Small resistance electronic switch

Discussion in 'General Electronics Chat' started by ecka333, Nov 30, 2010.

  1. ecka333

    Thread Starter Active Member

    Oct 1, 2009
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    I need to cumutate Ni-Mh 4,8V battery that is to say i need to connect or disconnect battery from 5V charging circuit. Tried to use 2N5551 bipolar transistor. I connected collector to +, emiter to load (battery), and base to +. Then measured voltage drop between collector and emiter - it was about 0,7V. Then measured base current - it was 0,5mA. Voltage drop is to big for my project. I think the big voltage drop is because of low base current. And base current is to little because supply voltage between collector and the load is to low (5V). Am i right? maybe someone knows simple electronis switch, which i can drive from microcontroller?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Are you using a base resistor?
     
  3. JDT

    Well-Known Member

    Feb 12, 2009
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    This voltage drop is because by connecting the base to the collector you are not fully saturating the transistor.

    One way to improve the situation is to use a PNP transistor. Emitter to +, Collector to the load (battery). Connect the base to 0V via a resistor to switch the transistor on. Select the resistor to give sufficient base current. As a rough guide find out the hfe value of the transistor. If, say, it is 100 then your base current needs to be a bit more than 1/100 of the collector current.

    If you connect your base resistor to the output of a micro-controller then by driving the output "low" (to 0V) you will switch your transistor on.
     
  4. ecka333

    Thread Starter Active Member

    Oct 1, 2009
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    I used base resistor, but with this resistor transistor base current was very small , so i tried to shorten base resistor. And what is the diference between npn and pnp transistors, i think complementar transistors have the saturation voltage.
     
  5. retched

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    Last edited: Nov 30, 2010
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I don't see any schematics posted.

    I suspect that ecka333's primary language is not English. Communication via schematic diagrams is vital to ensure that everyone understands what is going on.

    Retched,
    Your last statements are confusing. Transistors are current devices, not voltage devices.

    Current flow through the base controls a larger current through the collector; either NPN or PNP. What differs between PNP and NPN is the direction of current flow.

    In order to use either a PNP or NPN transistor as a saturated switch, the base current needs to be 1/10 of the desired collector current. If controlling the transistor is to be performed by a voltage level, the current must be limited by using a resistor of an appropriate value.

    Rbase = (Vsupply - Vbe) / (Ic/10)
    where:
    Rbase = The base current limiting resistor.
    Vsupply = The voltage relative to the transistors' emitter that will be applied to the base resistor.
    Vbe = The voltage on the base relative to the emitter when the transistor is conducting.
    Ic = The desired collector current.

    Note that most uCs' I/O pins are limited to 20mA source or sink current. If their Vcc is 5v, the smallest base resistor that can be used is 220 Ohms, and 200mA is the highest collector current that can be sunk or sourced from a single transistor and have it still be saturated, with a few exceptions (certain very high-hFE transistors).

    Logic-level power MOSFETs are voltage controlled. You charge or discharge the gate with respect to the source terminal.

    Now, it seems that our OP wants to charge a battery. The current flow into the battery must be limited; without current limiting the battery will overheat, possibly exploding and causing damage and/or injury.

    NiMH batteries are difficult to charge. In order to do so properly, the battery temperature must be monitored. Simple "charge at x current over time" methods don't work very well.

    Read here: http://www.powerstream.com/NiCd.htm
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    True. I will edit to reduce confusion.
     
  8. ecka333

    Thread Starter Active Member

    Oct 1, 2009
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    I know bipolar transistor theory. I measured initial battery charge current, when the battery was discharged to 4V and R2 was 0hm (shorted). It was about 60mA, then the current falled down after short time. After 1 hour it was 12mA, after 3 hours it was about 3mA. And there was no overheat. The battery wil be used in timer, so i want simple shematics, which will disconnect battery after some time, when the mains supply is lost: the schematics must protect battery from deep discharge. The variant shown in the drawing, is not functioning properly, because the voltage drop across transistor is 0.9V, even if VT1 base is connected directly to +5.5V supply wire ,when i need 0.2V drop at most.
     
    Last edited: Dec 1, 2010
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