transistor gate question

mikea · 2012-02-20 15:07

Im trying to understand how to select proper gate parameters for a transistor. I have a basic stamp 2. The max output of an i/o pin i think is 20ma, which doesnt seem like enough to "open" the transistor(2n3904) all the way. I can connect the solenoid to vdd,vss,(5 v), and it closes properly. The coil resistance is 108 ohms, 5 volts. which would need .05 amp to activate solenoid. The continous current rating of the transistor is .2 amps. My understanding of transistors is the more current applied to the gate or base , the more the "electrical valve"opens up. Is this correct?

qazxsw21000 · 2012-02-20 15:15

If I am understanding your question, the answer is yes. But do note that transistors have a maximum current rating for the gate.

Phil Pilgrim (PhiPi) · 2012-02-20 15:47

20mA is way more than enough gate base current to drive a 2N3904 into saturation. In fact, you should limit the gate base current with a series resistor. Given that a 5mA gate base current is ample to drive a 50mA collector load, you can compute the size of the series resistor as (5V - 0.6V) / .005A = 880 ohms. Anything in that neighborhood should work.

-Phil

PJAllen · 2012-02-20 16:59

C'mon Uncle Phil, educate these people.
2N3904, NPN, bipolar -- it's base current

Phil Pilgrim (PhiPi) · 2012-02-20 17:18

Oh, man! I'm asleep at the wheel here! 'Saw "gate" in the title, and ... oh, nevermind.

Thanks, PJ! (Is it beer time yet?)

-Phil

mikea · 2012-02-20 18:00

Thank you for the input!....Another rookie question thats related: on the back of the parts bag from radio shack there are absolute max ratings.

Vcbo-40v
Vceo-40v
Vebo-5v
Ic-3a
ft-3mgz
dissipation 20w
h(sub-e) min-10-50
.....are the gate threshold and saturation points in these figures?

Phil Pilgrim (PhiPi) · 2012-02-20 18:23

No. First of all there's no gate threshold, since there's no gate. (It's not a MOSFET, as PJ was quick to catch with a pointy stick to my ribs). For saturation figures, consult the datasheet:

www.fairchildsemi.com/ds/2N/2N3904.pdf

-Phil

mikea · 2012-02-21 19:29

Thank you phil...sorry, i did pick up the term error in the replies and used it again by mistake. Thank you for the link. -mike

Dr_Acula · 2012-02-21 20:21

One useful number is the "current gain", or hFE. For the 2904, gain at 5ma into the base is ? 80.

Gain values:

                       min max
hFE(1) VCE=1V, IC=0.1mA 40 - -
hFE(2) VCE=1V, IC=1mA   70 - -
hFE(3) VCE=1V, IC=10mA 100 - 300
hFE(4) VCE=1V, IC=50mA  60 - -
hFE(5) VCE=1V, IC=100mA 30 - -

As PhiPi says, 5ma into the base will give you more than enough gain to switch 50ma as this is only a gain of 10.

mikea · 2012-02-23 18:25

Thank you Dr-acula that helps ....I'm still unclear about the relationship between base current and emitter current as it relates to gain...I have a 2n222a, which says Hfe :200 . If 1 ma goes to the base does that mean 200ma will be passing through the emitter? -mike

Phil Pilgrim (PhiPi) · 2012-02-23 18:51

Remember: current drawn is a function of the load and is limited by the capacity of the driving element. You can't force more current into a load than its V/R (Ohm's law) characteristics allow. Also, it's bad design practice to rely on the "typical" h_FE of a transistor. If gain is an important consideration, use the minimum value.

-Phil

mikea · 2012-02-23 20:01

Thank you phil, so far i've been copying circuits from others and trying to understand how some of the components work for future projects. Thank you for the input. -Mike

transistor gate question

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