12v regulator to trigger 2n2222

[harps]

(oops, i meant a 2n3904 transistor, wrong title )

hello, hope all is well.

I'm wondering, if a 12v (3amp) regulator is the main power on a bread board and you need to to trigger a 2n3904 transistor at its base, would an 8k resistor be o.k? or is that too small a value?

thanks for any help .

 

[MikeMl]

Used as a switch, the base current should be between 1/20 to 1/10th of the collector current.

Ib = (12-Vbe)/(20*Ic), where Vbe ~= 0.65V, and only you know Ic

 

[vlad777]

If you are putting something in it's collector branch and using it as a switch then:

2N3904 has:
200 mA collector current
30..300 hfe

Which gives for 12 V :
1.6K .. 16K Ohm

 

[vlad777]

Used as a switch, the base current should be between 1/20 to 1/10th of the collector current.

Ib = (12-Vbe)/(20*Ic), where Vbe ~= 0.65V, and only you know Ic

Maybe I misunderstood something but your equation gives 3A for Ib and also
how can volts trough amps give amps?

Edit:
I think you meant Rb= (12-Vbe)/(Ic/20)

 

[Reloadron]

(oops, i meant a 2n3904 transistor, wrong title )

hello, hope all is well.

I'm wondering, if a 12v (3amp) regulator is the main power on a bread board and you need to to trigger a 2n3904 transistor at its base, would an 8k resistor be o.k? or is that too small a value?

thanks for any help .

I believe what you are saying is the power supply to the board is 12 volts rated at 3 amps, meaning the power supply can deliver 3 amps. I don't believe the transistor is going to be used to switch a 3 amp load. I also believe you are wanting to drive the transistor into saturation and use it as a switch.

This brings us to what has been asked in that you haven't said what the load current will be for the transistor?

I suggest that you read this link to understand what resistor you want on the base lead of the transistor. That will give you the basic idea of what everyone is telling you.

Hope that helps
Ron

 

[audioguru]

If you are putting something in it's collector branch and using it as a switch then:

2N3904 has:
200 mA collector current
30..300 hfe

Which gives for 12 V :
1.6K .. 16K Ohm

No.
hFE is never used for a transistor used as a switch (but some articles about it on the web are wrong). hFE is used for a linear transistor that has plenty of collector to emitter voltage so it is not saturated.
The max saturation voltage of a 2N3904 transistor and most others is listed when its base current is 1/10th its collector current.
You can test a few hundred transistors and maybe find one or two that need less base current.

 

[harps]

thanks i shall read the link.

yes its a regulator that can handle 3amps. i am to switch on two small 3mm led's and two 40ma relays. so a little over 120ma 's i beleive.

what i did in the mean time was use a variable resistor and reduce the ohms until the transistor turned on the relays and i then multi-metered the variable resistor and it read about 8k or there abouts so as i had a an 8k fixed resistor i used that.

but i shall read the link for more info

thanks for all your help everyone. : )