Re: Thanks to all - well most...


"Fred Bloggs" <nospam@xxxxxxxxxx> wrote in message
news:47F50EA0.1070003@xxxxxxxxxxxxx


Fred Bartoli wrote:
You don't need the ref source and get Vbatt independence by using the
LED as the ref voltage.

___ ___
.-|___|---+---|___|--.
| | |
=== | |\| |
GND '-|-\ |
| >-----+
.-|+/ |
| |/| .-.
| === | |
| GND | |
| '-' (*)
| | ___
'----------+----|___|--- +V
| NC
V ->
-
|
===
GND


(*) using a GND supplied opamp with a bipolar output stage ensures
startup, or add this resistor.



Right, we covered that earlier, and have been calling it the
self-referential circuit. That is the one with dI/dV=-I/V , I and V are
for the LED. The OA was a micropower type.


I made a self-referential circuit using a 2N3904 and 2N3906. It works in
the SwitcherCad simulator down to 2.1 volts. It may not be very stable with
temperature, but it's cheap and easy.

Paul

===========================================================================

Version 4
*** 1 1172 680
WIRE -16 0 -256 0
WIRE 80 0 -16 0
WIRE -16 32 -16 0
WIRE 160 80 80 80
WIRE 80 96 80 80
WIRE 160 112 160 80
WIRE -16 144 -16 112
WIRE 16 144 -16 144
WIRE -256 176 -256 0
WIRE -16 208 -16 144
WIRE 80 224 80 192
WIRE 160 224 160 192
WIRE 160 224 80 224
WIRE 80 256 80 224
WIRE 80 256 48 256
WIRE -16 320 -16 304
WIRE 80 320 80 256
WIRE -256 416 -256 256
WIRE -16 416 -16 400
WIRE -16 416 -256 416
WIRE 48 416 -16 416
WIRE 80 416 80 384
WIRE 80 416 48 416
WIRE 48 432 48 416
FLAG 48 432 0
SYMBOL pnp 16 192 M180
WINDOW 0 49 26 Left 0
WINDOW 3 38 52 Left 0
SYMATTR InstName Q2
SYMATTR Value 2N3906
SYMBOL res 0 416 R180
WINDOW 0 36 76 Left 0
WINDOW 3 36 40 Left 0
SYMATTR InstName R1
SYMATTR Value 50k
SYMBOL res 64 -16 R0
SYMATTR InstName R5
SYMATTR Value 1.5k
SYMBOL voltage -256 160 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 3
SYMBOL npn 48 208 M0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL LED 64 320 R0
WINDOW 3 23 71 Left 0
SYMATTR InstName D1
SYMATTR Value QTLP690C
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res -32 16 R0
SYMATTR InstName R2
SYMATTR Value 40k
SYMBOL res 144 96 R0
SYMATTR InstName R3
SYMATTR Value 4.7Meg
TEXT -224 440 Left 0 !.tran 1m startup


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