Hello, I am trying to interface with a device that outputs 24V when
off and switches to 0v when commanded on. The problem is, I need 0v
when off and 24V when commanded. I tried to use an opamp as a
comparator to swap the voltages but have not been successful. Is
there a circuit I can make or is there a device that exists to do what
I require? Thanks a million.

On Tue, 14 Aug 2007 20:12:04 -0700, robert.mcd wrote:

> Hello, I am trying to interface with a device that outputs 24V when
> off and switches to 0v when commanded on. The problem is, I need 0v
> when off and 24V when commanded. I tried to use an opamp as a
> comparator to swap the voltages but have not been successful. Is
> there a circuit I can make or is there a device that exists to do what
> I require? Thanks a million.

What sort of "device"? How often does the output change, and how much
current do you need to draw?

On Tue, 14 Aug 2007 20:12:04 -0700, robert.mcd@gmail.com wrote:

>Hello, I am trying to interface with a device that outputs 24V when
>off and switches to 0v when commanded on. The problem is, I need 0v
>when off and 24V when commanded. I tried to use an opamp as a
>comparator to swap the voltages but have not been successful. Is
>there a circuit I can make or is there a device that exists to do what
>I require? Thanks a million.

Nobody asks the relevant questions.

But if you are talking signal with not much current and on-off switch
a simple NPN transistor switch will invert the signal for you.

Emitter to ground, base connected to your signal with a current
limiting resistor in series, collector load resistor pulls the voltage
to 24 volts when the transistor is off (24 volts not present on base
resistor) and 0 volts on collector (when 24 VDC is present).
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On Aug 15, 7:00 am, default <defa...@defaulter.net> wrote:
> On Tue, 14 Aug 2007 20:12:04 -0700, robert....@gmail.com wrote:
> >Hello, I am trying to interface with a device that outputs 24V when
> >off and switches to 0v when commanded on. The problem is, I need 0v
> >when off and 24V when commanded. I tried to use an opamp as a
> >comparator to swap the voltages but have not been successful. Is
> >there a circuit I can make or is there a device that exists to do what
> >I require? Thanks a million.
>
> Nobody asks the relevant questions.
>
> But if you are talking signal with not much current and on-off switch
> a simple NPN transistor switch will invert the signal for you.
>
> Emitter to ground, base connected to your signal with a current
> limiting resistor in series, collector load resistor pulls the voltage
> to 24 volts when the transistor is off (24 volts not present on base
> resistor) and 0 volts on collector (when 24 VDC is present).
> --



Thanks, I will give this a go.

On Aug 15, 6:07 am, robert....@gmail.com wrote:
> On Aug 15, 7:00 am, default <defa...@defaulter.net> wrote:
>
>
>
>
>
> > On Tue, 14 Aug 2007 20:12:04 -0700, robert....@gmail.com wrote:
> > >Hello, I am trying to interface with a device that outputs 24V when
> > >off and switches to 0v when commanded on. The problem is, I need 0v
> > >when off and 24V when commanded. I tried to use an opamp as a
> > >comparator to swap the voltages but have not been successful. Is
> > >there a circuit I can make or is there a device that exists to do what
> > >I require? Thanks a million.
>
> > Nobody asks the relevant questions.
>
> > But if you are talking signal with not much current and on-off switch
> > a simple NPN transistor switch will invert the signal for you.
>
> > Emitter to ground, base connected to your signal with a current
> > limiting resistor in series, collector load resistor pulls the voltage
> > to 24 volts when the transistor is off (24 volts not present on base
> > resistor) and 0 volts on collector (when 24 VDC is present).
> > --
>
> Thanks, I will give this a go.- Hide quoted text -
>
> - Show quoted text -

Also, consider inverting the signal at the input of your device. I
can't tell you which way would work better though, because you gave us
so little information.

On Tue, 14 Aug 2007 20:12:04 -0700, robert.mcd@gmail.com wrote:

>Hello, I am trying to interface with a device that outputs 24V when
>off and switches to 0v when commanded on. The problem is, I need 0v
>when off and 24V when commanded. I tried to use an opamp as a
>comparator to swap the voltages but have not been successful. Is
>there a circuit I can make or is there a device that exists to do what
>I require? Thanks a million.

Single-ended or differental? Are you able to access the device
circuit and power supplies? Or must you connect to it, without
modifying the device? When it provides 24V, what impedance and
current compliance does it have? When it outputs 0V, is this at a
high impedance (effectively open, if single-ended?) What supplies do
you have for your own circuit? What are you then driving with your
output?

Jon

Since inquiring minds want to know, I will try to get more detailed.
The device I am trying to interface with is a PLC. On this PLC, I am
using two DO's, that at the off state, output 24VDC, max 2A. When a
DO is switched on, the output drops to 0V by connecting to an earth
ground in the PLC. I am inputting to an actuator controller that
requires the off state to be 0V and the on state to be 24VDC with max .
25A. So the signals are backwards. When the PLC is at 24V, the
controller needs 0V (at rest); then I flip a switch to create a move,
and the PLC is at 0V but the controller required 24V(move signal). I
hope this did not confuse everyone.

robert.mcd@gmail.com wrote:
> Since inquiring minds want to know, I will try to get more detailed.
> The device I am trying to interface with is a PLC. On this PLC, I am
> using two DO's, that at the off state, output 24VDC, max 2A. When a
> DO is switched on, the output drops to 0V by connecting to an earth
> ground in the PLC. I am inputting to an actuator controller that
> requires the off state to be 0V and the on state to be 24VDC with max .
> 25A. So the signals are backwards. When the PLC is at 24V, the
> controller needs 0V (at rest); then I flip a switch to create a move,
> and the PLC is at 0V but the controller required 24V(move signal). I
> hope this did not confuse everyone.

I think you need something like this solid state relay. and
you will have to mount it on a heat sink surface for it to
carry 25 amps.

http://www.newark.com/jsp/displayProduc ... YDOM+D1D40

Connect the control input between the +24 volt supply and
the PLC DO. Connect the output between the +24 volt supply
and the load.

> I think you need something like this solid state relay. and
> you will have to mount it on a heat sink surface for it to
> carry 25 amps.
>
> http://www.newark.com/jsp/displayProduc ... KNC-G100...
>
> Connect the control input between the +24 volt supply and
> the PLC DO. Connect the output between the +24 volt supply
> and the load.


Sorry, that was .25A or 1/4A. Word wrap

robert.mcd@gmail.com wrote:
>> I think you need something like this solid state relay. and
>> you will have to mount it on a heat sink surface for it to
>> carry 25 amps.
>>
>> http://www.newark.com/jsp/displayProduc ... KNC-G100...
>>
>> Connect the control input between the +24 volt supply and
>> the PLC DO. Connect the output between the +24 volt supply
>> and the load.
>
>
> Sorry, that was .25A or 1/4A. Word wrap

Well, that's a horse of a different color.

Try:
http://www.crydom.com//userResources/pr ... io_odc.pdf
plugged into this:
http://rocky.digikey.com/WebLib/Crydom/ ... s/MS-4.jpg
These are the mini I/O modules and screw terminal plug board
on this catalog page:
http://dkc3.digikey.com/PDF/T072/P1879.pdf

Use a cmos flywheel rectifier, parallel capacitors, and damp the
discharge to ground to generate the desired Amps (the cmos flywheel
element lends itself to steep enough rise times to use the ground
state in this manner. (It's more or less what a "ready bit" does to
regulate a computers phase steps... (or maybe I'm completely wrong)

On Aug 15, 10:24 pm, robert....@gmail.com wrote:
> Since inquiring minds want to know, I will try to get more detailed.
> The device I am trying to interface with is a PLC. On this PLC, I am
> using two DO's, that at the off state, output 24VDC, max 2A. When a
> DO is switched on, the output drops to 0V by connecting to an earth
> ground in the PLC. I am inputting to an actuator controller that
> requires the off state to be 0V and the on state to be 24VDC with max .
> 25A. So the signals are backwards. When the PLC is at 24V, the
> controller needs 0V (at rest); then I flip a switch to create a move,
> and the PLC is at 0V but the controller required 24V(move signal). I
> hope this did not confuse everyone.

On Wed, 15 Aug 2007 19:24:26 -0700, robert.mcd@gmail.com wrote:

>Since inquiring minds want to know, I will try to get more detailed.
>The device I am trying to interface with is a PLC. On this PLC, I am
>using two DO's, that at the off state, output 24VDC, max 2A. When a
>DO is switched on, the output drops to 0V by connecting to an earth
>ground in the PLC. I am inputting to an actuator controller that
>requires the off state to be 0V and the on state to be 24VDC with max
>.25A. So the signals are backwards. When the PLC is at 24V, the
>controller needs 0V (at rest); then I flip a switch to create a move,
>and the PLC is at 0V but the controller required 24V(move signal). I
>hope this did not confuse everyone.

I could guess, but how fast are these signals required to be? What
kind of capacitance are you driving? (Do you need active high and
active low drive? Or can you accept passive drive on one side of
this?) Have you tried 'default's suggestion to you?

You still haven't mentioned what your supplies are, but may I assume
you have access to the 24V and ground rails? Any other rails?

If you want inversion and active high and active low drive at fairly
good rates of change in both directions, you may need 3 BJTs.
Something like this: (read with fixed-spaced font such as Courier,
Courier New, etc.)

>: +24V +24V +24V +24V
>: | | | |
>: | | | |
>: | \ _|_ D1 |
>: | / R3 /_\ 1N4148 |
>: | \ 10k | |
>: | / | |
>: \ | | |
>: / R4 | | |<e Q1
>: \ 1.8k +----------+--------| PNP
>: / | |\c
>: | | |
>: | | |
>: | |/c Q3 |
>: +----+-------| NPN |
>: | | |>e |
>: | | | |
>: D2 --- --- C1 | |
>: /z\ --- 10n | +--> OUT
>: 6.2V --- | | |
>: zener | | | |
>: | | \ |
>: | | / R1 |
>: gnd gnd \ 1k |
>: / |
>: | |
>: | |
>: | R2 |/c Q2
>: IN >-----------------+-------/\/\--------| NPN
>: 4.7k |>e
>: |
>: |
>: |
>: gnd

I assumed about 5mA being enough drive for your .25A output. Easy to
adjust by changing R1 and R2. (R1 sees a little more than 5V and R2
sees most of the 24V. So figure from that.) The above is a pretty
standard construction of an active BJT inverter, so nothing special
here. You need to make sure that Q1 and Q2 can handle the 24V and
current requirements (not too hard to do.)

I've omitted various speed-ups (across R1, R2, and from IN to the base
of Q1), but in the small numbers of kHz it should look pretty good on
a scope. There is an overlapping period when both Q1 and Q2 are on
and that overlap period can be significantly shortened with the speed
ups added. But it may be just fine, as above.

I'm a hobbyist and don't have much experience with 24V industrial PLCs
or actuator controllers and the supplies you are likely to have. So
keep that in mind.

Jon