2n3904 bias resistor

[Colonel_Sanders]

Hi again,

I'm using K O'Connors Quad Relay Kit, which is a 40175 setup, to drive shunt jFets but I'd like to use it to drive some 2n3904's to switch relays on/off as well.

The 3904 has an minimum HFE of 30 @ 100mA and the relay I need to drive needs about 20mA from a 12V supply. That means I need at least .666mA at the base. Can somebody explain to me what value bias resistor I will need at the base of the transistor?

I would figure it out myself but I have no idea what voltage the 40175 can put out in high state

Thanks,
C_S

[Earl]

Most of the time (IME), you use a transistor in the ground leg of the relay coil circuit. 12V sits at one end of the coil, the transistor sits at the other. Emitter at ground.

Then you just hit the base with 12V through a suitable current limiting resistor.

[Steve Conner]

10k?

[Earl]

Oh, you're wanting to drive some transistors with a Flip-Flop. I reckon you need to find out how much current that IC can source.

Add - If you figger the coil is ~600 ohms, and your Vcc for the IC is 5V, a 10K resistor ought to get you about 0.425mA of base current...shoud work.

[Colonel_Sanders]

Legends! I'll give 10K a go.

Earl, what spec should I look for in the data sheet regarding current?

Cheers,
C_S

[Earl]

Uh........er.....well, it used to be that the old TTL gates had a "fan out" current of 10 gate inputs...which is about 10mA.

I looked at a data sheet for this Flip-Flop, but didn't see any input/output recs for it. I'm sure there's someone here that is more familiar with this.

When all else fails, put your finger on the case and see if it's hot. :-)

[Colonel_Sanders]

Sweet as, yeah I couldn't see anything either.

More like pull my finger out and get to work eh :-).

Cheers,
C_S

[Enzo]

I cheat. When I want to drive a relay or something I use a darlington - MPSA13 is common, cheap, and available. Like so many of my dates. Now drive current is not an issue at all.

[Colonel_Sanders]

haha, 10,000 HFE @ 100mA/5V! I'm made in the shade! Thanks Enzo.

C_S

[Enzo]

I started using that a long time ago because I could easily drive it from a TTL output.