Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Fri, 18 Feb 2022 22:29:11 +0100, Piotr Wyderski
<bombald@protonmail.com> wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a >constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp >"interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT >current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the >current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and >maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr

Depletion fet? They aren't accurate to 3%, but an LED shouldn't need
that sort of precision.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

fredag den 18. februar 2022 kl. 22.29.33 UTC+1 skrev Piotr Wyderski:

Hi,

I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

MOSFET and something like the lower voltage TL431, TLVH431 ?

maybe a diode and capacitor to provide a constant supply for the gate/reference

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

John Larkin wrote:

Depletion fet?

I love them, but they have pretty high V_TH and it is
temperature-dependent too. Too much voltage drop and doesn't solve the
tempco issue -- a regular 2 BJT limiter performs better.

They aren't accurate to 3%, but an LED shouldn't need that sort of precision.

Absolute accuracy is not required, but some value of the current should
remain relatively stable. What's the point of PWM dimming if the lamp
current can drift away by 20% just because the lamp has been turned on
for some time?

Below is one idea. The differential amplifier looks good, so at the sim
level it boils down to the tempco of the reference. Google says 5.6V
zeners are particularly good, but I don't know how good is good. Looks
like it would be worth torturing with a hotair gun with the Q5 and Q7
being a BCV61 and Q1 and Q2 a BCM847.

Best regards, Piotr

Version 4
SHEET 1 1700 1204
WIRE 0 -224 -64 -224
WIRE 160 -224 0 -224
WIRE 1360 -224 160 -224
WIRE 1536 -224 1360 -224
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SYMATTR Value BC557C
SYMBOL pnp 208 704 R180
SYMATTR InstName Q2
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WINDOW 123 0 0 Left 0
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SYMATTR InstName V1
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SYMATTR InstName R3
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WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
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SYMATTR Value 0.6
SYMBOL res -144 640 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
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SYMBOL npn 1296 688 R0
SYMATTR InstName Q3
SYMATTR Value BC547B
SYMBOL cap 1232 656 R0
WINDOW 3 -55 51 Left 2
SYMATTR InstName C1
SYMATTR Value 100p
SYMBOL res 1344 -64 R0
SYMATTR InstName R5
SYMATTR Value 4k7
SYMBOL zener 176 48 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value BZX84C6V2L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 144 -208 R0
SYMATTR InstName R6
SYMATTR Value 1k
SYMBOL npn 0 864 M0
SYMATTR InstName Q5
SYMATTR Value BC547B
SYMBOL npn 80 864 R0
SYMATTR InstName Q7
SYMATTR Value BC547B
SYMBOL res -80 384 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL nmos 1488 544 R0
SYMATTR InstName M1
SYMATTR Value FDS4885C_N
SYMBOL res 1520 688 R0
SYMATTR InstName R8
SYMATTR Value 1.2
SYMBOL res 1520 400 R0
SYMATTR InstName R9
SYMATTR Value 1
SYMBOL res 592 -16 R0
SYMATTR InstName R1
SYMATTR Value 9.6k
SYMBOL res 592 112 R0
SYMATTR InstName R7
SYMATTR Value 1k
TEXT -344 584 Left 2 !.tran 1

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Lasse Langwadt Christensen wrote:

MOSFET and something like the lower voltage TL431, TLVH431 ?

I was experimenting with the MAX8515 from Mouser, but it was a total
disaster. It looks as if the the physical part and the datasheet were completely unrelated. IMHO the origin excludes counterfeiting, but the
way the part misbehaves makes me think that even the pinout cannot be
right. If I just connect IN and PGND/GND and leave FB and OUT
unconnected, the chip draws 40mA, while the datasheet says 1mA max. When
I connect FB to GND, the current consumption goes to 0. The scope says
it is not oscillating. No idea where the 40mA goes, as the high-power
path is through the OUT pin. When connected to the FET, it jumps to the
40mA mode and starts oscillating at the OUT. 100nF kills the
oscillations, but the chip looks offended and keeps the FET almost off.
Same behaviour with 3 different chips.

I had similar experience last year with some Maxim boost converter --
the chip looked haunted, while a TI part did the job right out of the
box. Not going to use any Maxim part anymore.

I have suddenly started liking discretes.

maybe a diode and capacitor to provide a constant supply for the gate/reference

I considered this, but keep it a last resort solution.

