Power inlet with EMI filter: Do the values matter and how to pick the right one? - Page 1
Forum section: electronics / beginners

author: agent_power
author_rank: Regular Contributor
timestamp: September 12, 2022, 07:27:56 pm
content: Hi ForumI have to replace an exploded power inlet with filter capacitors and inductors inside and my favourite supplier doesn't have one with the exact values.How important is the filter in test equipment such as oscilloscopes and does it need to match the one I'm replacing?For example, the original values are 1A 2x2200pf Y, 47nf X2, 2x2,8mH and the replacement in stock is rated 6A and 0,8mH (see attached datasheet).How much does the change in inductance matter for an oscilloscope PSU? I would like to avoid to order the wrong one...Thanks for some hints.

author: srb1954
author_rank: Super Contributor
timestamp: September 12, 2022, 09:56:50 pm
content: Quote from: agent_power on September 12, 2022, 07:27:56 pmYou could use the suggested replacement and probably not observe any problems at all. The specification of the filter is generally not that critical for the operation of a piece of equipment but is more to more to do with the original manufacturer meeting the applicable EMI regulations. If you are not concerned with some EMI noise from your equipment you can even bypass the filter altogether and just use a plain inlet socket. However, since the filter also attenuates incoming noise from the mains, there is a greater risk of transient damage to your equipment if the filter is totally removed.If you do replace the filter on a normal piece of test equipment the main consideration is to get a filter from a name-brand manufacturer, such as Schurter, that has all the requisite safety approvals and an adequate current and voltage rating for the equipment. It is best to go with a filter that doesn't exceed the original current rating by too much. You will note from your datasheet that the higher current versions use smaller and smaller inductors and this significantly degrades the filter attenuation at the lower end of the frequency range. This may mean you get excessive noise from your oscilloscope power supply affecting other equipment connected to the same power source.All of the above is applicable for normal types of mains powered electronic equipment. If you are dealing with medical equipment or safety related equipment then it generally not possible to make substitutions of filter types without special approvals.

author: agent_power
author_rank: Regular Contributor
timestamp: September 13, 2022, 09:36:09 pm
content: Thanks a lot for the explanations! I'll try to get a lower amperage Schurter or equally good brand part then.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2022, 05:47:58 am
content: Considering it's a scope, I would argue about the "operational" aspect. A few mV of background noise on literally everything you measure, can be quite aggravating. Or, maybe you never notice it because you're always doing higher voltage stuff (digital logic, uh, audio amps, power supplies, Idunno?). Likewise for anything nearby -- extra EMI from other, poorly filtered equipment can upset measurements just as well, or introduce noise to analog (audio/video) stuff, or... Like, just last week I had to cycle all the connectors (plus contact cleaner) on my USB hub as I was getting extra (100s mV) noise on the bench here, one or more connectors wasn't grounding properly.But yeah, similar ratings will have similar values. The values shouldn't be critical, +/-30% kind of thing should be fine.Tim

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 14, 2022, 06:49:38 am
content: Generally, you want to use the smallest current rated filter you can for your application.For example, don't use a 10A rated filter for a device that only draws 2A, use a 3A filter instead.Reason being, the higher current part needs thicker wire in the inductors etc, so will often have less turns due to lack of physical space, and hence less filtering capability.As for specific values, it's not a big deal, use what you can get that fits the spot and it'll be fine.