Best way to ensure signal does not exceed 5v?

[Kaz Kylheku]

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:

I would like to add a little circuitry to cap the analog line at a max
of 5v.

The exact answer to this (or whether you even need a solution, for that
matter) depends on what kind of device you are driving.

For instance, if you're driving a TTL input, you need to be able to
handle current coming from the input when you drive it low. Can the
analog output do that?

Also, what kind of impedance does the output require from the next
device? Is it a fairly powerful driver that can source current, or
does it require pure low-current voltage bridging?

The voltage level is not the only problem. Suppose the output requires a
10 Kohm impedance (typical line impedance requirement for audio).
Suppose the next device only has a hundred ohm impedance or something.
You could fry your analog output, rather than the next device.

In other words, even as it is, you likely cannot simply plug "anything"
into this analog output! So if your requirement is to be able to plug in
anything, you may have to reinforce that output.

Another question is: will you be switching this output at a high
frequency? If you vary the output very slowly, then you can consider
everything just from a DC point of view (impedances, voltages, etc).
If you're going to be pulsing it at a high rate, then you have an AC
signal, which faces different impedances. (Almost always, lower
impedances: AC will leak into places where DC cannot go.)

It may be much better for you to just know what kind of device will be
hooked up and spec accordingly. If it's a high impedance device that can
take 10V, you have nothing to do.

Maybe you can simply dictate this requirement to whoever will be
interfacing with this, making it someone else's problem.

 

[Jasen Betts]

On 2011-09-03, krw@att.bizzzzzzzzzzzz <krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 19:15:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.

Mea Culpa.

Actually less than a US dollar,

$6.34 in thousands.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM136H-5.0-ND

yeah, if you need the ruggedised one.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM336Z-5.0-ND

96c for singles. the SMT version is slightly cheaper.

That is, if you're talking about an LM136-5 (that you can actually buy). An
LM286 is an opamp. You've really pulled your pud on this one.

dunno where you got LM286, he wrote LM386 (which is a low power
loudspeaker driver recently discussed in another thread and perhaps
the cause of the error) it's fairly obvoius he meant LM336 (which
is described in the PDF)

but what have you got that's less than
half a penny?

Two 0603 resistors. Two tenths each.

not that cheap unless you buy 5000

--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---

 

[Kaz Kylheku]

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:

if the user does the RIGHT thing and sets the DAC to 5v. So maybe my
question should be: How can I cap a 5-10v signal to 5v?

One of the simplest ways: send the output to ground through a resistor
and a 5V zener diode. This is a kind of voltage divider, and so the
resistor has to be chosen properly with regard to the impedance of the
next device. You take the output from the top of the Zener (between the
resistor and Zener).

How your Zener works is that it does not conduct current (or conducts a
very small amount of reverse leakage current) until its Zener voltage is
reached, and then its starts to conduct copiously. At this point,
current is limited by the resistor. Whatever voltage is in excess of
the Zener, that excess voltage is dropped across the resistor.

If your resistor is too high for the input impedance of the next device,
this Zener circuit will not exhibit a stiff voltage. For instance
suppose that the next device is disconnected and you have a 100K
resistor with the Zener, and the analog interface is programmed to
output 4V. This means that the Zener is not turned on (it is a 5V Zener,
recall). No current flows through the 100K resistor, and so the output
(top of Zener voltage) is 4V. Very good. Now suppose you hook up the
input, and the input has only a 1K resistance. Now the Zener becomes
irrelevant! Current flows across the 100K resistor, and through the 1K
input, forming a 100:1 voltage divider. Your actual output voltage is
now about 0.04V, not 4V. Oops!

Choose this resistor so that it's 5 to 10 times smaller than the
input impedance of the next device. But if the input impedance of the
next device is small already, this means that the resistor will be
small, which means that a lot of current can flow when the Zener diode
is turned on (10V worst case).

Without being able to assume anything about the next device, or about
the analog output device for that matter, this can't be 100% correctly
designed using just a couple of passive components.

If you cannot assume anything at all, you can end up way over-designing
it. Gee, let's see, 0 to 5V, rock solid, noise-free, into anything from 0.1
ohm, to 1Mohm. No problem, we will just need a couple of powerful
transistors bolted to a heat sink ...