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Sat, 19 Feb 2022 00:28:54 +0100, Piotr Wyderski
<bombald@protonmail.com> wrote:

John Larkin wrote:

Depletion fet?

I love them, but they have pretty high V_TH and it is
temperature-dependent too. Too much voltage drop and doesn't solve the
tempco issue -- a regular 2 BJT limiter performs better.

They aren't accurate to 3%, but an LED shouldn't need that sort of precision.

Absolute accuracy is not required, but some value of the current should >remain relatively stable. What's the point of PWM dimming if the lamp
current can drift away by 20% just because the lamp has been turned on
for some time?

Below is one idea. The differential amplifier looks good, so at the sim
level it boils down to the tempco of the reference. Google says 5.6V
zeners are particularly good, but I don't know how good is good. Looks
like it would be worth torturing with a hotair gun with the Q5 and Q7
being a BCV61 and Q1 and Q2 a BCM847.

Best regards, Piotr

It's just an LED! I doubt that you would notice 20% light output
change. LEDs get dimmer as the temp goes up anyhow.

There are cheap LED current limiter chips.

Or maybe an LM1117 and a resistor.

Version 4
SHEET 1 1700 1204
WIRE 0 -224 -64 -224
WIRE 160 -224 0 -224
WIRE 1360 -224 160 -224
WIRE 1536 -224 1360 -224
WIRE 0 -208 0 -224
WIRE 160 -192 160 -224
WIRE 0 -112 0 -128
WIRE 1360 -48 1360 -224
WIRE 160 -32 160 -112
WIRE 160 -32 -64 -32
WIRE 608 -32 160 -32
WIRE 160 -16 160 -32
WIRE 608 0 608 -32
WIRE 160 64 160 48
WIRE 608 96 608 80
WIRE 608 96 464 96
WIRE 608 128 608 96
WIRE 608 224 608 208
WIRE -64 400 -64 -32
WIRE 1536 416 1536 -224
WIRE 1536 544 1536 496
WIRE -64 576 -64 480
WIRE 144 576 -64 576
WIRE -64 608 -64 576
WIRE 144 608 144 576
WIRE 1360 624 1360 32
WIRE 1360 624 1248 624
WIRE 1488 624 1360 624
WIRE -240 656 -336 656
WIRE -128 656 -160 656
WIRE 288 656 208 656
WIRE 1248 656 1248 624
WIRE 1360 688 1360 624
WIRE 1536 688 1536 640
WIRE 1696 688 1536 688
WIRE 1536 704 1536 688
WIRE 144 736 144 704
WIRE 1248 736 1248 720
WIRE 1248 736 144 736
WIRE 1296 736 1248 736
WIRE 288 752 288 656
WIRE -64 800 -64 704
WIRE 32 800 -64 800
WIRE 1536 800 1536 784
WIRE 464 832 464 96
WIRE 464 832 288 832
WIRE -64 864 -64 800
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WIRE 32 912 32 800
WIRE 32 912 0 912
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WIRE 1360 992 1360 784
WIRE -64 1008 -64 960
WIRE 144 1008 144 960
WIRE -336 1200 -336 656
WIRE 1696 1200 1696 688
WIRE 1696 1200 -336 1200
FLAG 0 -112 0
FLAG 288 976 0
FLAG 160 64 0
FLAG 1360 992 0
FLAG -64 1008 0
FLAG 144 1008 0
FLAG 1536 800 0
FLAG 608 224 0
SYMBOL pnp -128 704 M180
SYMATTR InstName Q1
SYMATTR Value BC557C
SYMBOL pnp 208 704 R180
SYMATTR InstName Q2
SYMATTR Value BC557C
SYMBOL voltage 0 -224 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
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SYMATTR InstName R3
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WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 0.6
SYMBOL res -144 640 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 1
SYMBOL npn 1296 688 R0
SYMATTR InstName Q3
SYMATTR Value BC547B
SYMBOL cap 1232 656 R0
WINDOW 3 -55 51 Left 2
SYMATTR InstName C1
SYMATTR Value 100p
SYMBOL res 1344 -64 R0
SYMATTR InstName R5
SYMATTR Value 4k7
SYMBOL zener 176 48 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value BZX84C6V2L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 144 -208 R0
SYMATTR InstName R6
SYMATTR Value 1k
SYMBOL npn 0 864 M0
SYMATTR InstName Q5
SYMATTR Value BC547B
SYMBOL npn 80 864 R0
SYMATTR InstName Q7
SYMATTR Value BC547B
SYMBOL res -80 384 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL nmos 1488 544 R0
SYMATTR InstName M1
SYMATTR Value FDS4885C_N
SYMBOL res 1520 688 R0
SYMATTR InstName R8
SYMATTR Value 1.2
SYMBOL res 1520 400 R0
SYMATTR InstName R9
SYMATTR Value 1
SYMBOL res 592 -16 R0
SYMATTR InstName R1
SYMATTR Value 9.6k
SYMBOL res 592 112 R0
SYMATTR InstName R7
SYMATTR Value 1k
TEXT -344 584 Left 2 !.tran 1

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties. Francis Bacon

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Friday, February 18, 2022 at 1:29:33 PM UTC-8, Piotr Wyderski wrote:

I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

At lower voltage, there's integrated solutions for onesie white-output LEDs, like
AMC7135 (that one's about 350 mA, 6V), in three-pin packages. At 9V nominal, even the venerable uA723 with a pass transistor could do
something close to valid, with its overcurrent limit; maybe 10% accuracy.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

whit3rd wrote:

At lower voltage, there's integrated solutions for onesie white-output LEDs, like
AMC7135 (that one's about 350 mA, 6V), in three-pin packages. At 9V nominal,
even the venerable uA723 with a pass transistor could do
something close to valid, with its overcurrent limit; maybe 10% accuracy.

But would it like pulsed supply? This is the core issue, not how to
build a limiter itself. I have no idea what's inside a real
opamp/refrence/LDO and the models probably do no cover this aspect. Or
maybe they do, some randomly selected LTC opamps went monkey in LTspice.

Best regards, Potr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 19/02/2022 7:29 am, Piotr Wyderski wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

    Best regards, Piotr

A small resistor to swamp the tempco?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

lørdag den 19. februar 2022 kl. 03.02.24 UTC+1 skrev David Eather:

On 19/02/2022 7:29 am, Piotr Wyderski wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current limiter should be capable of being powered from a +12V/open drain source with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT current limiter. The voltage at the MOSFET source turns on a BJT that steals the MOSFET gate voltage, stabilizing the drain current. It works, but the V_BE tempco makes the current drift way beyond my comfort zone. OTOH, an opamp-based current limiter + a decent reference would keep the current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and maintaining stable current during the ON phase down to, say, 3% accuracy over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr

A small resistor to swamp the tempco?

or an NPN with two diodes as reference, all thermally connected

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 18/02/2022 11:06 pm, Lasse Langwadt Christensen wrote:

fredag den 18. februar 2022 kl. 22.29.33 UTC+1 skrev Piotr Wyderski:

Hi,

I am experimenting with white LED COBs and wanted to power them from a
constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp
"interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT
current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the
current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and
maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

MOSFET and something like the lower voltage TL431, TLVH431 ?

maybe a diode and capacitor to provide a constant supply for the gate/reference

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

<https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1>

piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 18/02/2022 11:56 pm, Piotr Wyderski wrote:

I have suddenly started liking discretes.

Haha! A sign of maturity and useful in a time of supply shortages and no
second sources!

piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

piglet wrote:

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

<https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1>

This is exactly how I wanted to use the MAX8515, but for some reason it
just doesn't work. It even does notwork without the FET - it looks like
it needs tons of capacitance to start behaving.

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 19/02/2022 2:16 pm, Piotr Wyderski wrote:

piglet wrote:

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

<https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1>

This is exactly how I wanted to use the MAX8515, but for some reason it
just doesn't work. It even does notwork without the FET - it looks like
it needs tons of capacitance to start behaving.

    Best regards, Piotr

I am not familiar with the MAX8515 but knowing the way they think it is probably massively over complicated and expects a proper unvarying
supply rail. The simple '431 can work to surprisingly high frequency. If
even a '431 is unbearable then since this is an undemanding application
you could make a two NPN diff amp with a reference somewhere from 0.9 to
1.5V made from a divided down zener?

piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Fri, 18 Feb 2022 22:29:11 +0100, Piotr Wyderski wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

I started my experiments with a boring two-transistor series MOSFET/BJT current limiter. The voltage at the MOSFET source turns on a BJT that
steals the MOSFET gate voltage, stabilizing the drain current. It works,
but the V_BE tempco makes the current drift way beyond my comfort zone.
OTOH, an opamp-based current limiter + a decent reference would keep the current rock-solid, but it can go nuts during the PWM edge transients.

