Re: Transistor questions

On 24 Oct 2006 12:33:14 -0700, "Chris" <cfoley1064@xxxxxxxxx> wrote:

Digitmode@xxxxxxxxx wrote:
Hey thanks for the help, the PNP transistor is sounding like a much
better idea. I run into one snag though, the PNP's I have can only
support a max of 100mA at the collector and my load is six leds at 20mA
each, so I need atleast +120mA. This is where I was heading asking
about using 2 transistors on one output to split the load. I need to
know how to calculate the resistor values need for the transistor.

IC specs: SN74LC148 http://focus.ti.com/lit/ds/symlink/sn74ls148.pdf

Hi, Digit. Calculating resistor values for transistors used as
switches isn't too difficult -- I wonder why your teacher hasn't walked
you through it.

OK. The information that we've gotten so far is that you have a TTL
gate (which assumes a +5V supply) and a +12V supply.

Elsewhere, he had written:
: Thanks for your help and for the diagram, one small problem though. My
: second power source is 9 volts not 4.5 volts. The chip is running of 5
: volts and leds are running off 9 volts operating at 4.5 volts. So is it
: better to use individual resistors to each led instead of one higher 1
: watt resistor?

I think this means 9V. It was a different poster talking about 12V, I
think.

Your LEDs have a
forward voltage of 4.5V to 5V (this indicates to me that they're
probably logic level LEDs with built-in resistors).

Some of the blue LEDs (I'm looking at one from RS here) operate at 5V
and 30mA. So 4.5 at 20mA wouldn't surprise me for this one. (300mcd
at 30mA device, RS part number 276-311.)

You haven't said
whether you want the LEDs to turn on when the gate output is high or
when it's low (this is useful to know, unless you happen to have spare
inverters floating around).

Other discussions gave me to understand that:

: What I need to happen is when the output goes high is to turns on 6
: leds (parralle) operating at 20ma each from a 9 volts source. I already
: know I need a 39 ohm resistor for the leds, what I really need to know
: is how to calculate the resistor value I need between the IC and the
: transistor.

I had missed the 9V statement, at that time. Anyway, there is the
description you were looking for, I think.

Let's assume to begin with that you want
the output to be active-low (which was very common back in those
thrilling days of yesteryear when men were men, women were glad of it,
the Lone Ranger and Tonto ruled the West and TTL ruled the globe).

Except that is not what he said. However, it is probably better that
way, if he has the option to control that choice.

I'll leave it at that. The rest must await further discussions.

Jon
.

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