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simplest way to invert + to -?

Discussion in 'General Electronics Discussion' started by Terje Høegh Sæverud, Aug 8, 2009.

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  1. Terje Høegh Sæverud

    Terje Høegh Sæverud

    3
    0
    Aug 8, 2009
    Hi all. I am putting an accessory in my car that is a mirror with lights that are to turn on with my dome lights. So I took my dome light wire for a signal, but it is + and the mirror lights turn on with -, so the mirror lights were working opposite the dome light.

    What is the most simple way to change the + to - so the mirror lights will work the correct way? (but not with a relay because that is too loud in the cabin)

    I was thinking to use a logic IC NOT gate, or a transistor inverter NOT gate. Which is more simple for a car environment?

    My mirror lights takes .43 amps. Is there a logic IC made that will work for me?

    Thanks you all
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    If you have two wires out of the mirror then you don't need anything more, just hook + up to +acc and hook the - to the dome grounding switch wire.

    An ordinary logic gate can't take more than 10mA but a transistor or a driver IC can.

    So if there's only one wire from the mirror then you'll need a PNP-transistor + a resistor, or else only a P-channel MOSFET. Hook the Emitter to +acc, the Base (via the resistor) to the dome light grounding switch wire, and the Collector to the mirror light.
     
  3. Terje Høegh Sæverud

    Terje Høegh Sæverud

    3
    0
    Aug 8, 2009
    The mirror is complex, multifunction and has seperate controls for the same lights I want triggered, so the only way I can do it is by triggering through the "-".

    Should this gate driver work? FAN3223TMXCT, datesheet www(DOT)fairchildsemi.com/ds/FA%2FFAN3223C.pdf

    Or this MC34167T? datasheet www(DOT)onsemi.com/pub_link/Collateral/MC34167-D.PDF

    ^Could I hook up either of those directly without other parts?



    If I did a transistor and resistors, a TIP31 NPN Transistor, and my math tell me 10k ohm 5 Watt Base resistor and 30 ohm 5 Watt Collector resistor? Trigger into Base, output to Collector. Do I not need Collector resistor?

    Thanks
    Terje
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Nope, the FAN thing is a surface mounted CMOS gate driver. Even if it says 5A it's the peak current into a capacitive load, doesn't say how much the static current is.
    And the MC thing is a switchmode controller. I wouldn't try to use that.
    If I were to use an IC I'd look at the ULN2000 series darlington drivers.

    If I understand you correctly now, the dome light gets +12V when switched on and the mirror has to ground a wire to turn on? If so you are correct in choosing an NPN.
    You don't want a Collector resistor, that is against the purpose. The mirror will take care of the current limiting there. No need for any 5W resistors anywhere either.
    The Base resistor for the TIP31 should be 270 Ohms 0.7W.
    If you chose a darlington transistor instead then the base resistor could be 10-15k 0.25W.
    I'd also consider a N-MOSFET, like the BUZ10 or 11. It wouldn't even need a base resistor, but could be susceptible to electrostatic damage during assembly.
     
  5. Terje Høegh Sæverud

    Terje Høegh Sæverud

    3
    0
    Aug 8, 2009
    Thanks for the help. I did the P = V × I ; and at around 13v with mirror lights having .42A draw I thought 5.5 Watts or so would be for the resistor.

    I like the looks of the ULN2000, like the ULN2002A, but is the input max voltage high enough at 13v? When the car's running it can be at 14v to 15v+. And it does not needs a resistor?

    The BUZ10 is available to me where I normally shop. I don't know what to connect to pins 1,2,3 for proper function.
     
  6. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    That 5.5 Watts is the power consumed by the lamps, having a resistance of 30 Ohms, not to be confused with any extra resistor needed - it's already there, in the lamps. An extra collector resistor would only dim the light.

    The 13V is the max ever needed to get down to 2V saturation voltage at 300mA.
    Look at the absolute max ratings, where it states 30V as being the max safe input voltage for all the ULN's (exept 2001).
    I'd rather use the ULN2003 as it gives the highest base current, ensuring the lowest saturation voltage - giving lower package dissipation. You might even want to connect all in's & out's in parallell. No need for extra base resistors (exept for the ULN 2001).

    You can think of the power MOSFET's as having the same pinout as power transistors; 1-2-3 = B-C-E. 1 = dome light, 2 = mirror, 3 = ground.
     
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