Any suggestion on how to make a limiter capable of being PWM-powered and maintaining stable current during the ON phase down to, say, 3% accuracy
over the 11-15V input voltage range and 25-80 degrees C temperature
would be appreciated. The nominal LED voltage is 9V and the required
current is ~500mA. Are there any particularly forgiving opamps?

Best regards, Piotr

For an op-amp based current limiter, you want one that comes out of
saturation fast, i.e. in a small fraction of your PWM period. Some are
orders of magnitude better than others in this respect, but it's rarely mentioned in the datasheet.

There was a list of good ones in AoE III IIRC. I'll have a look in my
copy when I'm back in the office next week.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2022 12:16 am, Piotr Wyderski wrote:

piglet wrote:

It can be even simpler, just use the TLV431 as a kind of NPN with
precisely defined Vbe, as in this sketch ...

<https://www.dropbox.com/s/c4l4hnnu6f2j5xx/TLV431_curlim.JPG?raw=1>

This is exactly how I wanted to use the MAX8515, but for some reason it
just doesn't work. It even does notwork without the FET - it looks like
it needs tons of capacitance to start behaving.

    Best regards, Piotr

Just thinking - some op amps misbehave when driving a capacative load.
Not any load but only a value between certain values. Does it still
misbehave if you isolate the 8515 and the fet with say 1k?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

piglet wrote:

The simple '431 can work to surprisingly high frequency.

Yes, its datasheet explicitly says it can work without capacitors. So
maybe instead of demanding low threshold voltage I should add 2.5V
offset with a second 431? To be prototyped:

https://i.postimg.cc/pXKzztfs/shifted-current-sense.png

If even a '431 is unbearable then since this is an undemanding application you could make a two NPN diff amp with a reference somewhere from 0.9 to
1.5V made from a divided down zener?

This is exactly what my sim does:

https://i.postimg.cc/1XrRRMFr/discrete.png

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2022 7:12 am, Piotr Wyderski wrote:

piglet wrote:

The simple '431 can work to surprisingly high frequency.

Yes, its datasheet explicitly says it can work without capacitors. So
maybe instead of demanding low threshold voltage I should add 2.5V
offset with a second 431? To be prototyped:

https://i.postimg.cc/pXKzztfs/shifted-current-sense.png

Sorry but I don't understand the need for a second 431?

If even a '431 is unbearable then since this is an undemanding
application you could make a two NPN diff amp with a reference
somewhere from 0.9 to 1.5V made from a divided down zener?

This is exactly what my sim does:

https://i.postimg.cc/1XrRRMFr/discrete.png

    Best regards, Piotr

I was thinking of something much simpler, like this ...

Version 4
SHEET 1 1700 1204
WIRE 464 -224 48 -224
WIRE 800 -224 464 -224
WIRE 976 -224 800 -224
WIRE 1184 -224 976 -224
WIRE 464 -192 464 -224
WIRE 1184 -48 1184 -224
WIRE 464 -32 464 -112
WIRE 608 -32 464 -32
WIRE 976 16 976 -224
WIRE 608 64 608 -32
WIRE 1184 80 1184 32
WIRE 464 112 464 -32
WIRE 976 160 976 96
WIRE 1136 160 976 160
WIRE 48 176 48 -224
WIRE 800 192 800 -224
WIRE 976 192 976 160
WIRE 608 240 608 144
WIRE 736 240 608 240
WIRE 1184 240 1184 176
WIRE 1184 240 1040 240
WIRE 800 304 800 288
WIRE 896 304 800 304
WIRE 976 304 976 288
WIRE 976 304 896 304
WIRE 608 352 608 240
WIRE 896 368 896 304
WIRE 1184 384 1184 240
WIRE 48 528 48 256
WIRE 464 528 464 176
WIRE 464 528 48 528
WIRE 608 528 608 432
WIRE 608 528 464 528
WIRE 896 528 896 448
WIRE 896 528 608 528
WIRE 1184 528 1184 464
WIRE 1184 528 896 528
WIRE 48 576 48 528
FLAG 48 576 0
SYMBOL voltage 48 160 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(11 15 0 1 0)
SYMBOL res 960 0 R0
SYMATTR InstName R5
SYMATTR Value 22k
SYMBOL zener 480 176 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value BZX84C6V2L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 448 -208 R0
SYMATTR InstName R6
SYMATTR Value 470
SYMBOL npn 1040 192 M0
SYMATTR InstName Q5
SYMATTR Value BC547B
SYMBOL npn 736 192 R0
SYMATTR InstName Q7
SYMATTR Value BC547B
SYMBOL nmos 1136 80 R0
SYMATTR InstName M1
SYMATTR Value FDS4885C_N
SYMBOL res 1168 368 R0
SYMATTR InstName R8
SYMATTR Value 2.2
SYMBOL res 1168 -64 R0
SYMATTR InstName R9
SYMATTR Value 1
SYMBOL res 592 48 R0
SYMATTR InstName R1
SYMATTR Value 22k
SYMBOL res 592 336 R0
SYMATTR InstName R7
SYMATTR Value 4700
SYMBOL res 880 352 R0
SYMATTR InstName R2
SYMATTR Value 470
TEXT 112 584 Left 2 !.tran 1
TEXT 336 584 Left 2 ;Piotr - EPW SED FEB 2022 - PWM-able current sink

piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

piglet wrote:

Sorry but I don't understand the need for a second 431?

This is to ensure low voltage drop at the current sense resistor.
It moves the 0-0.6V CS range to 2.5-3.1V where the second 431 can start regulating, as its V_REF is 2.5V.

I have just prototyped this one and its performance is stunning -- it
starts with 497mA and I cannot move it by more than 4mA with the hotair
gun. In the 11-15 V_IN range I can see no current change, it is only temperature-dependent. Problems: at 250mA it is inherently stable, at
500mA it starts oscillating, so a 22nF cap at the main REF is required
to kill it.

[snip your sim -- thank you, most interesting. It requires >1V reference though. My PNP diff amp was to allow <0.6V]

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 20/02/2022 11:30 am, Piotr Wyderski wrote:

piglet wrote:

Sorry but I don't understand the need for a second 431?

This is to ensure low voltage drop at the current sense resistor.
It moves the 0-0.6V CS range to 2.5-3.1V where the second 431 can start regulating, as its V_REF is 2.5V.

I have just prototyped this one and its performance is stunning -- it
starts with 497mA and I cannot move it by more than 4mA with the hotair
gun. In the 11-15 V_IN range I can see no current change, it is only temperature-dependent. Problems: at 250mA it is inherently stable, at
500mA it starts oscillating, so a 22nF cap at the main REF is required
to kill it.

[snip your sim -- thank you, most interesting. It requires >1V reference though. My PNP diff amp was to allow <0.6V]

    Best regards, Piotr

Thanks, yes I understood it within a few minutes of my first reply, it
should work fine - as you then proved!

piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 2022-02-18, Piotr Wyderski <bombald@protonmail.com> wrote:

Hi,

I am experimenting with white LED COBs and wanted to power them from a constant current source while still having the PWM dimming capability.
The current limiter should be located next to the diode and the lamp "interface" should be regular two wires. In other words, the current
limiter should be capable of being powered from a +12V/open drain source
with the PWM frequency of ~400Hz.

Putting two high impedances in series is asking for trouble.

perhaps shunt the LED current instead of cutting the power to the
current regulator, or arrange the current limiter to tune its limit
according to a PWM input

--
Jasen.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Jasen Betts wrote:

Putting two high impedances in series is asking for trouble.

The trouble can be caused only by energy stored in stray inductance or
an EMP from a nearby lightning. TVSes on both ends will handle both
cases nicely.

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 21/02/2022 7:16 pm, Piotr Wyderski wrote:

piglet wrote:

Thanks, yes I understood it within a few minutes of my first reply, it
should work fine - as you then proved!

Fine is an understatement. I didn't expect this sort of performance from
a couple of dirt cheap components. It works perfectly well even at 10V
and there is no (reasonable) upper limit - just the regular cooling constraints apply. Selected for implementation. :)

Thank you all for your help.

    Best regards, Piotr

You are very welcome, I enjoyed it and it is good to have on-topic posts
to read!

The 431 packs a lot of gain into a dirt cheap part, folk even use them
as op-amps with a well defined input offset voltage!

piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

piglet wrote:

Thanks, yes I understood it within a few minutes of my first reply, it
should work fine - as you then proved!

Fine is an understatement. I didn't expect this sort of performance from
a couple of dirt cheap components. It works perfectly well even at 10V
and there is no (reasonable) upper limit - just the regular cooling
constraints apply. Selected for implementation. :)

Thank you all for your help.

Best regards, Piotr

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)