200kHz EMI from BRX units

[Cactuar]

We've used BRX units for a wide variety of tasks around the lab with great success. Recently, we had an experiment involving particularly sensitive electronics and discovered a mysterious noise signal repeating every 5usec (so, 200khz.)

Ruled out the 24V supply and other components. Using a lab supply to provide a clean power rail, we confirmed it was related to the BRX chassis, in this case a BX-DM1E-18ED13-D. Seems that the noise is radiating from the 24V input connector of the PLC, and also coupling onto all IO lines connected (in particular some 4-20mA sensor lines coming from pressure transducers) and being picked up in the experimental system.

I was able to confirm this was present on another DM1E-18ED13-D. Using near-field probes it seems that this is likely to be originating from the flyback transformer used for the DC input isolation circuit; it matches the switching timing on Q3. (We do not have a calibrated reference setup so I cannot comment on the absolute magnitude of signals observed.)

Granted, this is not normally a problem in most cases, but in this particular setup it is quite an issue and we would like to avoid it in the future. Are there any particular recommendations for mitigation of this signal? We can apply ferrites to some extent but for systems of a certain size that may become impractical. Wondering if other chassis models include different power circuitry, for example, or there is some external filtering we can apply to help reduce this effect.

Any suggestions would be appreciated, thanks!

[BobO]

We've used BRX units for a wide variety of tasks around the lab with great success. Recently, we had an experiment involving particularly sensitive electronics and discovered a mysterious noise signal repeating every 5usec (so, 200khz.)

Ruled out the 24V supply and other components. Using a lab supply to provide a clean power rail, we confirmed it was related to the BRX chassis, in this case a BX-DM1E-18ED13-D. Seems that the noise is radiating from the 24V input connector of the PLC, and also coupling onto all IO lines connected (in particular some 4-20mA sensor lines coming from pressure transducers) and being picked up in the experimental system.

I was able to confirm this was present on another DM1E-18ED13-D. Using near-field probes it seems that this is likely to be originating from the flyback transformer used for the DC input isolation circuit; it matches the switching timing on Q3. (We do not have a calibrated reference setup so I cannot comment on the absolute magnitude of signals observed.)

Granted, this is not normally a problem in most cases, but in this particular setup it is quite an issue and we would like to avoid it in the future. Are there any particular recommendations for mitigation of this signal? We can apply ferrites to some extent but for systems of a certain size that may become impractical. Wondering if other chassis models include different power circuitry, for example, or there is some external filtering we can apply to help reduce this effect.

Any suggestions would be appreciated, thanks!

The AC supply topology is a bit different, but not sure it's enough to be helpful.

A line input filter near the terminals might help a bit.

If you didn't care to lose the isolation, we might be able to do a minor mod to bypass the front end. The unit would need to be powered by 12VDC. There is still some 710kHz noise that would come from the 3.3VDC regulator, but it would eliminate the largest source of noise.

[Cactuar]

It'd be a useful option to have in a pinch. Can I inject +12V at J3, or is there additional modification required to do that safely?

[BobO]

It'd be a useful option to have in a pinch. Can I inject +12V at J3, or is there additional modification required to do that safely?

Engineer thinks it should be fine, assuming the input power is disconnected. Obviously this one would be the absolute pinnacle of warranty voiding, but we're pretty helpful regardless. 

[BobO]

Actually, this may not work. We're thinking the CPU will hang because of the EPF detection.

[Cactuar]

Hmm, OK. Thanks for checking! I'll give the other filtering methods a try as well.

[Controls Guy]

Obviously this one would be the absolute pinnacle of warranty voiding, but we're pretty helpful regardless. 

Worse than the one I drilled a hole in to place a temp probe?

BTW, what is this 'warranty' you speak of??