Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!elroy.jpl.nasa.gov!swrinde!mips!pacbell.com!pacbell!osc!jgk From: jgk@osc.COM (Joe Keane) Newsgroups: sci.electronics Subject: Re: IR Reception - Modulation and Quality Summary: Add a choke. Keywords: impedance Message-ID: <4729@osc.COM> Date: 9 Apr 91 18:50:41 GMT References: <2766.27E40773@ofa123.fidonet.org> <159639@felix.UUCP> <160142@felix.UUCP> Reply-To: jgk@osc.COM (Joe Keane) Organization: Versant Object Technology, Menlo Park, CA Lines: 35 In article <160142@felix.UUCP> dennisg@felix.UUCP (Dennis Griesser) writes: >[Thanks again for your response, John. Anybody else care to step up to bat?] OK, i'll take a swing. The problem is that high-frequency signals are coming off the power line into the circuit. The reason they're coming through is that there's a low-impedance path at those frequencies. I don't think that increasing the filter capacitor does much good at blocking these signals. I mean, it probably works with large enough capacitors, but it's not the right way to do it. It increases the peak current through the transformer and rectifier, and also makes your power supply less `friendly' in terms of generating harmonic currents. I think the right solution is to add a series choke (inductor). Chokes are good idea for a power supply, in order to smooth out current flow into the filter capacitor. The problem is that they're expensive and take up space, so people often leave them out. Then you're just relying on the leakage inductance of the transformer, which often isn't very large. It doesn't matter whether you put the choke in the primary or secondary, it just means a different value for inductance and current. Use whatever's cheaper. I don't know how much power your equipment draws, so i can't give exact numbers on what to do. I would say that a choke in the primary would be something like 10 millihenries at 1 ampere, while in the secondary it would be 100 microhenries at 10 amperes. You can get these pretty cheap at some places, or you can just make your own. Again these are just somewhat conservative order-of-magnitude estimates, you'll have to see what works. Note that, especially with a large filter capacitor, the peak current can be a lot higher than the average current if the rectifier only conducts during a small portion of a cycle. The choke should reduce this problem somewhat, but you should make sure the choke is rated for the peak current. -- Joe Keane, amateur electronics hacker jgk@osc.com (...!uunet!stratus!osc!jgk)