dsl@portia.Stanford.EDU (Dave Lauben) (07/20/89)
Hello netlanders! I need to protect a differential discrete logic receiver
(54LS32) against fault overvoltage of +/- 32 volts (continuous).
I am not a circuit designer by trade, but this task has fallen to me. My plan
is to use two zeners, but I can't decide which of two configurations would be
more appropriate.
Can anybody point out the relative merits of the two configurations?
My concern is not only how well each would work but also reliability and
failure mode effects...
____ ____
--/\/\/\/--o--------|+ | --/\/\/\/--o----o---|+ |
| | | | | | |
,---' | 5 | | | | 5 |
/ \ | 4 | | | | 4 |
--- | L | ,---' --- | L |
| | S | / \ \ / | S |
--- | 3 | --- ,---' | 3 |
\ / | 2 | | | | 2 |
,---' | | | | | |
| | | | | | |
--/\/\/\/--o--------|- | --/\/\/\/--o----o---|- |
---- ----
Config. 1 Config. 2
If Config. 1 is preferrable, are back-to-back Zeners in a single package the
best way to implement it? How 'bout TRANSORBs -- can they handle continous
overvoltage? Thanks in advance for replys...
-Dave
lauben@star.stanford.edubill@videovax.tv.Tek.com (William K. McFadden) (07/22/89)
In article <3780@portia.Stanford.EDU> dsl@portia.Stanford.EDU (Dave Lauben) writes: >Hello netlanders! I need to protect a differential discrete logic receiver >(54LS32) against fault overvoltage of +/- 32 volts (continuous). >Can anybody point out the relative merits of the two configurations? >My concern is not only how well each would work but also reliability and >failure mode effects... > > ____ ____ > --/\/\/\/--o--------|+ | --/\/\/\/--o----o---|+ | > | | | | | | | > ,---' | 5 | | | | 5 | > / \ | 4 | | | | 4 | > --- | L | ,---' --- | L | > | | S | / \ \ / | S | > --- | 3 | --- ,---' | 3 | > \ / | 2 | | | | 2 | > ,---' | | | | | | > | | | | | | | > --/\/\/\/--o--------|- | --/\/\/\/--o----o---|- | > ---- ---- > > Config. 1 Config. 2 > Config. 2 won't work because zeners behave like normal diodes when forward biased. Config. 1 will work as long as you don't exceed the zener's ratings. The resistors, if chosen appropriately, will limit current through the zener. >If Config. 1 is preferrable, are back-to-back Zeners in a single package the >best way to implement it? How 'bout TRANSORBs -- can they handle continous >overvoltage? As far as I know, transorbs are no different than back-to-back zeners except they can dissipate a lot more power. BTW my data book shows a 54LS32 is a quad 2-input NOR gate. You probably don't want to use the zeners anyway. Since what you're really trying to do is protect the inputs of the line receiver, the best way to do this is with a pair of diodes on each input to clamp it to the supply rail: +5V | _|_ +------------- ^ | --- | R | | -----/\/\/\-------+----| input | | _|_ | ^ | --- +------------- | | GND Do this on each input. The diodes can be switching diodes or rectifiers, depending on the size of R and the response time needed. (Switching diodes are a lot faster, but can't handle as much current, but rectifiers are probably too slow to effectively protect the input. If needed, you could put a rectifier in parallel with a switcher to get the best of both worlds. Also, shottky diodes are available that have a much lower forward voltage drop.). The way the circuit works is that when the input voltage exceeds the supply voltage or goes negative, one of the diodes turns on and keeps the input from going more than a diode drop beyond the rails. In your application, R = 32 volts / If_max. (If_max is the maximum forward diode current from the diode spec sheet). BTW, the IC will have similar diodes built-in, but these can't handle much current. Good luck. -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 UUCP: bill@videovax.Tek.com, {hplabs,uw-beaver,decvax}!tektronix!videovax!bill GTE: (503) 627-6920 "The biggest difference between developing a missle component and a toy is the 'cost constraint.'" -- John Anderson, Engineer, TI