 

[Kaz Kylheku]

On 2011-09-02, Tim Wescott <tim@seemywebsite.com> wrote:

All the suggestions so far, or a transistor with built-in bias and a
resistor (like this one: http://search.digikey.com/scripts/DkSearch/
dksus.dll?Detail&name=FJY3013RCT-ND). Emitter to ground, base to the DAC,

Hmm, page says: "Limited quantity available, not recommended for new design".

collector to a 10k resistor to +5V. Take your output off of the
transistor collector. It'll invert, but that's OK -- software is
supposed to fix things like that.

Invert?

Emitter to +5V, collector to resistor to ground: PNP!

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=FJY4013RCT-ND

"Obsolete item", unfortunately. But there many such devices in the catalog.

 

[Kaz Kylheku]

On 2011-09-03, Kaz Kylheku <kaz@kylheku.com> wrote:

Emitter to +5V, collector to resistor to ground: PNP!

Hahaha! *rubeyes* I can't belive I just wrote this bull.

Sorry about that! Well, Back to sleep.

 

[Kaz Kylheku]

On 2011-09-03, Kaz Kylheku <kaz@kylheku.com> wrote:

On 2011-09-03, Kaz Kylheku <kaz@kylheku.com> wrote:
Emitter to +5V, collector to resistor to ground: PNP!

Hahaha! *rubeyes* I can't belive I just wrote this bull.

Sorry about that! Well, Back to sleep.

Actually, there is a way to have a single PNP non-inverting switching
stage for this, which is what I must have been thinking of, since
I worked with such a circuit not long ago. Suppose we want 0V output,
until the input is somewhere above 3V, in which case our circuit is
to swing to +5V.

One trick to get non-inverting behavior in a single transistor stage is
to convert the input voltage, or portion thereof, to current and sense
it by the BE junction. A PNP transistor is required because it
has non-inverting behavior (under a positive supply) with
respect to current. Since the load is ground-side, the more current
flows through the collector, the higher the load voltage rises above
ground. We just have to make collector current follow input voltage.

What we do is bias it like this: the input drops across a
current-sensing resistor, which is placed across the PNP base-emitter
terminals, then another resistor to help limit current, and finally
across a 3.3V Zener to ground. (Or maybe a stack of regular diodes to
get some lower voltage, as desired.) When the input voltage is below
3.3, no current flows across the resistors and so the PNP is cut off.
As the input voltage rises from there, current flows across the
resistors. When about 0.7V builds in the current sensing resistor, the
PNP turns on. We put no feedback resistor between the transistor's +5V
and the emitter, so it saturates easily. When it does saturate, it pulls
the load to +5V (minus the small saturation VCE). Thus load voltage
polarity follows input polarity: no inversion.

 

[krw@att.bizzzzzzzzzzzz]

On 3 Sep 2011 06:19:53 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2011-09-03, krw@att.bizzzzzzzzzzzz <krw@att.bizzzzzzzzzzzz> wrote:
On Fri, 02 Sep 2011 19:15:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.

Mea Culpa.

Actually less than a US dollar,

$6.34 in thousands.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM136H-5.0-ND

yeah, if you need the ruggedised one.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM336Z-5.0-ND

96c for singles. the SMT version is slightly cheaper.

He said LM136. That's what I searched on. Still, $1 vs $.004; you pick.

That is, if you're talking about an LM136-5 (that you can actually buy). An
LM286 is an opamp. You've really pulled your pud on this one.

dunno where you got LM286, he wrote LM386 (which is a low power
loudspeaker driver recently discussed in another thread and perhaps
the cause of the error) it's fairly obvoius he meant LM336 (which
is described in the PDF)

Typo. Yes, but John's post was so screwed up there's no telling what he
meant.

but what have you got that's less than
half a penny?

Two 0603 resistors. Two tenths each.

not that cheap unless you buy 5000

At $.002 each that's a whole $10 for a reel. Wow! ...like you'll never use
another.

 

[John Fields]

On Sat, 03 Sep 2011 07:56:00 -0500, "krw@att.bizzzzzzzzzzzz"
<krw@att.bizzzzzzzzzzzz> wrote:

On 3 Sep 2011 06:19:53 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2011-09-03, krw@att.bizzzzzzzzzzzz <krw@att.bizzzzzzzzzzzz> wrote:
On Fri, 02 Sep 2011 19:15:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.

Mea Culpa.

Actually less than a US dollar,

$6.34 in thousands.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM136H-5.0-ND

yeah, if you need the ruggedised one.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM336Z-5.0-ND

96c for singles. the SMT version is slightly cheaper.

He said LM136. That's what I searched on. Still, $1 vs $.004; you pick.

That is, if you're talking about an LM136-5 (that you can actually buy). An
LM286 is an opamp. You've really pulled your pud on this one.

dunno where you got LM286, he wrote LM386 (which is a low power
loudspeaker driver recently discussed in another thread and perhaps
the cause of the error) it's fairly obvoius he meant LM336 (which
is described in the PDF)

Typo. Yes, but John's post was so screwed up there's no telling what he
meant.

---
How so? You can't get around a typo?

It's just a low-power shunt reference to ground being used like a
Zener.
---

but what have you got that's less than
half a penny?

Two 0603 resistors. Two tenths each.

---
Dumb idea if he sets the DAC to output 5V.

Much better idea someone posted earlier:


.. +5
.. |K
.. DAQ [DIODE]
.. +-------+ |
.. | OUT|--[R]--+-->To logic
.. +-------+

The diode's a Schottky and the R is chosen to limit output current
from the DAQ DAC.

--
JF

 

[Michael A. Terrell]

Kaz Kylheku wrote:

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:
if the user does the RIGHT thing and sets the DAC to 5v. So maybe my
question should be: How can I cap a 5-10v signal to 5v?

One of the simplest ways: send the output to ground through a resistor
and a 5V zener diode.

5.1 volt zeners have a soft knee, and rarely do what you described.


--
You can't have a sense of humor, if you have no sense.

 

[Jamie]

krw@att.bizzzzzzzzzzzz wrote:

On Fri, 02 Sep 2011 21:45:37 -0400, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


John Fields wrote:


On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:



On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:



On Thu, 1 Sep 2011 22:27:29 -0700 (PDT), SklettTheNewb
steveklett@gmail.com> wrote:



I'm the "software guy" but am trying to help out on the hardware side
for a small project we have going on. It's fun, I like to learn new
stuff. I'm currently working on a circuit that is connected to a
piece of data acquisition hardware that provides 8 digital bits (1
port) and these 8 bits are wired up to a set of decoder ICs. Great,
that all works fine. The problem is that I need ONE MORE digital bit
to control a SSR - but I don't have any more available digital pins so
I'm going to use one of the analog outputs on the DAQ hardware.

OK, finally my question: The analog out can be software controlled to
output 0 - 10v. If I connect this straight to a logic pins this means
that if someone screws up (people always do) they could fry things by
accidentally sending 10v to a part that is expecting a maximum of 5v.
I would like to add a little circuitry to cap the analog line at a max
of 5v. I have NO IDEA how to do this and was hoping someone here
could suggest a simple solution to cap a signal at a max of 5v.

---
View with a fixed-pitch font:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.


---
Mea Culpa.

Actually less than a US dollar, but what have you got that's less than
half a penny?









10 V
+
|
|
|
1k +
___ |/
IN -|___|----+----|
| |> OUT
| +-----------+
+
|
\ o
\ Jumper Select
\.
o
|
|
.-.
| |
| | 1k
'-'
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

I Like this idea better.


Why bother with the follower? It's slower than crap, and may not work,
without a pull-down (leakage). In any case, it's a useless part.

I guess speed wouldn't matter since I doubt the DAC converter is going

to respond any faster than this circuit can handle..

If would be simple to add a R load if needed.

Jamie

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 03 Sep 2011 11:33:04 -0400, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

Kaz Kylheku wrote:

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:
if the user does the RIGHT thing and sets the DAC to 5v. So maybe my
question should be: How can I cap a 5-10v signal to 5v?

One of the simplest ways: send the output to ground through a resistor
and a 5V zener diode.


5.1 volt zeners have a soft knee, and rarely do what you described.

Right, might just as well use a Schottky clamp to the 5V rail or use the
input's ESD clamp, if available (though I don't do it unless it's suggested in
the spec).

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 03 Sep 2011 12:15:08 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

krw@att.bizzzzzzzzzzzz wrote:
On Fri, 02 Sep 2011 21:45:37 -0400, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


John Fields wrote:


On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:



On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:



On Thu, 1 Sep 2011 22:27:29 -0700 (PDT), SklettTheNewb
steveklett@gmail.com> wrote:



I'm the "software guy" but am trying to help out on the hardware side
for a small project we have going on. It's fun, I like to learn new
stuff. I'm currently working on a circuit that is connected to a
piece of data acquisition hardware that provides 8 digital bits (1
port) and these 8 bits are wired up to a set of decoder ICs. Great,
that all works fine. The problem is that I need ONE MORE digital bit
to control a SSR - but I don't have any more available digital pins so
I'm going to use one of the analog outputs on the DAQ hardware.

OK, finally my question: The analog out can be software controlled to
output 0 - 10v. If I connect this straight to a logic pins this means
that if someone screws up (people always do) they could fry things by
accidentally sending 10v to a part that is expecting a maximum of 5v.
I would like to add a little circuitry to cap the analog line at a max
of 5v. I have NO IDEA how to do this and was hoping someone here
could suggest a simple solution to cap a signal at a max of 5v.

---
View with a fixed-pitch font:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.


---
Mea Culpa.

Actually less than a US dollar, but what have you got that's less than
half a penny?









10 V
+
|
|
|
1k +
___ |/
IN -|___|----+----|
| |> OUT
| +-----------+
+
|
\ o
\ Jumper Select
\.
o
|
|
.-.
| |
| | 1k
'-'
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

I Like this idea better.


Why bother with the follower? It's slower than crap, and may not work,
without a pull-down (leakage). In any case, it's a useless part.

I guess speed wouldn't matter since I doubt the DAC converter is going
to respond any faster than this circuit can handle..

If there is any capacitance at the input, it could hang for some time. There
is nothing but leakage to bleed the charge outta there. Who knows what the
leakage is, or even direction.

If would be simple to add a R load if needed.

Unless we know a lot more about the circuit, the R is needed. Unless it's a
low impedance input, there is no need for the transistor at all. Both are a
bad idea for a TTL type input, though.

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 03 Sep 2011 10:00:53 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 03 Sep 2011 07:56:00 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On 3 Sep 2011 06:19:53 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2011-09-03, krw@att.bizzzzzzzzzzzz <krw@att.bizzzzzzzzzzzz> wrote:
On Fri, 02 Sep 2011 19:15:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.

Mea Culpa.

Actually less than a US dollar,

$6.34 in thousands.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM136H-5.0-ND

yeah, if you need the ruggedised one.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM336Z-5.0-ND

96c for singles. the SMT version is slightly cheaper.

He said LM136. That's what I searched on. Still, $1 vs $.004; you pick.

That is, if you're talking about an LM136-5 (that you can actually buy). An
LM286 is an opamp. You've really pulled your pud on this one.

dunno where you got LM286, he wrote LM386 (which is a low power
loudspeaker driver recently discussed in another thread and perhaps
the cause of the error) it's fairly obvoius he meant LM336 (which
is described in the PDF)

Typo. Yes, but John's post was so screwed up there's no telling what he
meant.

---
How so? You can't get around a typo?

Typo? Sure, but it wasn't a typo.

It's just a low-power shunt reference to ground being used like a
Zener.

Sure, but it's a crap idea. As I said, expensive and an all-around PITA.

---

but what have you got that's less than
half a penny?

Two 0603 resistors. Two tenths each.

---
Dumb idea if he sets the DAC to output 5V.

So? What happens if he sets it to 2.5V? Think!

Much better idea someone posted earlier:


. +5
. |K
. DAQ [DIODE]
. +-------+ |
. | OUT|--[R]--+-->To logic
. +-------+

The diode's a Schottky and the R is chosen to limit output current
from the DAQ DAC.

That's a *great* idea! (I was the first to suggest a Schottky. ;-)

 

[Tim Wescott]

On Sat, 03 Sep 2011 06:02:11 +0000, Kaz Kylheku wrote:

On 2011-09-02, Tim Wescott <tim@seemywebsite.com> wrote:
All the suggestions so far, or a transistor with built-in bias and a
resistor (like this one: http://search.digikey.com/scripts/DkSearch/
dksus.dll?Detail&name=FJY3013RCT-ND). Emitter to ground, base to the
DAC,

Hmm, page says: "Limited quantity available, not recommended for new
design".

OK, not quite like it in exactly that way -- but there's tons of pre-
biased transistors out there to be had.

--
www.wescottdesign.com

 

[John Fields]

On Sat, 03 Sep 2011 12:11:19 -0500, "krw@att.bizzzzzzzzzzzz"
<krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 03 Sep 2011 10:00:53 -0500, John Fields
jfields@austininstruments.com> wrote:

On Sat, 03 Sep 2011 07:56:00 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On 3 Sep 2011 06:19:53 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2011-09-03, krw@att.bizzzzzzzzzzzz <krw@att.bizzzzzzzzzzzz> wrote:
On Fri, 02 Sep 2011 19:15:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.

Mea Culpa.

Actually less than a US dollar,

$6.34 in thousands.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM136H-5.0-ND

yeah, if you need the ruggedised one.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM336Z-5.0-ND

96c for singles. the SMT version is slightly cheaper.

He said LM136. That's what I searched on. Still, $1 vs $.004; you pick.

That is, if you're talking about an LM136-5 (that you can actually buy). An
LM286 is an opamp. You've really pulled your pud on this one.

dunno where you got LM286, he wrote LM386 (which is a low power
loudspeaker driver recently discussed in another thread and perhaps
the cause of the error) it's fairly obvoius he meant LM336 (which
is described in the PDF)

Typo. Yes, but John's post was so screwed up there's no telling what he
meant.

---
How so? You can't get around a typo?

Typo? Sure, but it wasn't a typo.

---
Well, since there aren't any LM386-5.0's around and there are _lots_
of LM336-5.0's out there, I'd have to say it appears you're wrong.
---

It's just a low-power shunt reference to ground being used like a
Zener.

Sure, but it's a crap idea. As I said, expensive and an all-around PITA.

---
It may be a bit expensive, but I can't imagine why you'd think it's an
all-around PITA, especially compared to that crap divider of yours.
---

but what have you got that's less than
half a penny?

Two 0603 resistors. Two tenths each.

---
Dumb idea if he sets the DAC to output 5V.

So? What happens if he sets it to 2.5V? Think!

---
That's even dumber.
---

Much better idea someone posted earlier:


. +5
. |K
. DAQ [DIODE]
. +-------+ |
. | OUT|--[R]--+-->To logic
. +-------+

The diode's a Schottky and the R is chosen to limit output current
from the DAQ DAC.

That's a *great* idea! (I was the first to suggest a Schottky. ;-)

---
But not the first to suggest the circuit, so that's just an
incremental improvement; hardly anything worth dislocating your
shoulder while patting yourself on the back about.

--
JF

 

[Kaz Kylheku]

On 2011-09-03, Michael A. Terrell <mike.terrell@earthlink.net> wrote:

Kaz Kylheku wrote:

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:
if the user does the RIGHT thing and sets the DAC to 5v. So maybe my
question should be: How can I cap a 5-10v signal to 5v?

One of the simplest ways: send the output to ground through a resistor
and a 5V zener diode.


5.1 volt zeners have a soft knee, and rarely do what you described.

I consciously chose this approximation of the Zener because it
fits the application.

The knee behavior is not important, because there is no requirement to
clamp voltage to exactly 5.000 volts.

 

[Michael A. Terrell]

Kaz Kylheku wrote:

On 2011-09-03, Michael A. Terrell <mike.terrell@earthlink.net> wrote:

Kaz Kylheku wrote:

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:
if the user does the RIGHT thing and sets the DAC to 5v. So maybe my
question should be: How can I cap a 5-10v signal to 5v?

One of the simplest ways: send the output to ground through a resistor
and a 5V zener diode.


5.1 volt zeners have a soft knee, and rarely do what you described.

I consciously chose this approximation of the Zener because it
fits the application.

The knee behavior is not important, because there is no requirement to
clamp voltage to exactly 5.000 volts.

A low voltage zener has a lot of other failings, but if you want to
half ass something you can't say that people didn't try to tell wy it
was a bad idea.


--
You can't have a sense of humor, if you have no sense.

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 03 Sep 2011 16:18:22 -0400, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

Kaz Kylheku wrote:

On 2011-09-03, Michael A. Terrell <mike.terrell@earthlink.net> wrote:

Kaz Kylheku wrote:

On 2011-09-02, SklettTheNewb <steveklett@gmail.com> wrote:
if the user does the RIGHT thing and sets the DAC to 5v. So maybe my
question should be: How can I cap a 5-10v signal to 5v?

One of the simplest ways: send the output to ground through a resistor
and a 5V zener diode.


5.1 volt zeners have a soft knee, and rarely do what you described.

I consciously chose this approximation of the Zener because it
fits the application.

The knee behavior is not important, because there is no requirement to
clamp voltage to exactly 5.000 volts.


A low voltage zener has a lot of other failings, but if you want to
half ass something you can't say that people didn't try to tell wy it
was a bad idea.

Right. Zener diodes (as opposed to avalanche diodes - ~6V and up) are really
crappy. Avoid them whenever possible.

In this case, the 5V rail is handy, so unless there are some really strange
mitigating factors, adding any sort of reference diode is dumb.

