4/20mA current loop filtering .

Dear All,

I need suggestions for filtering any noise on the current loop from my sensor before it enters my PLC . I know about combination of Ferrite and capacitor at both ends but not sure , PL help .



Rakesh

4-20 ma filtering

What specific noise problem are you trying to filter?

What brand of PLC? A lot of PLC's have built in filters.

Thank you for the reply , I want to filter radiations arising from RF waves from mobile telephones ,and 50-60Hz AC harmonice on my 4-20mA current loop .

Thank you

Ok, First, what do you consider harmonics specifically? Or are you describing bursts of EMI from the de-energizing of inductive loads?

Twisted pair shielded cable that is properly grounded with clean connections, should minimize EMI/RFI signals from the phones. This will be relatively ineffective with a poor grounding scheme. Are the cables with this signal physically routed away from any power type conductors/sources?

Have used ferrite beads with mixed results.

Capacitive coupling of the may be effective.

Either if used improperly, can magnify the problem. Trouble is within high gain signal amplifiers, this may be a current loop, but it still operates on millivolt signal level. Noise level may only be in the mv range.

What brand of PLC is it? Most mfgs. offer some type of filtering on the front end of their analog modules.

I agree that solution is to correct wiring instead of
"fixing" signal. Do you have any evidence of
interference or this is shot in the dark?

Current Loop Wiring.

Notice that there is a shield, a common as well as the 4-20ma control signal.

4-20ma

hi
For noise filteration you should do the following:-
1-use twisted pair cable with shield
2-connect the shield from on side to the earth and let it free on air from the other side.
3- lay the signal cables in a seperate duct or cable tray away from telephone & power cables.
i hope that this could help
Thx

All the above replies are correct. But it is worth remembering that the reason why 4-20mA current loops are so popular and effective is that the 250ohm resistor at the input is a relatively low impedance, and this fact alone makes the loop relatively insensitive to induced noise.

Simply pay attention to the cable sheilding, earthing and routing suggestions made above and there is no need for any extra filtering components at any frequencies below 10kHhz at least.

At the very worst if > 100kHz RFI is shown to be a problem then maybe winding the whole cable for 2-5 turns around a small ferrite toroid would be a help...but I have never seen this to be a problem in any normal industrial site I have been on.

The worst creators of RFI are welding machines, induction heaters, some older types of VSD, and any large DC soleniod, such as an electromagnetic brake on a motor. If any of these are a problem it is much better to reduce the RFI at the source of the problem, rather than trying to sheild every sensitive circuit on site.

All the above replies are correct. But it is worth remembering that the reason why 4-20mA current loops are so popular and effective is that the 250ohm resistor at the input is a relatively low impedance, and this fact alone makes the loop relatively insensitive to induced noise.

Simply pay attention to the cable sheilding, earthing and routing suggestions made above and there is no need for any extra filtering components at any frequencies below 10kHhz at least.

At the very worst if > 100kHz RFI is shown to be a problem then maybe winding the whole cable for 2-5 turns around a small ferrite toroid would be a help...but I have never seen this to be a problem in any normal industrial site I have been on.

The worst creators of RFI are welding machines, induction heaters, some older types of VSD, and any large DC soleniod, such as an electromagnetic brake on a motor. If any of these are a problem it is much better to reduce the RFI at the source of the problem, rather than trying to sheild every sensitive circuit on site.

Dear All,
Thanks a lot for your answers :I will take care with the shielding however it would be difficult to pass the loop from a steel conduit .


PhilipW (others ), Could you please specify me if the shield is tied to one end of the system electrical ground then how much FREQUENCY induction due to Cell phone is expected i.e from its transmitter in GSM band 900Mhz , 1.2 GHz etc . Should I design a low pass LC filter with a cutoff Fc=10KHZ ,As I dont know about where my system could be used ,I should be ready for the worst condition . Any help Most Welcomed .


Thanks

banghead
Please try to be a little less vague with your application, and maybe we can be more specific with the possible solutions. Are you presently experiencing a problem, or trying to protect yourself worst case?

Thanks All ,

I want to take precaution against Cell phone radiations 900Mhz ,1.2GHz , can you suggest me some filter Low Pass for both ends of the loop .

Thanks