Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rochester!pt!speech1.cs.cmu.edu!phd From: phd@speech1.cs.cmu.edu (Paul Dietz) Newsgroups: sci.electronics Subject: Re: TTL Questions Message-ID: <1009@speech1.cs.cmu.edu> Date: Mon, 20-Jul-87 11:37:48 EDT Article-I.D.: speech1.1009 Posted: Mon Jul 20 11:37:48 1987 Date-Received: Tue, 21-Jul-87 04:46:45 EDT References: <1395@crash.CTS.COM> <426@bucket.UUCP> Distribution: na Organization: Carnegie-Mellon University, CS/RI Lines: 51 Keywords: No, no, no, no, no. In article <426@bucket.UUCP> chrisb@bucket.UUCP (Chris Bradley) writes: >In article <1395@crash.CTS.COM> rpluth@pnet01.CTS.COM (Ron Pluth) writes: >> 1. What's the easiest way to go about interfacing a TTL >>output (from, say, a standard S or LS AND gate) to a relay, for >>controlling anything from automobile (12VDC) to house (120VAC) > >Well, it all depends on how much currentthe coil in the relay needs to >activate. I'll assume that your digital output is around 5 volts, and ^^^^^^^^^^^^^^ TTL only guarentees 2.4V! Granted, it is generally higher than this, but it is significantly less than 5V when drawing any current. >that your relay coil needs to support at least 3 amps. If you can, use ^^^^ Don't you mean the contacts? >relay (Cat. # 275-214 - $4.69). Then, you need some sort of transistor >that will support (E) 5 / (R) 150 = (I) 33.3 Ma. The transistor doesn't ^^^^^^^^^^^^^^^^^^^^^ 33.3 mA for a 5 volt relay! This seems too low by a factor of 3! Maybe this is a "typical" spec, but I sure would be shocked if this worked in general! (For that matter, is this a 5V relay?) >have to be very powerful in terms of wattage, so we'll use the NPN transistor >(Cat. # 276-2030 - $.79). Now, connect the collector at +12V. Connect the >emitter to one of the coil leads on the relay. Connect the remaining lead >to ground. Now, connect the base to the output wire of your digital logic >circuit... This circuit is loaded with problems!. First of all, the voltage applied to the coil when "on" can easily be below 2 volts! This is because the emitter can't go higher than about .6 volts above the base. In practice, you might typically get between 2 and 3 volts. Hardly enough to drive most 5 volt relays. Next problem is the current. TTL has almost no sourcing capability. I don't have the spec.s for you transistor, but I doubt beta is much bigger than 100 at even 33 mA. Thus, your poor little gate would have to provide .33 mA. This is definitely pushing TTL and LSTTL which are only rated at .4 mA. Given the other fudges, you're well beyond that. Finally, you provide no protection to the poor little NPN when you shut off the current in the coil. (This works just like the ignition coil in a car: you shut off the current, and the voltage shoots way up!) A free wheeling diode will solve this problem. As I said in my post, there are about a zillion ways of solving this problem. But this is not one of them... Paul H. Dietz Carnegie Mellon University