Power supplies need to have low impedance. But in the frequency range where the ferrite bead has high impedance the power supply distribution on the circuit board controls the impedance seen by the load, not the output impedance of the power supply itself. Tom Dagostino Teraspeed Consulting Group LLC Device Modeling Division 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@xxxxxxxxxxxxx <mailto:tom@xxxxxxxxxxxxx> Teraspeed Consulting Group LLC 2926 SE Yamhill St. Portland, OR 97214 http://www.teraspeed.com -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Chris Chalmers Sent: Monday, March 01, 2004 1:53 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Ferrite Bricks on power Supplies Gents, I have seen on certain power supply circuits the use of ferrite bricks. These have been put on the end of power supply before it goes on to the plane of a PCB. I.E Unregulated power supply-> regulated supply-> output caps-> ferrite brick.. The impedance of these bricks at 100MHz is around 30ohms. These are to control and absorb HF current draws from the supply changing them in to heat and reducing the emissions. I was always under the impression that the power supply impedance should be as low as possible across a broad range of frequencies. All that you are doing by adding the ferrite brick is adding inductance. What is the real answer? Best Regards Chris ********************************************************************** This communication contains information which is confidential and may also be privileged. It is for the exclusive use of the intended recipient(s). Please note that any unauthorised distribution, copying or use of this communication, or the information in it, is strictly prohibited. If you have received this communication in error, please notify us by email, by telephone: +44 1383 828800, or fax: +44 1383 828801, then delete the email and any copies of it. This communication is from CRL Opto Ltd., whose registered office is at 1 St David's Business Park, Dalgety Bay, Dunfermline, KY11 9PF, Scotland. This footnote also confirms that this email message has been checked for the presence of computer viruses. ********************************************************************** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu