Identification of mains filter choke - Page 1
Forum section: electronics / repair

author: Strand17
author_rank: Contributor
timestamp: August 21, 2020, 05:55:37 pm
content: Hello,Can someone help me find an equivalent (or just the relevant specification) of a mains filter choke used in an audio preamplifier from the early 1990's?The choke is labeled AT4043/91A OPO22 and is used before a 47nF capacitor for mains filtering.I do not have the hardware available, only the schematics, part list and a few low resolution photos.Thanks in advance!

author: bob91343
author_rank: Super Contributor
timestamp: August 21, 2020, 07:48:30 pm
content: It depends on the purpose of the choke. It may be there to filter incoming noise or to attenuate outgoing noise. If it's audio, maybe it's for RFI.Once you know that you can see what will filter what you want. You can then decide whether to use air or iron or ferrite core, etc.

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2020, 07:52:58 pm
content: Quote from: Strand17 on August 21, 2020, 05:55:37 pmThe low res photos (and the relevant portion of the schematic) would certainly help us to understand the type of choke .

author: Strand17
author_rank: Contributor
timestamp: August 21, 2020, 10:06:19 pm
content: Thanks for the quick responses!Attached are the schematic and the most useful picture. The choke is the L01 in the schematic and it can be seen in the left hand side of the picture.

author: Gyro
author_rank: Super Contributor
timestamp: August 21, 2020, 10:20:20 pm
content: It looks like a standard split bobbin common mode choke. Unless you can find a reference for the actual manufacturer's part number, it's impossible to tell the value. These things are not critical, but given that the supply current is low, it is probably in the 2x10mH to 2x20mH range. As the last part of the part number is "OPO22", it might even be a 2x22mH part.As long as you get something with a rating that exceeds the maximum 160mA supply current and fits the footprint then it should be fine.Common mode chokes don't often fail, but if you don't have physical access to it then maybe you are just trying to duplicate the circuit.

author: bob91343
author_rank: Super Contributor
timestamp: August 22, 2020, 04:42:41 am
content: It's obviously a common mode choke. The fuse I think is 750 mA so the choke would be wound with wire around 22 AWG. As mentioned, it's not critical so get all the inductance you can in the available space with sufficient current rating.

author: srb1954
author_rank: Super Contributor
timestamp: August 22, 2020, 05:41:54 am
content: It is a standard common-mode choke made by Philips. It has a high inductance for such a small choke and must be wound with quite fine wire. Consequently the current rating is only 250mA.See the attached datasheet.

author: Strand17
author_rank: Contributor
timestamp: August 22, 2020, 08:58:39 am
content: Thanks a lot! This is exactly the information I was looking for. This forum is an amazing source of help and knowledge!To be more clear - I am trying to replicate the circuit, or rather the functionality of the circuit. Changing footprint is not an issue but understanding more about the function and performance of the original circuit is important. Information about the choke is very helpful indeed.A follow up question, how does this input filter compare to a filter like for instance a FN9244B-1-06 from Schaffner EMC Inc.?https://www.digikey.de/products/en?keywords=817-1593-NThe inductance appears to be in the same ballpark but presumably the original filter mainly dampens common mode disturbances while the FN9244B-1-06 is probably more effective for asymmetrical disturbances(?).Again, many thanks!