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Sci.Electronics.Basics -> TTL input circuit to drive diode

There are 6 messages in this thread.
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Author: Gary H
Date: 20:45 22-11-06

Hi. I need a little help with something that should be pretty basic. I
have a control board that has a TTL output to turn on and off a laser. The
laser is nothing more than a diode that consumes about 60mA. So, I want to
be able to turn the diode on or off with the TTL output. I don't really
know much about TTL requirements. But, would a NPN transistor with resistor
between the base and the TTL Output work? (Collector to + side of diode,
emitter to GND.)? If not, what would be the best thing to do?

Thanks!
Gary

Date: 21:23 22-11-06

"Gary H" <dontsend@memail.com> writes:
> Hi. I need a little help with something that should be pretty basic.
> I have a control board that has a TTL output to turn on and off a
> laser. The laser is nothing more than a diode that consumes about
> 60mA. So, I want to be able to turn the diode on or off with the TTL
> output. I don't really know much about TTL requirements. But, would
> a NPN transistor with resistor between the base and the TTL Output
> work? (Collector to + side of diode, emitter to GND.)? If not, what
> would be the best thing to do?

That's what I do. Or an N-channel MOSFET, which requires less math.

For a BJT (NPN) transistor, the math goes like this:

Consider the NPN with Rb between its base and the TTL output, and Rc
in series with the diode load, between Vcc and the collector. The
emitter is simply grounded.

The goal current is Ic. There will be a voltage drop across the
transistor (Vce) and across the diode (Vf) at current Ic. Subtract
those from Vcc to get the voltage across Rc (Vrc). Compute R given Ic
and Vrc (V=IR R=V/I, so Rc = Vrc/Ic).

Now, look up Hfe for the transitor. This is the "gain" of the
transistor. Divide Ic by Hfe to get Ib. This is a minimum Ib, of
course. Now, there will be a voltage drop across the transistor again
- Vbe. Subtract that from the TTL Voh to get the voltage drop across
Rb, and calculate Rb from Vrb and Ib. This resistance is a maximum,
you can use smaller values if you want.

For a MOSFET, the math is easier. The voltage drop across the MOSFET
(Vds) is negligible, you can just calculate Rc (er, Rd now) by (Vcc -
Vf)/Ic. As for the base, any medium resistor is sufficient, since the
gate is capacitive. You probably could omit the resistor completely.

Author: Chris
Date: 00:01 23-11-06

Gary H wrote:
> Hi. I need a little help with something that should be pretty basic. I
> have a control board that has a TTL output to turn on and off a laser. The
> laser is nothing more than a diode that consumes about 60mA. So, I want to
> be able to turn the diode on or off with the TTL output. I don't really
> know much about TTL requirements. But, would a NPN transistor with resistor
> between the base and the TTL Output work? (Collector to + side of diode,
> emitter to GND.)? If not, what would be the best thing to do?
>
> Thanks!
> Gary

Hi, Gary. If you actually have a TTL output, you might not know that
they sink a lot more current than they can source. That means you
might want to use a PNP transistor, with the output active low (the
diode goes on when the TTL output is low) like this (view in fixed font
or M$ Notepad):

|
| VCC
| +
| |
| .-. VCC
| | | +
| | | |
| '-' |
| |\ ___ | |<
| -| >O--|___|-o-|
| |/ |\
| |
| .-.
| | |
| | |
| '-'
| |
| V ~
| - ~
| |
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Thus was it done in days of yore when TTL ruled the earth.

As is mentioned in a prior post, if you're stuck with an active high
output (diode must go on when the TTL output is high), you might want
to go with a logic level MOSFET. If you do, make sure to use a pullup
at the TTL output to ensure the high voltage goes over the typical 3.5V
you'll see as a TTL high. That might not fully turn on your MOSFET.

Good luck
Chris

Author: Gary H
Date: 00:10 23-11-06

Thanks. I appreciate you taking time to show the equations and how they
work. It makes sense.

