On Thu, 30 Apr 1998 19:35:01 -0700 Mark Devin Newland writes: >David VanHorn wrote: > >> >how about puting a diode in series with the base resistor. It'll >make the >> vbe increase to >> >app.1,4 V. The resistor value mast be recalculated for the current >necesary >> to swith the transistor. >> >> That works, but just adding one resistor is the least expensive >option. It's >> what we use in production. > > Unless you have a noisy environment. Sometimes the filtering is >more than >the diode. A series diode (alone) seems like a bad idea. Any collector-base leakage current would reverse bias the diode and have nowhere to go except through the base-emitter junction, where it would be amplified and cause more undesired collector current. The diode could also rectify RF fields that are present (to a greater extent than just the transistor would) and cause false turn-on. A resistor from base to emitter (ground) is conventionally used and effective at keeping the base voltage low enough that transistor action does not occur from leakage currents. With this resistor in place, the series diode could then also be used to increase the voltage from the logic circuit that is required to turn the transistor on. The two resistors also form a voltage divider so more voltage is required to reach the conduction threshold of the transistor. I've had no problem leaving both out because low PIC outputs are accurately modeled as small resistances to Vss. Unless the internal Vss bus is raised by large currents through other outputs, a single series resistor from the PIC to the transistor will also serve to drain leakage currents away from the transistor when it is supposed to be off. If the circuit has to operate at extreme high temperatures or it is important that the transistor stay off while the PIC is in reset (all pins become inputs then), the resistor to ground should be used. As someone else noted, "digital transistors" which integrate this resistor and the series limiting resistor are available. The ULN2003 type transistor array devices also have integrated resistors. Being Darlington transistors, they also require 2 Vbe voltage to turn on, so direct drive from "not quite zero" TTL outputs is effective. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]