Newbie BJT Question--Using low base voltage to switch high Collector-emittor Voltage

Discussion in 'General Electronics Chat' started by Conan, Mar 6, 2008.

  1. Conan

    Thread Starter New Member

    Mar 5, 2008
    Hi. I'm having trouble understanding the gate voltage of a BJT transistor.

    Let's say I have a TIP48 power transistor, and I'm planning to switch a high voltage through the Collector-Emitter (~330V).

    1. Can I use 5V at the gate terminal to switch on the transistor?

    2. Is there a risk of the high voltage from the Collector-Emitter flowing into the portion of the circuit connected to the base pin? (since the emitter will join the 5v and 330v grounds)

    Thanks in advance.
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    According to the datasheet ( ) you should be fine.

    Please exercise caution when dealing with such voltages. You might consider experimenting with 48V or less until you have some experience built up.
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    BJTs don't have gates, they have bases. They are switched by current, not voltage, so you will need a current-limiting resistor in series with the base. The base-emitter voltage will be about 0.7V when the transistor is ON, so you can calculate the base resistor Rb:
    Rb=(5-0.7)/Ib, where Ib is the base current. The base current for this transistor, in order to saturate it, should be about Ic/5 (according to Fig. 2 in the Fairchild datasheet), where Ic is your collector current.
    How much collector current do you expect?
  4. Conan

    Thread Starter New Member

    Mar 5, 2008
    My mistake, base pin.

    So that means, as long as supply a voltage greater than .7 volts to the base, I can switch it on? Can I fully saturate the transistor with, say, 1.5 Volts if I give the base enough current via a proper resistor?

    Ok, these datasheets always confuse me. On the top, it says hfe=([30,10]min, [150]max). Isn't this the gain? I also see the fig. 2 you're reffering to (Ic=5Ib), but how is this different for the hfe specified? A gain of 5 seems really low.

    Ok, good question. I'm planning to discharge a 350v .022mf capacitor into a transformer (no resistor -- it's the high voltage trigger circuit of a camera flash if you're wondering). The cap is just being shorted into the transformer input coil--how much current will pass through the transistor in this scenario if the transistor is used as a switch for the 'shorting'?
  5. Audioguru


    Dec 20, 2007
    The hFE of a transistor is used when the transistor is a linear amplifier with plenty of voltage from collector to emitter.
    You are using the transistor as a awitch that has a very low collector to emitter voltage. You need to look at Maximum Saturation Voltage on the datasheet to see that it needs a lot of base current to saturate with a fairly low voltage loss.

    If you use the transistor to switch the capacitor into a high current load then the transistor will try to exceed its max allowed current then it won't work anymore.
    Or if you don't supply enough base current to the transistor then it won't saturate and it will have a high current and a substantial voltage across it. That makes a lot of heat and will cause the tramnsistor to melt.