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EMI / EMC Design practices

D

Devendra

Jan 1, 1970
0
Hello All,

I am designing a controller based on 24VDC supply voltage. It is a
process indicator based on a Freescale 16 bit controller and
communicates with external devices using CAN or RS485.

I want to subject the product to CE testing.

I just want to know that whether the X and Y suppression capacitors
will be required for a 24VDC powered controller also!? I have put
across the line (X) capacitors as 0.1uF and 0.01uF.

Also whether my circuit ground should be isolated from the chassis
ground, because my unit will be subjected to some tests in which the
transient voltages are applied between supply lines (230Vac, 50Hz) and
Earth (connected to Chassis of my controller).

Kindly let me know your suggestions about standard design practices,
for EMI EMC compliance.

Thanks in advance.

Best Regards,
Devendra Jogdeo
 
E

Eeyore

Jan 1, 1970
0
Devendra said:
I just want to know that whether the X and Y suppression capacitors
will be required for a 24VDC powered controller also!?

X and Y caps are safety ratings for capacitors to be connected to the AC line.
There's no need for any on 24v DC circuits.

I have put across the line (X) capacitors as 0.1uF and 0.01uF.

Where ?

Graham
 
E

Eeyore

Jan 1, 1970
0
Devendra said:
Kindly let me know your suggestions about standard design practices,
for EMI EMC compliance.

You want someone to write a book for you for free ?

Graham
 
P

Paul Mathews

Jan 1, 1970
0
Hello All,

I am designing a controller based on 24VDC supply voltage. It is a
process indicator based on a Freescale 16 bit controller and
communicates with external devices using CAN or RS485.

I want to subject the product to CE testing.

I just want to know that whether the X and Y suppression capacitors
will be required for a 24VDC powered controller also!? I have put
across the line (X) capacitors as 0.1uF and 0.01uF.

Also whether my circuit ground should be isolated from the chassis
ground, because my unit will be subjected to some tests in which the
transient voltages are applied between supply lines (230Vac, 50Hz) and
Earth (connected to Chassis of my controller).

Kindly let me know your suggestions about standard design practices,
for EMI EMC compliance.

Thanks in advance.

Best Regards,
Devendra Jogdeo

Capacitors for EMI reduction are used to provide intentional paths for
parasitic currents which would otherwise cause stronger radoated or
conducted EMI. They are never required, per se, except in the sense
that, if you use capacitors for this purpose and they are conductively
connected across the mains or from mains to protective ground or any
accessible conductive components, then those capacitors must have the
proper safety rating such as X, Y, X2, etc. In some cases, equipment
powered by 'safety' low voltages may still require safety rated
capacitors. A DC/AC inverter is a good example.
As a good starting point for a couple of years of intensive study of
design practices, I recommend Keith Armstrong's 'Compliance Club' EMI
tutorial pages, easily found via web search.
Paul Mathews
 
B

Barry Lennox

Jan 1, 1970
0
Hello All,

I am designing a controller based on 24VDC supply voltage. It is a
process indicator based on a Freescale 16 bit controller and
communicates with external devices using CAN or RS485.

I want to subject the product to CE testing.

I just want to know that whether the X and Y suppression capacitors
will be required for a 24VDC powered controller also!? I have put
across the line (X) capacitors as 0.1uF and 0.01uF.

X and Y caps are not used in 24 V DC PSUs.

You need to get a book. Try "EMC for Product Designers" by Tim
Williams.
 
E

Eeyore

Jan 1, 1970
0
Barry said:
X and Y caps are not used in 24 V DC PSUs.

PSUs ? I'm sure they are used on the mains input filter.

Graham
 
D

Devendra

Jan 1, 1970
0
Regarding X and Y capacitor, I mean to say....

X : Across 24V incoming power supply and
Y: 24VDC line to Earth and Circuit Ground to Earth....

Also Circuit Ground and Earth should be isolated or not?
 
B

Barry Lennox

Jan 1, 1970
0
PSUs ? I'm sure they are used on the mains input filter.

Graham

I'm not about to spend too much time analysing the OPs text, I'm
confused if the input is 24 vdc or 240 vac. But it also appears that
he wants to put them across the 24 v line. It's all too hard.
 
E

Eeyore

Jan 1, 1970
0
Devendra said:
Regarding X and Y capacitor, I mean to say....

X : Across 24V incoming power supply and
Y: 24VDC line to Earth and Circuit Ground to Earth....

It is totally unnecessary to use X and Y parts here. They are not subjected to
mains voltages. Ordinary capacitors would be fine.

More problematic would seem to be that you placed them here in the circuit
without any reason.

Also Circuit Ground and Earth should be isolated or not?

Depends on your design strategy. For good EMC behaviour circuit ground should be
firmly connected to equipment chassis (frame ground) *in many places* in fact.

Graham
 
D

Devendra

Jan 1, 1970
0
Also Circuit Ground and Earth should be isolated or not?
Depends on your design strategy. For good EMC behaviour circuit ground should be
firmly connected to equipment chassis (frame ground) *in many places* in fact.



Thank you very much for your suggestions.

Best Regards,
Devendra
 
M

MooseFET

Jan 1, 1970
0
On Apr 20, 6:01 am, Eeyore <[email protected]>
wrote:
[....]
Depends on your design strategy. For good EMC behaviour circuit ground should be
firmly connected to equipment chassis (frame ground) *in many places* in fact.

I disagree with that suggestion. You never want to provide a path by
which the circuit can directly drive currents through the chassis.
The enclosure should be well connected to its self so that there are
no gaps or bad electrical connections.

I have had to resort to making special mounting hardware to mount a
power supply that connected its internals to the chassis in more than
one place. The chassis had about 1.5Vp-p on it from corner to
corner. When it was bolted into the rack it screwed up everything
else in the rack.
 
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