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 03 Sep 2011 14:13:55 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 03 Sep 2011 12:11:19 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 03 Sep 2011 10:00:53 -0500, John Fields
jfields@austininstruments.com> wrote:

On Sat, 03 Sep 2011 07:56:00 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On 3 Sep 2011 06:19:53 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2011-09-03, krw@att.bizzzzzzzzzzzz <krw@att.bizzzzzzzzzzzz> wrote:
On Fri, 02 Sep 2011 19:15:07 -0500, John Fields
jfields@austininstruments.com> wrote:

On Fri, 02 Sep 2011 09:32:59 -0500, "krw@att.bizzzzzzzzzzzz"
krw@att.bizzzzzzzzzzzz> wrote:

On Fri, 02 Sep 2011 08:49:22 -0500, John Fields
jfields@austininstruments.com> wrote:

DAQ
. +-------+
. | OUT|--[4.7K]--+-->To logic
. +-------+ |
. [LM386-5.0]
. |
. GND

http://www.national.com/ds/LM/LM136-5.0.pdf


Oh, that's good. A $6 solution where a $.004 solution is better and smaller.

Mea Culpa.

Actually less than a US dollar,

$6.34 in thousands.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM136H-5.0-ND

yeah, if you need the ruggedised one.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM336Z-5.0-ND

96c for singles. the SMT version is slightly cheaper.

He said LM136. That's what I searched on. Still, $1 vs $.004; you pick.

That is, if you're talking about an LM136-5 (that you can actually buy). An
LM286 is an opamp. You've really pulled your pud on this one.

dunno where you got LM286, he wrote LM386 (which is a low power
loudspeaker driver recently discussed in another thread and perhaps
the cause of the error) it's fairly obvoius he meant LM336 (which
is described in the PDF)

Typo. Yes, but John's post was so screwed up there's no telling what he
meant.

---
How so? You can't get around a typo?

Typo? Sure, but it wasn't a typo.

---
Well, since there aren't any LM386-5.0's around and there are _lots_
of LM336-5.0's out there, I'd have to say it appears you're wrong.
---

You're getting as good as your pal, AlwaysWrong, here John. It was a brain
fart, sure, but not a typo. Your excuses are instructive, though.

It's just a low-power shunt reference to ground being used like a
Zener.

Sure, but it's a crap idea. As I said, expensive and an all-around PITA.

---
It may be a bit expensive, but I can't imagine why you'd think it's an
all-around PITA, especially compared to that crap divider of yours.
---

I guess now we all know why you don't even call yourself an engineer. You
want to spend over $6 on a half-cent solution. ...and then whine at the
half-cent solution because it's "crap". Very telling.

but what have you got that's less than
half a penny?

Two 0603 resistors. Two tenths each.

---
Dumb idea if he sets the DAC to output 5V.

So? What happens if he sets it to 2.5V? Think!

---
That's even dumber.
---

You're the king of dumb, here, John.

Much better idea someone posted earlier:


. +5
. |K
. DAQ [DIODE]
. +-------+ |
. | OUT|--[R]--+-->To logic
. +-------+

The diode's a Schottky and the R is chosen to limit output current
from the DAQ DAC.

That's a *great* idea! (I was the first to suggest a Schottky. ;-)

---
But not the first to suggest the circuit, so that's just an
incremental improvement;

It's an important improvement. It keeps current out of the ESD diodes (often
important) and smaller Schottkys (and arrays, if need be) can usually be
found. In this case, the resistor divider is the right solution and the OP
agrees.

hardly anything worth dislocating your
shoulder while patting yourself on the back about.

Then why did you pretend it was such a great improvement, in your response to
me? It seems you're the one twisting in the wind, John. Now admit that
you're full of shit, or just skulk away. Your choice.

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 03 Sep 2011 13:32:37 -0500, Tim Wescott <tim@seemywebsite.com> wrote:

On Sat, 03 Sep 2011 06:02:11 +0000, Kaz Kylheku wrote:

On 2011-09-02, Tim Wescott <tim@seemywebsite.com> wrote:
All the suggestions so far, or a transistor with built-in bias and a
resistor (like this one: http://search.digikey.com/scripts/DkSearch/
dksus.dll?Detail&name=FJY3013RCT-ND). Emitter to ground, base to the
DAC,

Hmm, page says: "Limited quantity available, not recommended for new
design".

OK, not quite like it in exactly that way -- but there's tons of pre-
biased transistors out there to be had.

Pre-biased resistors are slick but every time I've considered them, they turn
out to be too expensive or a better way to do things shows up (steal part of
another circuit). It's hard to beat a MMBT3904 and a couple of resistors
(less than two cents).