DC supply noise - instrument interface

DaleP

Member
Join Date
May 2007
Location
Ann Arbor, MI
Posts
10
Hi,
I am trying to interface a 24 Vdc PLC output to a TTL input on an instrument. The 24 Vdc supply connected to the PLC output module is floating. The common on the instrument input is earth grounded.
I am running the DC output and DC common over a 10 ft. cable to 1 Kohm resistor (for current limiting) to the LED of an opto isolator (Motorola 4N26)on a breadboard. The open collector opto isolator output is connected to a 4.7 Kohm pullup resistor and the input of a 7414 Schmitt Trigger inverter. The 7414 output is driving my TTL input. (The Schmitt Trigger was necessary because my transition time was too slow for the TTL input). The opto isolator and 7414 are being supplied by the 5 Vdc supply of the instrument.

My problem is the noise on the PLC output. Every time a solenoid or motor that's being driven by a 24 Vdc PLC output turns on, there is a spike on the DC output that controls the instrument, and I get extra trigger signals. This seems to be true of all the DC modules connected to this supply.

Grounding the PLC DC supply doesn't seem to help.

Any ideas about how to solve this would be appreciated!
 
One question are you saying that any device on another PLC output causes the spike on the output you have the TTL device on?
 
mordred said:
One question are you saying that any device on another PLC output causes the spike on the output you have the TTL device on?
mordred,
Yes, I have a few DC pneumatic valves and a couple small relays that are controlled by the same DC PLC module. The relays each control a small DC motor. The module and the motors all share the same supply. Both solenoids and and motor cause noise on the supply. This is not a problem when running the PLC, but the TTL input is falsely triggered by the noise. I'll try putting a diode across the solenoids as rsdoran suggested, I'm not sure how to limit the noise from the motor.
 
If the diode suppression does not work as expected.

May want to use shielded cable to your TTL device, just ground one end. The shielding may prevent any induction into the signal wire and the shield being grounded at one end will provide a path to remove "noise" etc but not a path for a loop to form.
 
you may try seperating your TTL circuit from the other circuits using Opto isolators. I ran into a similar problem when I was trying to run 6 stepper motors via the expansion slot on a PC with a homebuilt controller
the noise generated from the AC power caused the PC I was using to continually crash until I used the Isolators to seperate the two power sources. If you have a schematic of your system It may help others here to visualize your problem and you will most likely get a more expediant solution. I realize you already have one installed however it should be giving you some form of isolation.
 
Last edited:
You need to be above the noise floor. With your current setup, just a couple of volts is all that's needed to fire the LED in the opto. The proper circuit would be a comparator, with a 10 or 15 volt threshold. Put a Zener in series with the 1K. Try 5.1V, 9.1V, or even 15V. The idea is to switch when you have greater than 15 volts or so, using just the top portion of the 24 volts. That should rule out the noise.
 
You could also connect pin 2 of the 4N26 to +5 or +15 if you have it. That would do the same thing as above. The 1K is giving about 22 mA to the opto. I would prefer about 10 mA. The new circuits will put it about 9. (with 1K, voltage drop=mA).
 
Last edited:
Thanks for your help.

Thanks for the ideas everyone. My circuit seems to be operating pretty reliably now. After a little more investigation with a 'scope I realized 99% of my noise was from the DC motor, with noise spikes as high as +/-30 V, well up into the MHz. I already had the cable shield grounded at one end, and I was already isolating the signal with an opto-isolator. Using a separate supply on the motor totally eliminated the noise, but I'm trying to avoid that extra work and expense. What seemed to help was 0.1 uF cap across the motor terminals, and then a filter at the input of the opto-isolator
optofilter.gif

Now my noise low enough that it doesn't give me false triggers.
Here are some sites I found helpful regarding motor noise.
http://www.superdroidrobots.com/product_info/motor_wiring.htm
http://www.x2y.com/techlib.htm#dcpub
http://www.x2y.com/publications/dcmotors/feb14-05.pdf
Thanks again,
Dale
 

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