Analog signals passing thru a relay

flyers

Member
Join Date
Sep 2006
Location
Amk
Posts
295
Hi,

In one of our projects, they client is requesting us to design a system where, 4-20mA is coming from PLC (Analog output module), into another panel. The panel has selector switch to switch between PLC or LOCAL control. When switch to "PLC", the 4-20mA signal will pass thru a relay contact and send to the VFD.

My concern is when the 4-20mA (or 0-10V) pass thru a relay, will there be any problem? Signal drops, instability, etc...?

Thanks, guys.
 
Generaly, try to avoid using analog signals with relays.
You will have some transition proccesses.
Due to switchover time of a relay, maybe output device will detect a signal loss.
If you want to multiplex analog signals, use multi-input AIN cards then use software mutiplexing in PLC rather.
Best regards.
 
Do not allow open circuit. Can you use a selector with shorting (Make-Before-Break) contacts? Prefer Gold or Gold over Silver contacts. C&K.
If you must use relays, use 2. Turn the other one on first, then drop out the first one. Low current, bifuricated contacts. Silver is okay. Potter Brumfield (P&B Tyco)
4-20mA driver circuit can handle the temporary overload.
 
Last edited:
Hi,

What do you mean by this?:confused:
Best example is a current transformer. You NEVER open the secondary on a CT (unless you KNOW system is completely deenergized). CT MUST always be connected to a load.
So when switching a CT secondary you must shut switch 2 (dummy load) first THEN open switch 1 (meter).
Commonly called make before break. Used where CTs are used to dervive a current signal one CT per phase. On switch gear the ammeter can be selected for phase A or B or C.

So to get to the 4 to 20 mA signal - the 4 20 source may not handle an open circuit. You are connected to PLC and when switch to "other" the other contact shuts before the PLC contact opens to avoid open circuit.

Dan Bentler
 
So you guys want to put two outputs together for a short time.
Please do not try this at home.
Just switch over the signal when it is 4-20 mA.
When possible put the manual also as input to the PLC this way you can do the switch in software.
When switching volts, the resistance of the relais could be a problem, better to switch a current signal.
( I use the voltage signal as switch but that is because i have a small meter switching over between many outputs.
 
So you guys want to put two outputs together for a short time.
Please do not try this at home.
Just switch over the signal when it is 4-20 mA.
Low current signals with low current devices is still the same. Semiconductor devices blow when you go open circuit under a load.

Crank up your stereo about half volume, then unhook the speakers.
Output devices will go shorted. However, if you connect a second set of speakers first, then disconnect the first, nothing will happen.

The other option would be to install a minimum load, say 1K, but that would put an offset (non-linear?) on the 4-20mA loop.
 
humm been there trying that and get very bad results after a year while it was working fine at first.
A relay need some current passing thru to clean the contacts....4-20mA should work better for you (I did try with voltage) but it is like installing a loose connection in the loop...
Make sure to go with a silver or gold contact relay !

Like said, you would need to short with a 1000 ohm each source signal
before opening it's circuit...You can do it with few expensive small relay or with early make#break gold contact design...
Must of the time the 4-20mA source won't blow if it stay open for 5microsecondes so just using the NC side of each source relay connected to 500-1000 ohms load resistor should make it...(Commons connected to source and NO connected to the VFD (2 relay, not 1 using NO#NC)
Make sure you set the VFD speed ramp or input filtering to get rid of the noise created with the open circuit.
Make sure both 4-20mA source are isolated or refferenced at the same common 0v

In both case you are making a weak point in the chain....The first place to look at when something goes wrong with this equipement...I would rather enter the local 4-20mA to your plc and choose if you copy it to your 4-20mA output or you use your plc logic to control it...
 
Last edited:
We have a similar setup to this and do it with a master slave kind of set up, the 4-20ma comes from a S7-300 and is fed into a Siemens Logo analog input, then fed to the output of the logo unit. this is enabled using a standard input to the logo via a selector, when we want to change to local control we flick a switch, this then changes the logic internally and disables the external input from the s7-300 and enables intrenal logig which is directly connected to the analog output the same as the s7-300 then the control is then done with a Pot locally.

To sum up, we have two analog inputs selectable by a switch and one output.

Regards

John
 
Last edited:
Most of the time the 4-20mA source won't blow if it stay open for 5 microsecond...
You can leave it open for 5 uS or days. That won't hurt it. The initial spike during disconnect is the problem.

Use Tyco Potter Brumfield R10 series. Contact type Z.
Bifuricated contacts. These have wiping action, and are designed for low current.
 
I second Mrslipmat. Use a cheap programmable relay to avoid all the pitfalls of relays etc and this will give you a reliable system
 
Hi,

In one of our projects, they client is requesting us to design a system where, 4-20mA is coming from PLC (Analog output module), into another panel. The panel has selector switch to switch between PLC or LOCAL control. When switch to "PLC", the 4-20mA signal will pass thru a relay contact and send to the VFD.

My concern is when the 4-20mA (or 0-10V) pass thru a relay, will there be any problem? Signal drops, instability, etc...?

Thanks, guys.


What make of VFD?. Some VFDs allow 2 speed references and two start/stop commands determined by an input.

We do this all the time with PowerFlex 40s/70s. Hand - OFF - Auto switch, in HAND (input to VFD) VFD starts and speed reference is via keypad, in AUTO (Input to VFD), start/stop command is via PLC as is speed reference.
 
Dear guys,

Really appreciate all the info given based on you guy's experience. The VFD is Danfoss.

Due to many reason of unreliable with this design, the client will go with other way - to adjust the speed at the keypad of the VFD, no switching relay of 4-20mA is required. To safe all the hassle.

Thanks again.
 

Similar Topics

I'm looking for some opinions here. Typically for analog signals, we use a twisted pair wire with a drain shield. However, am I correct in...
Replies
5
Views
2,147
Using RSLogix 5000 version 20.05 with a 1769-IF8 analog card set to 4ma-20ms which is set to engineering units with a 10hz filter. I am wondering...
Replies
4
Views
2,987
I would like to enquire for a product as a solution for our on-going project. We need to transfer some analogue signals (QTY:08 / 4-20mA ) from...
Replies
11
Views
3,780
We worked in a sewage treatment plant and take care of their plc work. They have an issue with noise some of the analog signals in the plant. When...
Replies
19
Views
6,936
For the first time I have a load of analog signals being transferred into my Control Logix system via Modbus. I have sent analog to a DCS via...
Replies
1
Views
2,896
Back
Top Bottom