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help with low cost current detector

Discussion in 'Electronic Design' started by TT_Man, Jul 3, 2008.

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  1. TT_Man

    TT_Man Guest

    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.
     
  2. Hammy

    Hammy Guest


    ZXCT1009FCT

    1 0.98000 $0.98
    25 0.84160 $21.04
    100 0.77150 $77.15

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=ZXCT1009FCT-ND


    Or

    FAN4010IL6X_F113CT

    1 0.86000 0.86
    25 0.64760 16.19
    100 0.48560 48.56

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=FAN4010IL6X_F113CT-ND

    1% resistor 0.25w
    RHM.22MCT-ND 10 for $1.5

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=RHM.22MCT-ND
     
  3. Hi TT_Man,
    LM311 is your friend. You can tune the offset to a few mV in your most
    wanted direction, so you need only a milliohm resistor in the ground loop

    A cheaper way may be a 1N4001 as high side shunt device parallel to a
    pnp-transistors BE diode that switches on a LED with resistor in series to
    ground.

    Have a nice day

    Marte
     
  4. How about a reed relay?

    Wind the charger lead around it until it is "enough" to close it. You could
    adjust the current threshold by adding/removing turns.
     
  5. Paul G.

    Paul G. Guest

    If you have enough voltage to spare (open ckt - battery voltage),
    you can go real low-tech and put a 1-2 amp rated auto lamp in series.
    It should glow with .5 amps. If you short the charger terminals it
    will light up very brightly, but protect your charging circuitry. It
    is pretty reliable! You should be able to get one at any decent auto
    store or garage.
    The filament has a non-linear v vs. i characteristic, that should
    give you lower than expected voltage drop.

    -Paul
     
  6. 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)
     
  7. JosephKK

    JosephKK Guest

    I suppose you could run an operating point solution, Ib=10uA, Vce 3.3v
    with small inductive load. You should use all of the model values.
     
  8. I could not find any more information on the SiGe transistors, except that
    they are super high frequency (up to 350 GHz), and very low voltage, wuth a
    combination of bipolar and CMOS operating as low as 1.2 V. But I don't know
    what the input characteristic is. It might be worth a dollar to order one
    of these beasts and check it out, although I don't do much with RF, and
    it's probably easy to blow out a device that has a BVceo of 3.5 volts. Here
    is some interesting info:

    http://www.jazzsemi.com/process_technologies/sige.shtml

    http://www.eng.auburn.edu/~guofu/sige_intro.htm

    It's probably a CMOS gate with about 0.5-1 volt threshold.

    Paul
     
  9. Guest

    Voltage, current values would be nice. A transistor would do fine.
    pass the charging current through the base-emitter junction. Led-
    resistor between collector and supply return.
     
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