Marco said:
Il 10/10/2012 04:03, Jamie ha scritto:
Great!
The actual circuit should be closer to this:
Version 4
SHEET 1 880 680
WIRE 160 -128 -304 -128
WIRE 160 -96 160 -128
WIRE 160 32 160 -16
WIRE 256 32 160 32
WIRE 160 48 160 32
WIRE -304 144 -304 -128
WIRE 160 144 160 128
WIRE 160 144 112 144
WIRE 256 176 256 32
WIRE 112 192 112 144
WIRE 192 224 160 224
WIRE -304 272 -304 224
WIRE 112 288 112 256
WIRE 256 288 256 272
WIRE 256 288 112 288
WIRE 256 320 256 288
WIRE 256 320 224 320
WIRE 224 336 224 320
WIRE 224 448 224 416
FLAG 224 448 0
FLAG -304 272 0
SYMBOL npn 192 176 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL res 144 32 R0
SYMATTR InstName R1
SYMATTR Value 15k
SYMBOL References\\LT1634-2.5 112 224 R0
WINDOW 3 -154 3 Left 2
WINDOW 0 -50 33 Left 2
SYMATTR InstName R2
SYMBOL res 144 128 R0
SYMATTR InstName R3
SYMATTR Value 18k
SYMBOL res 208 320 R0
SYMATTR InstName R4
SYMATTR Value 200
SYMATTR SpiceLine ""
SYMBOL voltage -304 128 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 24V
SYMATTR Value2 ""
SYMATTR SpiceLine ""
SYMBOL current 160 -96 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 28 Left 2
SYMATTR InstName I1
SYMATTR Value 10m
SYMATTR Value2 ""
SYMATTR SpiceLine load
TEXT 320 -80 Left 2 !.dc I1 0 50m
and the simulation shows the curve I want to see!
I'm sorry for my ignorance, but why it not depends on the physical
characteristic of the bjt?
Thanks again!
Marco
Not sure as to what you mean by that last statement? The operation of
that circuit does depend on a bjt characteristics. The Vbe and
Vce(sat) combined will give much less over loss than what you would get
with a jFet or linear regulators
Vce(Sat) should be ~170mV and Vbe being around 0.7v to a sum of 0.870v
loss in this type of circuit.
R1 supplies minimum current in series with R3 to achieve ~ 20mA at
around 6 volts in the loop supply. WHen loop supply is up to 24 volts,
the 2.5V ref will clamp that and attempt to give a steady reference to
the base via R3.
bjt's work in the same manner as fets do with the exception of how
they are biased of course. With a fet, it's all about the Vgs and the
V(Th) with the Vgd that turns it into a constant current device, but the
problem his is, you normally have to get up in the Vg(th) region for it
to start working, that could be higher than the losses you wish to
sacrifice.
With bjt's, it's always current biased instead of voltage and the
voltage here starts ~0.7v and is repeatable within reason between batches.
Jamie