Converter: 24V from PLC to TTL devices
jodleren
I have a 6-input device, which will be connected to a PLC with 24V
outputs. My circuit uses 74HC and one 74LS, so I need to convert my
inputs to 5V. My device will consume 40mA at max.
Are there any special (and cheap) devices for this?
Or are optocouplers the best?
Then again a 10Kohm and 39 Kohm can do it, with e.g a diode for
protection.
WBR
sonnich
Nico Coesel
Use a resistor, a 4.7V zener and a 100nf capacitor (capacitor parallel
to the zener). Schmidt trigger inputs are advisable (74hc14).
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------
Jan Panteltje
<son...@hot.ee> wrote in
<d4bda3c2-afb0-4404...@q14g2000vbf.googlegroups.com>:
optoAndormeda deep space network CRC error
best.
Spehro Pefhany
wrote:
>Hi all
>
>I have a 6-input device, which will be connected to a PLC with 24V
>outputs. My circuit uses 74HC and one 74LS, so I need to convert my
>inputs to 5V. My device will consume 40mA at max.
>
>Are there any special (and cheap) devices for this?
>.. are optocouplers the best?
Yes.
John Fields
wrote:
>Hi all
---
View with a fixed-pitch font:
+5V
|
[10k]
|
PLCC>--[<SHOTTKY]--+--[HC/LS]
---
JF
Raveninghorde
<spef...@interlogDOTyou.knowwhat> wrote:
>On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren <son...@hot.ee>
>wrote:
>
>>Hi all
>>
>>I have a 6-input device, which will be connected to a PLC with 24V
>>outputs. My circuit uses 74HC and one 74LS, so I need to convert my
>>inputs to 5V. My device will consume 40mA at max.
>>
>>Are there any special (and cheap) devices for this?
>
>
>>.. are optocouplers the best?
>Yes.
Not much point in an optocoupler if there is a common ground.
Spehro Pefhany
<raveninghorde@invalid> wrote:
>On Fri, 25 Feb 2011 11:17:26 -0500, Spehro Pefhany
><spef...@interlogDOTyou.knowwhat> wrote:
>
>>On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren <son...@hot.ee>
>>wrote:
>>
>>>Hi all
>>>
>>>I have a 6-input device, which will be connected to a PLC with 24V
>>>outputs. My circuit uses 74HC and one 74LS, so I need to convert my
>>>inputs to 5V. My device will consume 40mA at max.
>>>
>>>Are there any special (and cheap) devices for this?
>>
>>
>>>.. are optocouplers the best?
>>Yes.
>
>Not much point in an optocoupler if there is a common ground.
In environments where PLCs are frequently used, there's no such thing.
upsid...@downunder.com
>On Fri, 25 Feb 2011 11:17:26 -0500, Spehro Pefhany
><spef...@interlogDOTyou.knowwhat> wrote:
>
>>On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren <son...@hot.ee>
>>wrote:
>>
>>>Hi all
>>>
>>>I have a 6-input device, which will be connected to a PLC with 24V
>>>outputs. My circuit uses 74HC and one 74LS, so I need to convert my
>>>inputs to 5V. My device will consume 40mA at max.
>>>
>>>Are there any special (and cheap) devices for this?
>>
>>
>>>.. are optocouplers the best?
>>Yes.
>
>Not much point in an optocoupler if there is a common ground.
"Common ground" is just an approximation in most complex electrical
systems.
In the OP's case, things might work well if the TTL side is truly
floating (battery powered or isolated power supply) and there are no
other connections from the TTL side.
For instance, connecting the "common ground" inside the TTL system (or
in a third device connected to the TTL system) to the (mains) ground
can cause various currents, such as relay coil return currents flowing
through the TTL system ground plane, causing voltage drops in the
ground plane, reducing the noise margin _inside_ the TTL system.
Even if each subsystem is well designed with appropriate grounding
arrangements and the whole system is designed with proper grounding
hierarchy,things can still go wrong. An outsourced, not so well
trained installer, will note that the drawing requires separate
grounding wires to the specified grounding points, will also notice
that there are several adjacent screw terminals with "ground" and just
use short jumpers between them, destroying the grounding hierarchy:-(.
In industrial control systems, in which PLCs are typically use, you
should use optoisolators at the TTL systems, not just to protect the
actual input pin, but also keeping any unwanted ground currents out of
your TTL system ground plane.
jodleren
> On Fri, 25 Feb 2011 20:30:36 +0000, Raveninghorde
>
>
>
> <raveninghorde@invalid> wrote:
> >On Fri, 25 Feb 2011 11:17:26 -0500, Spehro Pefhany
>
> >>On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren <sonn...@hot.ee>
Thanks for all the comments.
I have common ground, as I plan to feed my item with the same power -
that either by 7805 (max 0,8W) or a 24/5 swich power block. Either
way, I can get common ground.
I think that the solution with a Zener is the one I will go for.
John Fields
wrote:
>I have common ground, as I plan to feed my item with the same power -
>that either by 7805 (max 0,8W) or a 24/5 swich power block. Either
>way, I can get common ground.
>I think that the solution with a Zener is the one I will go for.
---
What did you find wrong with my resistor/Schottky diode solution?
---
JF
ChrisQ
>
> ---
> What did you find wrong with my resistor/Schottky diode solution?
>
> ---
> JF
Nothing and that's the way I would do it. Cheaper than a zener clamp and
works for any input voltage up to the diode breakdown voltage.
Why make it more complicated than it needs to be etc ?...
Regards,
Chris
Jan Panteltje
<me...@devnull.com> wrote in <xxuap.26691$To7....@newsfe12.ams2>:
Optos are the best solution.
Then the diode solution has the disadvantage that it can lift the supply,
and you need 2 diodes to protect against negative (for example mains shorts) too.
The zener limits negative, AND positive at or just below the supply voltage.
Zeners (or transzorbs) have a higher capacitance, and reduce some transients.
but really use an opto, ground faults will else get you sooner or later in a big way
in industrial equipment.
Also the opto gives a very good isolation for any incoming RF.
jodleren
> On Sat, 26 Feb 2011 04:19:41 -0800 (PST), jodleren <sonn...@hot.ee>
> wrote:
>
> >I have common ground, as I plan to feed my item with the same power -
> >that either by 7805 (max 0,8W) or a 24/5 swich power block. Either
> >way, I can get common ground.
> >I think that the solution with a Zener is the one I will go for.
>
> ---
> What did you find wrong with my resistor/Schottky diode solution?
That I did not understand it at first - I am thinking of that now, due
to the font here :) Later when I noticed that and copied into notepad,
I could see it.
It is simple and probably cheaper too, so I will try that.
Sonnich
John Fields
wrote:
>On Feb 26, 4:42 pm, John Fields <jfie...@austininstruments.com> wrote:
---
Thank you, :-)
---
JF
ChrisQ
Optos are a good solution for a very noisy environment, or where they
may be ground loops, but the op has already said that this is not the
case. You could also take a look at isolation devices like the Si Labs
Si8420/50/60 series devices, which i'm currently looking at to build a jtag
isolator for a debug adapter.
btw, John's solution has the diode facing *outwards*, with a pullup at
the other side, so doesn't lift the supply at all up to the diode breakdown
voltage...
Regards,
Chris
John Fields
<pNaonSt...@yahoo.com> wrote:
>On a sunny day (Sun, 27 Feb 2011 15:59:24 +0000) it happened ChrisQ
><me...@devnull.com> wrote in <xxuap.26691$To7....@newsfe12.ams2>:
>
>>John Fields wrote:
>>
>>>
>>> ---
>>> What did you find wrong with my resistor/Schottky diode solution?
>>>
>>> ---
>>> JF
>>
>>Nothing and that's the way I would do it. Cheaper than a zener clamp and
>>works for any input voltage up to the diode breakdown voltage.
>>
>>Why make it more complicated than it needs to be etc ?...
>>
>>Regards,
>>
>>Chris
>
>Optos are the best solution.
---
Depends on the application and the rules.
For instance, if you're looking for simplicity and there's a common
ground between the driving and driven systems, (as the OP said there
was) then an opto would probably be overkill and the minimalist
circuit I posted would be ideal.
---
>Then the diode solution has the disadvantage that it can lift the supply,
---
How so?
As I see it, that would require the blocking diode to be blown
shorted, the PLC's output(s) to be stiff enough to move logic Vcc
high, and to do it through the logic's ESD diodes.
Not at all likely, in my view.
---
>and you need 2 diodes to protect against negative (for example mains shorts) too.
---
If the PLC outputs are driven by a supply where the mains are
rectified, filtered, and regulated, and that output is used to
generate the 24VDC outputs of the PLC, I fail to see how those outputs
could go negative with a mains short fault.
Perhaps you could alleviate my confusion with an explanation of what
I've missed?
---
>The zener limits negative, AND positive at or just below the supply voltage.
