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Voltage drop across transistor — Parallax Forums

Voltage drop across transistor

JTBJTB Posts: 9
edited 2005-03-09 20:30 in BASIC Stamp
As a beginner, I am using the BS2 as count down timer to control the off/on status of an NPN transistor (2N3904)·that in turn controls a 5v relay.· I am using the HIGH pin 4 of the BS2 to trigger the·transistor.· I am measuring a 2.2 voltage drop across the collector/emitter while it is on.· I am using the Vdd of the BS2 as voltage for the collector side of the transistor.· This voltage on the emitter side to ground is 2.7 volts which is below the trigger level of the relay (spec. sheet says "must operate· VDC" is 3.5).· Is this a normal drop for a transistor?· If so, what options are there to raise the voltage to the relay triger level?

Julian

Comments

  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-03-09 17:08
    normally npn's are used with the emmitter connected to ground and the collector tied to the gnd terminal of the load with Vdd connected to the Vdd terminal of the load.

    pnp's are used for the configuration you've specified where the emitter is connected to Vdd, collector is tied to Vdd supply of load, and Gnd is connected to Gnd terminal of the load.

    This is because the transitor state is defined by the voltage between the base and emitter, so you tie the emitter to a known voltage (one of the supplies). Since the on voltage for an npn Vbe is positive you tie the emmiter to ground, for a pnp Vbe is negative so you tie the emitter to Vdd.
  • JTBJTB Posts: 9
    edited 2005-03-09 20:30
    Paul,

    Thanks for the information.

    Julian
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