logjam said:
Do I need a resistor when driving a transistor from a HC273? It will
be on for 1ms and off for 10ms.
Transistor will be a 2N2222 type driving an LED at a maximum of 60ma or
so.
On my breadboard this works fine, but I don't want to make any stupid
engineering mistakes for the final project@! It involves 900-1800 of
the buggers!
The short answer for a BJT is yes.
As you are driving the base of a bipolar, yes. As the bipolar is
sinking LED current then the resistor should be roughly (Vcc-0.6)/6mA.
For 3.3V systems, that would give ~ 4.7k (for a standard size) in
series with the base. You should, of course, have a collector resistor
to limit the LED current.
For the base resistor, I didn't take into account outputs being less
than Vcc due to the output current.
Are you using the 2N2222 because you have them? In such a situation (to
minimize parts) I would use a small FET such as the venerable VN2222LL
for through hole, 2N7002 surface mount. No gate resistor required as
the '273 is always driving it's outputs. Of course, some use gate
resistors to limit the charging current in the FET, but the value is
not critical (somewhere between 2k and 39k would probably work just
fine). In this case, that really shouldn't be an issue.
On your breadboard, it's probably working fine because the '273 outputs
are being limited to about 0.7 ~ 0.8V out (in the high state) and the
internal drivers can (so far) handle that current (and abuse). Don't
count on it lasting too long, though. Without looking at the datasheet,
I can't say whether it will damage the '273, but it's not a good
practise.
On the LED, 60mA is *very* bright. I use LEDs for panel information,
and I don't run those over 10mA. In either case, you will need series
resistors in the LED current path.
Cheers
PeteS