henry@utzoo.UUCP (Henry Spencer) (10/30/84)
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!henryrpw3@redwood.UUCP (Rob Warnock) (11/03/84)
+--------------- | ...The mystery part is, there is a 22 megohm resistor across | the "inside" ends of the 100k resistors. Any idea what it's for? +--------------- Just a guess... in case the ring detector gets connected to a long telephone line that is NOT connected at the other end (the C.O.), the 22 meg resistor will prevent static charge buildup which could eventually puncture the .01 capacitor (which has only a finite breakdown voltage). It may also be (and this is more likely) to protect the whole thing during packing and shipping (in plastic wrappings). I have done similar things designing interface cards (controllers) which had MOS pins straight on the bus (and no TTL) -- put a very high resistor between the pin and +5 (which is == 0 during shipping) to prevent static buildup during shipping and handling. Rob Warnock UUCP: {ihnp4,ucbvax!amd}!fortune!redwood!rpw3 DDD: (415)572-2607 Envoy: rob.warnock/kingfisher USPS: 510 Trinidad Ln, Foster City, CA 94404