Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!henry From: henry@utzoo.UUCP (Henry Spencer) Newsgroups: net.micro Subject: mysterious resistor Message-ID: <4559@utzoo.UUCP> Date: Tue, 30-Oct-84 15:33:08 EST Article-I.D.: utzoo.4559 Posted: Tue Oct 30 15:33:08 1984 Date-Received: Tue, 30-Oct-84 15:33:08 EST Organization: U of Toronto Zoology Lines: 19 I'm looking at a phone-line ring detector from an HP ap note, and am puzzled by one minor feature of it. The circuit is fairly simple: the two input lines each go through a 100k resistor, then one goes through a small capacitor, then they go into an opto-isolator with a small diode connected "in reverse" across its inputs. The 100k's give a high-impedance input and current-limiting, the capacitor blocks DC and provides some more AC impedance, and the diode prevents the opto-isolator from being fried by the reverse half of the AC ringing voltage. The mystery part is, there is a 22 megohm resistor across the "inside" ends of the 100k resistors. Any idea what it's for? Its impedance is two orders of magnitude higher than anything else in the circuit, so I can't imagine it being significant during normal operation. The orthodox use for something like that is as a bleed resistor, but damned if I can figure out why it's there. The capacitor would seem too small (0.02 uF) to store enough charge to be worth bleeding off. Anybody know what gives? -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry