B current conveyor on the left side, driven by an op amp. The right side is voltage-driven, so that the outputs are centred on Vcc/2.

Because of the asymmetric limits, I need at least two separate current limiters, one for each polarity. Seems like I only need to limit one side, right?

However, if the left side of the TEC ever got shorted to ground, the right side could supply enough current to turn it to lava in short order. The lasers are expensive, not to mention service calls and down time, so it seems like a good idea to put current limits in all four legs of the bridge.

(Class D is unattractive due to noise coupling into the laser.)

Cheers

Phil Hobbs


Guest

Sun Jan 08, 2017 1:46 am   



Quote:
Because of the asymmetric limits, I need at least two separate
current limiters, one for each polarity. Seems like I only
need to limit one side, right?

Won't asymmetric current limits screw with the symmetry implied by
voltage drive?

 
Sure, but only in fault conditions, and it simulates well as it is.;)

I could maybe do two current conveyors fighting each other, and apply voltage FB to the slave side to keep that within bounds. I don't think that saves any parts, though.

Cheers

Phil Hobbs


Guest

Sun Jan 08, 2017 2:37 am   



On Sunday, January 8, 2017 at 9:21:05 AM UTC+11, pcdh...@gmail.com wrote:
Quote:
Depending on the voltages involved... and the switching speed... you
might simply make the bridge switches part of the current control.

More info about what you're attempting?

It's a fairly simple bridge driver for a thermoelectric cooler. Normally I'd put a single current limiter in series with the input, but in this case I need different current limits for positive and negative.

The most recent iteration has a class B current conveyor on the left side, driven by an op amp. The right side is voltage-driven, so that the outputs are centred on Vcc/2.

Because of the asymmetric limits, I need at least two separate current limiters, one for each polarity. Seems like I only need to limit one side, right?

However, if the left side of the TEC ever got shorted to ground, the right side could supply enough current to turn it to lava in short order. The lasers are expensive, not to mention service calls and down time, so it seems like a good idea to put current limits in all four legs of the bridge.

(Class D is unattractive due to noise coupling into the laser.)


My 1996 paper describes a class-D driver, and the filtering that meant that hardly any of the high frequency current made it into the TEC, let alone the laser.

The original design wasn't temperature-controlling a laser, but there was a diode laser elsewhere in the system, and the boss finally accepted that that had to be temperature stabilised too, so Paul Buggs replicated the design for that job (leaving out some of the space-consuming filtering on the temperature sensing side).

--
Bill Sloman, Sydney

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