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Electronics Forum / Basics / November 2006Logic probe, pulsar and tracer
I am looking to get a set of logic probes and need some recomendations. The only set I see with the current tracer is the old hp 547a. There is a set on ebay for $299.00 which seems high. I mainly need the tracer. Is there another Current tracer out there that might be recomended? Is the HP 545 along with the 546 pulsar and 547a tracer the best three digital probes to get or should I go with something else? I have a radio shack probe and pulsar are these any good for late model pcbs? Thank you >I am looking to get a set of logic probes and need some >recomendations. The only set I see with the current tracer is the old [quoted text clipped - 5 lines] >good for late model pcbs? >Thank you Logic probes aren't very useful. Get a decent oscilloscope, perferably one of the low-end Tek or HP digitals, and a good DVM. John What am I missing here? If you want to test a gate don't you need a logic pulsar to inject the pulse? And if you want to check for a short get don't you use the pulsar to drive the node and the tracer to see the current flow. How do you use a scope and DVM to check a stuck gate and short? I can see a scope help with floating levels but it doesn't point to the right node. A DVM can check voltage levels and shorts between pins doesn't how does it handle: Shorts, Opens within the chip and Shorts and open in the trace and Shorts and opens coming from the other gates. Thank you > Logic probes aren't very useful. Get a decent oscilloscope, perferably > one of the low-end Tek or HP digitals, and a good DVM. > > John >What am I missing here? If you want to test a gate don't you need a >logic pulsar to inject the pulse? Dunno, I don't often test gates in isolation. I build a circuit that's supposed to work, and if it doesn't I observe the signals with a scope and see if they make sense. If I truly want to characterize a gate by itself, I use a real pulse generator and a regular or sampling oscilloscope, depending on speed. >And if you want to check for a short >get don't you use the pulsar to drive the node and the tracer to see >the current flow. How do you use a scope and DVM to check a stuck gate >and short? A decent DVM can measure the voltage drop in traces if a reasonable amount of short-circuit current is flowing. That's usually enough to figure out where a short is. You can also pump current into a short from a power supply, and measure trace drops. But shorts are usually solder bridges, and they're usually visible. >I can see a scope help with floating levels but it doesn't >point to the right node. A DVM can check voltage levels and shorts >between pins doesn't how does it handle: Shorts, Opens within the chip >and Shorts and open in the trace and Shorts and opens coming from the >other gates. HP/Agilent doesn't make logic probes any more; they're not very useful in real-life logic systems. Ditto signature analyzers. Neither provides much real information. *The* fundamental instrument is the oscilloscope. Does anybody here use logic probes? John John Larkin wrote: > *The* fundamental instrument is the oscilloscope. A scope is big. A scope is expensive. A scope requires AC power and finding an undamaged probe. And most importantly of all, a scopre requires taking your eyes of what you are poking around in to look somewhere else. > Does anybody here use logic probes? Unfortunately, no. Radio Shack discontinued theirs, which had much better AUDIO than the others, so these days I'm using a plain old LED most of the time. Yes, I have two scopes next to my desk, but most of the time that LED gets the job done more quikckly and simply. There are of course many things you can't do with a logic probe. But the things they could do they did better than almost everything else. Anyone else ever put a diode on the front of the RS probe to catch edges going just one way? >John Larkin wrote: > >> *The* fundamental instrument is the oscilloscope. > >A scope is big. The nice Tek low-end 100 MHz digitals are about as big as a shoebox. >A scope is expensive. Around $1000 new. That's not expensive. >A scope requires AC power and finding an undamaged probe. The lights on your bench need AC power, too. And you can get a battery-powered scope if you do field work. And fix or replace bad probes! >And most importantly of all, a scopre requires taking your eyes of what >you are poking around in to look somewhere else. True. But that's not hard. >> Does anybody here use logic probes? > >Unfortunately, no. Radio Shack discontinued theirs, which had much >better AUDIO than the others, so these days I'm using a plain old LED >most of the time. Yes, I have two scopes next to my desk, but most of >the time that LED gets the job done more quikckly and simply. A logic probe or an LED convey very little information. In a digital system, stuck-high and stuck-low and "pulsing" don't tell you a lot. It's the fast *patterns* that hold the information. And in an analog system, a logic probe is useless. A scope makes a useful DVM, too, like for checking power supply rails on a board. One cool thing you can do with a scope probe is just wave it around a board without touching anything. You can spot all sorts of interesting stuff. You can also test, and check the polarity of, a diode, or test a transistor, with just a scope. John > I am looking to get a set of logic probes and need some > recomendations. The only set I see with the current tracer is the old [quoted text clipped - 5 lines] > good for late model pcbs? > