current transducers

Hello! I'm checking into utilizing an AB-Micro Logix's 1500 PLC to monitor the milliampheres current draw from 9vdc,12vdc,& 16vdc while inspecting 12vdc motors. Question #1 is where to start looking for small inexpensive current transducers, (will need approx, 25). What I really would like to do is show the reading on an HMI (Maple unit), for the QA Tech to view. Outside of looking like a fence post, does anyone have any ideas, of the type of input card required to pickup the transducers readings Analog or what?

I picked up a brochure from these guys at a show I went to last week. I haven't tried their products or know the costs but it looks like they have something you could use. They show the outputs as 4-20ma, 0-5V or 0-10V so to answer you question, yeah it's analog.

The micrologix has a 4 point input card for the 1500 line, and can take upto 8 modules I think, so you should be able to read 25 inputs. Its been a couple years since I worked with them tho. Keep in mind no online edits with ML. Sure doent leave alot of spare input for other things tho (the base still will have its in and outs).

I have used a few of the NK tranducers that Greg mentioned and can recommend them...

I don't know that they're 'inexpensive'. You get what you pay for though.

What current range are you looking to measure. I know, milliamps, but how many?

beerchug

-Eric

The milliampheres will be max at 350mil, but it could exceed 500mil, if found bad. I appreicate the input, thanks!

ohm's law..

If the current is DC, then a simple resistor would do the trick.

Measure the voltage drop, and know the current!

The 1769-IF4 has 4 channels, so, you'd need 7 of them to measure
25 channels of data.

You might save some $$ if you multiplex the data from the resistors
through a few signal-quality relays...

Milliamps is hard to measure with most commercial current transducers. jdbrandt is right, a simple resistor or current shunt may be the best.

I have had difficulty finding any isolated commercial current transducer that will measure in the milliamp range. The resistor you use should be small in value to avoid loading the circuit and generating a lot of heat. The signal will be quite small which may require amplification to provide good resolution. (A .1 ohm resistor at 0.5 amps will have a voltage drop of 0.05 volts). If you use a 10 volt input with a 12 bit A/D then the voltage per bit is 10/4096=0.0024 volts. This means a 0.5 amp current will have a raw A/D count of around 20 (0.05/0.0024). The resolution will be very poor.

WARNING! Make sure that each channel is completely isolated from each other. Remember you are measuring a difference in voltage, but the common mode voltage (the voltage of each point with respect to ground) may be quite high. If the measurements share a common ground, then you are shorting one side of the current shunt to ground. The results are spectacular (and expensive!)

With most pass through current transducers you can "multiply" the value by passing the curent carrying leads through the window several times. This is done all the time with C/Ts (Current Transformers) for AC power, for example. If you pass the leads through the transmitter 5 times, your transmitter will see 1.75 to 2.5 amps, and with the right transmitter you should be able to get a range you need.

I'v had good luck with NK transmitters for AC but never used them for DC.

Oh, you want to measure DEE CEE current, huh?

Tom Jenkins said:

I'v had good luck with NK transmitters for AC but never used them for DC.

Click to expand...

Um, yeah, me too...

Seems I failed to see the fact that we were talking DC here...

beerchug

-Eric

I really appreicate the input, Its great to view different opinions oo ways to resolving issues. Have a great (1) I'm off to the races with some great ideas.

I use current transducer all the time and you should know your
requirements to select proper transducer.
Devices with current transformers will only work for AC.
Deviced with hall sensors can work with both AC and DC but have
slow response time. Usually rising edge is in the 0.5sec range
and falling edge can be much worse. Response time also depends
on measured current so if you need precise reading you have to use
timers to get stable reading. If you want fast response just
forget about this type. Nothing stops you from reading the input
as fast as PLC can scan but the readings of current pulses etc
will be very poor.

For example If you have situation where you have:
a) big current (80% of your input range for example - or more),
b) then no current for short period (ca. 1/2sec)
c) and then again you have current but much less (say 25-30% of the
input range),

...you probably won't even see that curent went to zero (case b).

For DC measurements that require fast response your best bet
is using DC/DC converter and in-circuit resistor.
There are nice current shunts for this purpose.
Transducers we normally use are DC/DC 3way isolated converters from
Weidmuller (they can go as fast as 10kHz) but anyone should have
something like that (Carlo Gavazzi, Phoenix Contact...).

BTW. ML1500 is probably very poor choice for this application as well.
You can have max 8 additional modules and with only 4channels per
module you are getting very close to limits of what this PLC can
do using only analogue inputs (7 cards).
If you want to add some outputs to turn those motors on/off, you
will need more outputs than what's integrated into largest ML base.
That would mean one more output card and in this case you are cornered.

For example input setting of 0-150mV and in-circuit resistor
of 0.2 Ohm would give you current range of 0-750mA.
Standard 1/4Watt resistor with 1% tolerance would be adequate.
You can put all your resistors and terminals on small PCB with
DIN-rail mount clip like the ones sold by Weidmuller.