--- On Sat, 9/26/09, n...@no6b.com <n...@no6b.com> wrote:
> From: n...@no6b.com <n...@no6b.com>
> Subject: RE: [Repeater-Builder] Busy Signal (COS) inverter Circuit.
> To: Repeater-Builder@yahoogroups.com
> Date: Saturday, September 26, 2009, 11:16 AM
> At 9/25/2009 14:08, you wrote:
> >So did I at first.
> >
> >Hope it works for him
>
> The problem I see with this circuit is that there's nothing
> limiting
> current on the LED other than the transistor itself.
> The datasheet for the
> 2N3905 specs a MINIMUM current gain of 30 at 50 mA of
> collector current,
> but it can be as high as 150. Assuming a gain of 100,
> you're going to put
> 43 mA of current through that poor LED. It will light
> very brightly but
> only last a few months. Better to put a current
> limiting resistor in
> series with the diode & let the transistor go to
> saturation. Assuming 2 V
> drop across the LED, a 2.7k resistor will yield just over
> 10 mA through the
> diode, regardless of the transistor's beta. That way
> you can also use a
> far more common PNP transistor, the 2N3906, & not worry
> about its higher gain.
>
> Bob NO6B
>
>
You seldom rely on the gain of a transistor. Just make sure its minimum gain
is enough or more to do what you want and use external components to set the
overall gain or current limit.
As you pointed out , the gain can vary over a range of 3 to 1 or more.