---
State how the input voltage to the series resistor/Zener interface
could go negative and maybe we'll talk.
An ASCIImatic (thanks, Steve :)) stating your position would be nice.
---
>Zeners (or transzorbs) have a higher capacitance, and reduce some transients.
---
Used as clamps, if they exhibit parallel capacitance, then they should
reduce _all_ transients to some extent, wouldn't you agree?
---
>but really use an opto, ground faults will else get you sooner or later in a big way
>in industrial equipment.
---
Pure conjecture.
---
>Also the opto gives a very good isolation for any incoming RF.
---
How much?
---
JF
Jan Panteltje
<jfi...@austininstruments.com> wrote in
<p8enm651m5qg3b5dg...@4ax.com>:
I was talking about the common case of somebody wiring the mains
to an input designed for 24 V.
That happens in industrial, sure there are cases where one wires it all up
onself, and even then... I remember the case where a technician connected the 24V
PLC supply to the 230 V AC, several thousand dollars of damage, basically
all new cards in the Siemens PLC.
In industrial ANYTHING can happen and does.
Sometimes the cable numbering is wrong, I remember a technician coming to
me with " This design does not work'.
What it turned out to be was that he was using the wrong signal cables :-)
I mean those went through the whole factory, he lost track of the right numbers somewhere.
So opto is usually safer, especially as you have huge ground differences,
sometimes when big machines start up etc.
upsid...@downunder.com
>Jan Panteltje wrote:
>> On a sunny day (Sun, 27 Feb 2011 15:59:24 +0000) it happened ChrisQ
>> <me...@devnull.com> wrote in <xxuap.26691$To7....@newsfe12.ams2>:
>>
>>> John Fields wrote:
>>>
>>>> ---
>>>> What did you find wrong with my resistor/Schottky diode solution?
>>>>
>>>> ---
>>>> JF
>>> Nothing and that's the way I would do it. Cheaper than a zener clamp and
>>> works for any input voltage up to the diode breakdown voltage.
>>>
>>> Why make it more complicated than it needs to be etc ?...
>>>
>>> Regards,
>>>
>>> Chris
>>
>> Optos are the best solution.
>> Then the diode solution has the disadvantage that it can lift the supply,
>> and you need 2 diodes to protect against negative (for example mains shorts) too.
>> The zener limits negative, AND positive at or just below the supply voltage.
>> Zeners (or transzorbs) have a higher capacitance, and reduce some transients.
>> but really use an opto, ground faults will else get you sooner or later in a big way
>> in industrial equipment.
>> Also the opto gives a very good isolation for any incoming RF.
>
>Optos are a good solution for a very noisy environment,
i.e. typical industrial environment.
>or where they may be ground loops, but the op has already said that this is not the
>case.
Why would anyone believe this ?
Tim Wescott
> On Fri, 25 Feb 2011 20:30:36 +0000, Raveninghorde
> <raveninghorde@invalid> wrote:
>
>> On Fri, 25 Feb 2011 11:17:26 -0500, Spehro Pefhany
>> <spef...@interlogDOTyou.knowwhat> wrote:
>>
>>> On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren<son...@hot.ee>
>>> wrote:
>>>
>>>> Hi all
>>>>
>>>> I have a 6-input device, which will be connected to a PLC with 24V
>>>> outputs. My circuit uses 74HC and one 74LS, so I need to convert my
>>>> inputs to 5V. My device will consume 40mA at max.
>>>>
>>>> Are there any special (and cheap) devices for this?
>>>
>>>
>>>> .. are optocouplers the best?
>>> Yes.
>>
>> Not much point in an optocoupler if there is a common ground.
>
> In environments where PLCs are frequently used, there's no such thing.
Aw man, you're letting reality intrude.
Unless it's on a lab bench -- use an opto.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Raveninghorde
wrote:
>On 02/25/2011 12:56 PM, Spehro Pefhany wrote:
>> On Fri, 25 Feb 2011 20:30:36 +0000, Raveninghorde
>> <raveninghorde@invalid> wrote:
>>
>>> On Fri, 25 Feb 2011 11:17:26 -0500, Spehro Pefhany
>>> <spef...@interlogDOTyou.knowwhat> wrote:
>>>
>>>> On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren<son...@hot.ee>
>>>> wrote:
>>>>
>>>>> Hi all
>>>>>
>>>>> I have a 6-input device, which will be connected to a PLC with 24V
>>>>> outputs. My circuit uses 74HC and one 74LS, so I need to convert my
>>>>> inputs to 5V. My device will consume 40mA at max.
>>>>>
>>>>> Are there any special (and cheap) devices for this?
>>>>
>>>>
>>>>> .. are optocouplers the best?
>>>> Yes.
>>>
>>> Not much point in an optocoupler if there is a common ground.
>>
>> In environments where PLCs are frequently used, there's no such thing.
>
>Aw man, you're letting reality intrude.
>
>Unless it's on a lab bench -- use an opto.
I make a series of a board for a customer, both opto and non opto
versions. They wire the grounds of both sides of the opto together.
They can't see the point of running more wires than necessary. The
non opto version actually works better.
Forget the theory, how will it be wired?
John Fields
<pNaonSt...@yahoo.com> wrote:
---
Ah, then, no wonder I was confused.
I was talking about a simple way of interfacing two circuits with
common grounds, while it appears you were, and are, talking about how
the Peter Principle can manifest itself in a blue-collar world,
something totally unrelated!
---
>So opto is usually safer, especially as you have huge ground differences,
>sometimes when big machines start up etc.
---
in applications where the isolation is required, of course an opto is
a good choice, but when it isn't, it isn't.
---
JF
Jan Panteltje
<jfi...@austininstruments.com> wrote in
<n41qm69f5p0ec4670...@4ax.com>:
>>What it turned out to be was that he was using the wrong signal cables :-)
>>I mean those went through the whole factory, he lost track of the right numbers somewhere.
>
>---
>Ah, then, no wonder I was confused.
>
>I was talking about a simple way of interfacing two circuits with
>common grounds, while it appears you were, and are, talking about how
>the Peter Principle can manifest itself in a blue-collar world,
>something totally unrelated!
>---
>
>>So opto is usually safer, especially as you have huge ground differences,
>>sometimes when big machines start up etc.
>
>---
>in applications where the isolation is required, of course an opto is
>a good choice, but when it isn't, it isn't.
An important point to notice, is that in industrial,
and he was talking ->PLC<-, one day you may install something the way
you want, correct for the current situation,
but then a while later things get moved around, somebody else
will place your device hundreds of meters away, with different
cables and a differnet ground, this happens.
Modifications are quite common.
upsid...@downunder.com
<jfi...@austininstruments.com> wrote:
>
>I was talking about a simple way of interfacing two circuits with
>common grounds, while it appears you were, and are, talking about how
>the Peter Principle can manifest itself in a blue-collar world,
>something totally unrelated!
Unless you are designing a one off project and you are prepared to go
to the other end of the world to install it, you really have to design
your device in such a way that a hardly literate local worker can
install it.
Spehro Pefhany
A trip to another city, or even across town, can pay for an awful lot
of prevention. And if you assume illiterate idiots, you won't be too
far off a significant portion of the time. That portion can cost you
all your profit as well all your reputation.
John Fields
---
Yes, of course, and when the modifications are done they should be
done by someone who has the competence, and documentation, to do it
correctly instead of by some illiterate grunt who can't tell the
difference between 240VAC and 24VDC.
My solution fulfilled the OP's request for something simpler than an
opto, and used under the conditions he specified should function
satisfactorily and present no problems.
Of course setting up straw men is your prerogative, but it has
precious little to do with the OP's query.
---
JF
Jan Panteltje
<jfi...@austininstruments.com> wrote in
<gjgqm6lbk64j8ra9u...@4ax.com>:
>Of course setting up straw men is your prerogative, but it has
>precious little to do with the OP's query.
I thought John Larkin was your sparring partner?
John Fields
---
It's hardly within your bailiwick to comment on how I should prosecute
a design, but in this instance I'm not designing a product.
This is USENET, and I was posting advice to a querant who wanted an
answer to a specific question.
He was afforded that answer, and it should be one of a number of
solutions to the problem he described and, arguably, the least
expensive.
Whether or not he chooses to use my solution is his decision to make,
of course, but I'd be interested in your critique of my design bearing
in mind the OP's request.
---
JF
John Fields
---
When you argue like he does you also become fair game.
---
JF
Jim Thompson
<jfi...@austininstruments.com> wrote:
>On Tue, 01 Mar 2011 19:38:22 +0200, upsid...@downunder.com wrote:
>
>>On Tue, 01 Mar 2011 09:14:23 -0600, John Fields
>><jfi...@austininstruments.com> wrote:
>>
>>>
>>>I was talking about a simple way of interfacing two circuits with
>>>common grounds, while it appears you were, and are, talking about how
>>>the Peter Principle can manifest itself in a blue-collar world,
>>>something totally unrelated!
>>
>>Unless you are designing a one off project and you are prepared to go
>>to the other end of the world to install it, you really have to design
>>your device in such a way that a hardly literate local worker can
>>install it.
>
>---
>It's hardly within your bailiwick to comment on how I should prosecute
>a design, but in this instance I'm not designing a product.
>
>This is USENET, and I was posting advice to a querant who wanted an
>answer to a specific question.
"querant" ?? What is that, a gay ant ?:-)
>
>He was afforded that answer, and it should be one of a number of
>solutions to the problem he described and, arguably, the least
>expensive.
>
>Whether or not he chooses to use my solution is his decision to make,
>of course, but I'd be interested in your critique of my design bearing
>in mind the OP's request.
>
>
>---
>JF
upsid...@downunder.com is rapidly approaching killfile status.
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Remember: Once you go over the hill, you pick up speed
Spehro Pefhany
<To-Email-Use-Th...@On-My-Web-Site.com> wrote:
>On Tue, 01 Mar 2011 13:53:32 -0600, John Fields
><jfi...@austininstruments.com> wrote:
>
>>On Tue, 01 Mar 2011 19:38:22 +0200, upsid...@downunder.com wrote:
>>
>>>On Tue, 01 Mar 2011 09:14:23 -0600, John Fields
>>><jfi...@austininstruments.com> wrote:
>>>
>>>>
>>>>I was talking about a simple way of interfacing two circuits with
>>>>common grounds, while it appears you were, and are, talking about how
>>>>the Peter Principle can manifest itself in a blue-collar world,
>>>>something totally unrelated!
>>>
>>>Unless you are designing a one off project and you are prepared to go
>>>to the other end of the world to install it, you really have to design
>>>your device in such a way that a hardly literate local worker can
>>>install it.
>>
>>---
>>It's hardly within your bailiwick to comment on how I should prosecute
>>a design, but in this instance I'm not designing a product.
>>
>>This is USENET, and I was posting advice to a querant who wanted an
>>answer to a specific question.
>
>"querant" ?? What is that, a gay ant ?:-)
His uncle's lesbian sister?
Jim Thompson
Bwahahahaha ha! ROTFLMAO!
Michael A. Terrell
Or his dad's lesbian brother?
--
You can't fix stupid. You can't even put a Band-Aid™ on it, because it's
Teflon coated.
John Fields
---
Why?
---
JF
Jim Thompson
Fred Bloggs
wrote:
> On Fri, 25 Feb 2011 03:56:14 -0800 (PST), jodleren <sonn...@hot.ee>
> wrote:
>
> >Hi all
>
> >I have a 6-input device, which will be connected to a PLC with 24V
> >outputs. My circuit uses 74HC and one 74LS, so I need to convert my
> >inputs to 5V. My device will consume 40mA at max.
>
> >Are there any special (and cheap) devices for this?
>
> >Then again a 10Kohm and 39 Kohm can do it, with e.g a diode for
> >protection.
>
> >WBR
> >sonnich
>
> ---
> View with a fixed-pitch font:
>
> +5V
> |
> [10k]
> |
> PLCC>--[<SHOTTKY]--+--[HC/LS]
>
> ---
> JF
If the OP is concerned about 24VDC outputs then chances are his PLC is
a simple 'source only' output which pulls the output to the 24 supply
for activation and shuts off with no pulldown for deactivation. In
such a case the diode circuit will not work very well.
Please view in a fixed-width font such as Courier.
.
.
.
.
. Logic supply
.
. +5V
. -----
. |
. /
. \ 1K
. /
. \
. | _____
. +------> INPUT
. |
. 10K c
. 24VDC PLC 1/4W |/
. SOURCE OUTPUT >--+--/\/\--+-------+----b-| NPN
. | | | |\
. / / --- e
. \ 560 \ 1.2K / \ |
. / 2W / 1/4W --- |
. \ \ |1N4148 |
. | | | |
. | | | |
. <--+--------+-------+--------+------>
.
. COMMON
.
.
.
.
.
He can juice things up with some nickel-dime low impedance stuff for
both pulldown and PLC line crosscoupling attenuation and also some
attenuation at the input for speedup and logic switching around mid-
swing of the PLC output transition- no filtering or Schmitt triggers
needed in most cases. The 50mA loading is winging it- it's up to the
OP to find out what he's working at a more detailed level.