What kind of signals get attenuated at "iron" core transformer ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: August 06, 2014, 10:05:04 am
content: I mean the common transformer like those used at step-down from mains line (50/60 Hz) AC -> low volt AC used in linear DC power supply.Do noise or voltage spikes get attenuated such as AC motor spikes or high freq noise from other switcher connected at the mains line ? May be others ?Aware what a line filter is and it's purpose, its just I'm curious whether an iron core transformer naturally has some filtering capability for high frequency signals.

author: David Hess
author_rank: Super Contributor
timestamp: August 06, 2014, 12:22:43 pm
content: Higher frequencies do get attenuated by a couple of different means including leakage inductance but iron core transformers also have additional eddy current losses because the core material is conductive which is why they are made from individual insulated laminations. For lower loss at higher frequencies like you would find in switching power supplies, powdered iron is used.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2014, 05:08:03 pm
content: Bank wound (split bobbin) transformers typically poop out above a few kHz. The attenuation depends on source and load impedance, and transformer materials and construction, but it should still vary between -6 to -12dB/octave past there.If that's sufficient for your purposes, then yes, it does filtering.Surges will still come through, to some extent, though limited in current by the transformer's resistance, and limited in energy by its flux (time * voltage). An 8/20us pulse might come through quite strongly, whereas a 10/1000us pulse probably won't look much longer because much of the amplitude disappears due to saturation. A 5/50ns pulse probably won't even show up, given its fast speed.Split bobbin transformers also have relatively low capacitance from primary to secondary, which helps reduce common mode interference. Do beware that common mode noise into or out of the attached circuit is still a concern! One transformer won't magically cure all ills.Tim

author: BravoV
author_rank: Super Contributor
timestamp: August 07, 2014, 01:55:37 am
content: Thanks for the replies.Ok, maybe I didn't put it clearly, I'm in the middle of helping a friend to build a simple mcu based + minor analog processing circuit that uses a small AC step down iron transformer to power the circuit in DC, very basic and down to earth linear power supply.It will be placed inside a really-really tight space in a enclosure box and I just can not use larger one, and currently there is no more space for more components like line filter, even a diy one.This circuit will be used in a machinist workshop which is full of high power AC motors like drill/welding/lathe and etc. My worry is will the heavy noise mains line affect my circuit ? Also I can not trouble shoot at the field like probing the voltage rails with a scope to see how it affects my circuit, as it will be sent hundreds km away.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 02:42:36 am
content: Yeah, I'd be more worried about what connects to it than what happens at the transformer side (beyond a TVS protection diode or something simple like that).Where you're more likele to have problems is with long cables and grounded equipment, especially if multiple grounds connect to the same device. Often the best solution for such issues is outright isolation, but filtering (and ESD / surge protection as applicable) can prove very handy.Tim

author: planet12
author_rank: Regular Contributor
timestamp: August 07, 2014, 05:54:45 am
content: Also consider what will happen when the mains supply droops for a time, which may well happen during big machine start ups. Give yourself a bit of headroom with your DC filter caps and regulator drop-out voltages.