Gary

"DJ Delorie" <dj@delorie.com> wrote in message
news:xnmz6ic35c.fsf@delorie.com...
>
> "Gary H" <dontsend@memail.com> writes:
>> Hi. I need a little help with something that should be pretty basic.
>> I have a control board that has a TTL output to turn on and off a
>> laser. The laser is nothing more than a diode that consumes about
>> 60mA. So, I want to be able to turn the diode on or off with the TTL
>> output. I don't really know much about TTL requirements. But, would
>> a NPN transistor with resistor between the base and the TTL Output
>> work? (Collector to + side of diode, emitter to GND.)? If not, what
>> would be the best thing to do?
>
> That's what I do. Or an N-channel MOSFET, which requires less math.
>
> For a BJT (NPN) transistor, the math goes like this:
>
> Consider the NPN with Rb between its base and the TTL output, and Rc
> in series with the diode load, between Vcc and the collector. The
> emitter is simply grounded.
>
> The goal current is Ic. There will be a voltage drop across the
> transistor (Vce) and across the diode (Vf) at current Ic. Subtract
> those from Vcc to get the voltage across Rc (Vrc). Compute R given Ic
> and Vrc (V=IR R=V/I, so Rc = Vrc/Ic).
>
> Now, look up Hfe for the transitor. This is the "gain" of the
> transistor. Divide Ic by Hfe to get Ib. This is a minimum Ib, of
> course. Now, there will be a voltage drop across the transistor again
> - Vbe. Subtract that from the TTL Voh to get the voltage drop across
> Rb, and calculate Rb from Vrb and Ib. This resistance is a maximum,
> you can use smaller values if you want.
>
> For a MOSFET, the math is easier. The voltage drop across the MOSFET
> (Vds) is negligible, you can just calculate Rc (er, Rd now) by (Vcc -
> Vf)/Ic. As for the base, any medium resistor is sufficient, since the
> gate is capacitive. You probably could omit the resistor completely.

Author: Robin
Date: 03:38 23-11-06

Author: Martin
Date: 13:03 24-11-06

DJ Delorie wrote:
> "Gary H" <dontsend@memail.com> writes:
> > Hi. I need a little help with something that should be pretty basic.
> > I have a control board that has a TTL output to turn on and off a
> > laser. The laser is nothing more than a diode that consumes about
> > 60mA. So, I want to be able to turn the diode on or off with the TTL
> > output. I don't really know much about TTL requirements. But, would
> > a NPN transistor with resistor between the base and the TTL Output
> > work? (Collector to + side of diode, emitter to GND.)? If not, what
> > would be the best thing to do?
>
> That's what I do. Or an N-channel MOSFET, which requires less math.
>
> For a BJT (NPN) transistor, the math goes like this:
>
> Consider the NPN with Rb between its base and the TTL output, and Rc
> in series with the diode load, between Vcc and the collector. The
> emitter is simply grounded.
>
> The goal current is Ic. There will be a voltage drop across the
> transistor (Vce) and across the diode (Vf) at current Ic. Subtract
> those from Vcc to get the voltage across Rc (Vrc). Compute R given Ic
> and Vrc (V=IR R=V/I, so Rc = Vrc/Ic).
>
> Now, look up Hfe for the transitor. This is the "gain" of the
> transistor. Divide Ic by Hfe to get Ib. This is a minimum Ib, of
> course. Now, there will be a voltage drop across the transistor again
> - Vbe. Subtract that from the TTL Voh to get the voltage drop across
> Rb, and calculate Rb from Vrb and Ib. This resistance is a maximum,
> you can use smaller values if you want.

I suspect we are in violent agreement since you say:
"you can use smaller values if you want"
But I think that is not quite strong enough ...

Good design practice is to ensure the transistor is well saturated.
Do this by dividing the Rb above by 10
(thus increasing the base current by a factor of 10)

<snip>

Martin

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