TT_Man said:
I need a simple/cheap method of detecting current ( battery charging). Max
current is 500mA. All it has to do is turn on an led if battery is
charging. I did try just a simple resistor + transistor, but it gets too
hot. Looked at maxim high side current detectors but uses expensive lo ohm
resistors and maxim stuff is expensive and hard to get.
Following is an LTspice ASCII file of a simple indicator which detects
current over about 350 mA. The 2 ohm resistor will dissipate 1/2 watt at
500 mA. As current increases, some of it is shared by the BE junction of
the transistor, so the resistor will see no more than about 1 watt. You can
add a base resistor to eliminate this. You can use a different value sense
resistor for other current ranges. A germanium transistor will allow even
smaller sense resistors, but they are rare.
Actually, Mouser stocks a variety of SiGe transistors at about $1 each, but
all are NPN and they are 60 GHz devices, and I did not see any forward
voltage spec for Vbe:
http://www.cel.com/pdf/datasheets/nesg2030m04.pdf.
You can also use a MOSFET with a low Vto, such as the Si4467DY (which is in
the LTspice library), and is available from Mouser for about $2. But the
FDC604P is only $0.36 each, and has typical Vto of 0.7 V, so it should work
well.
I made a Spice circuit for that but it is identical except for the MOSFET.
Paul
==============================================================================
Version 4
SHEET 1 880 680
WIRE -16 0 -48 0
WIRE 112 0 48 0
WIRE 192 0 112 0
WIRE 112 48 112 0
WIRE -48 96 -48 0
WIRE 192 112 192 0
WIRE 112 160 112 128
WIRE 128 160 112 160
WIRE 112 176 112 160
WIRE 64 192 -160 192
WIRE -160 208 -160 192
WIRE 192 224 192 208
WIRE -48 272 -48 176
WIRE 112 272 112 256
WIRE 192 320 192 288
WIRE -160 416 -160 288
WIRE -48 416 -48 352
WIRE -48 416 -160 416
WIRE 64 416 64 240
WIRE 64 416 -48 416
WIRE 112 416 112 352
WIRE 112 416 64 416
WIRE 192 416 192 400
WIRE 192 416 112 416
WIRE 192 464 192 416
FLAG 192 464 0
SYMBOL res 96 32 R0
SYMATTR InstName R1
SYMATTR Value 2
SYMBOL LED 176 224 R0
SYMATTR InstName D2
SYMATTR Value QTLP690C
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 176 304 R0
SYMATTR InstName R2
SYMATTR Value 470
SYMBOL voltage 112 256 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 6
SYMBOL voltage -48 256 R0
WINDOW 3 29 91 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 12
SYMBOL res -64 80 R0
SYMATTR InstName R3
SYMATTR Value 12
SYMBOL diode -16 -16 M90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D4
SYMATTR Value MURS120
SYMBOL pnp 128 208 M180
SYMATTR InstName Q1
SYMATTR Value 2N2907
SYMBOL sw 112 272 M180
WINDOW 0 -51 100 Left 0
WINDOW 3 -150 54 Left 0
SYMATTR InstName S1
SYMATTR Value MySwitch
SYMBOL voltage -160 192 R0
WINDOW 3 -341 113 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value PWL(0 0 .1 0 .101 14 .2 14 .201 0)
TEXT -82 440 Left 0 !.tran .5 startup
TEXT -880 352 Left 0 !.model MySwitch SW(Ron=.1 Roff=1Meg Vt=0 Vh=-.5
Lser=10n Vser=0)
