> 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
Reply by Piotr Wyderski●February 21, 20222022-02-21
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
Reply by Piotr Wyderski●February 21, 20222022-02-21
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
Reply by Jasen Betts●February 21, 20222022-02-21
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.
Reply by piglet●February 20, 20222022-02-20
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
Reply by Piotr Wyderski●February 20, 20222022-02-20
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
Reply by piglet●February 20, 20222022-02-20
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
Reply by Piotr Wyderski●February 20, 20222022-02-20
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?
> 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?
Reply by piglet●February 19, 20222022-02-19
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