simple DC current sensing

R

Rob Votin

Guest
I am trying to find a simple way to detect the presence of a DC
current. Its an industrial application where I need a PLC to be able
to know if current is flowing to several 24 volt electromagnetic
brakes (about 1 or 2 amps each).

Cost is not really an issue, but ideally this would be some small off
the shelf device (inductive or hall effect?) that could directly send
a signal to 24 volt PLC input card. I dont want to use shunts, and I
dont want to have to add an analog input card to the PLC rack.
Everything I've found on the web is either for AC only or puts out an
analog signal proportional to the current. I just want a simple on or
off. Any ideas? Thanks...

Rob
 
If you can afford a 2 volt drop in the power line to the load, just add a
'module' consisting of 3 power diode in series ,paralleled with an
optocoupler an 10 r current limiting resistor.
Feed the opto transistor to the PLC as a 'current to load' indicator.
have used this setup for years, rock stable,small,cheap,etc. made my own
load indicating SSRs as well......

hth
jay
 
"Rob Votin" <votin@verizon.net> wrote in message
news:bcugn09u33b6894tpqg834u0rai0t4hjaq@4ax.com...
I am trying to find a simple way to detect the presence of a DC
current. Its an industrial application where I need a PLC to be able
to know if current is flowing to several 24 volt electromagnetic
brakes (about 1 or 2 amps each).

Cost is not really an issue, but ideally this would be some small off
the shelf device (inductive or hall effect?) that could directly send
a signal to 24 volt PLC input card. I dont want to use shunts, and I
dont want to have to add an analog input card to the PLC rack.
Everything I've found on the web is either for AC only or puts out an
analog signal proportional to the current. I just want a simple on or
off. Any ideas? Thanks...

Rob
Click to expand...
I received some samples not to long ago for a hall IC device. It will take
up to 50A DC, and gives a linear 0-5V DC output isolated to something like
3KV. I don't remember the name, but I can get it when I return to the office
on Monday if your interested. I received like four samples, they sell for
somewhere around $8 if I remember right.
 
On Fri, 22 Oct 2004 03:02:29 GMT, Rob Votin <votin@verizon.net> wrote:

I am trying to find a simple way to detect the presence of a DC
current. Its an industrial application where I need a PLC to be able
to know if current is flowing to several 24 volt electromagnetic
brakes (about 1 or 2 amps each).

Cost is not really an issue, but ideally this would be some small off
the shelf device (inductive or hall effect?) that could directly send
a signal to 24 volt PLC input card. I dont want to use shunts, and I
dont want to have to add an analog input card to the PLC rack.
Everything I've found on the web is either for AC only or puts out an
analog signal proportional to the current. I just want a simple on or
off. Any ideas? Thanks...
Click to expand...
You already have a signal; the voltage on the solenoid coil. Perhaps
you think the signal connection for this is too long, and you're
worried about open connections to the solenoid itself, or the sensing
voltage connection.

I'm not sure how you envision this (these) device(s) fitting into your
wiring. It would seem to make more sense if the PLC solenoid driver
card could sense this directly, without external interference.

Over-current and under-current sensors are available from most relay
mfrs. These have all of the problems of getting accurate and
repeatable results from any electromagnetic actuator, unless they
include other powered circuitry for measurement and control of the
terminals. Clare offers app notes on winding your own reed relays,
that could produce the lowest series voltage drop.

A diode drop is sufficient to generate a 24V signal from a bipolar
transistor that current is flowing in the solenoid wire, using the
same voltage that powers the solenoid to get the voltage swing that
the PLC input module expects. Two diode drops would be enough to power
an opto isolator, three; an fet, four; a conventional 5V relay coil.

Having to make an assembly of these sensors, with contacts, is
something that the use of the PLC and its available modules is
intended to avoid in the first place.

If you actually want to know that the solenoid has done its job,
you're talking motion sensors, actuators or auxiliary switches built
into the solenoid assembly.

RL
 
Hi Rob,

Maybe you could have a chat with CR Magnetics. This is one version that
also comes in 4-20mA:

http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR5210

Or this one:

http://www.rke.com/dccurr.htm

Otherwise search Google for a "current sensing relay".

Regards, Joerg

http://www.analogconsultants.com
 
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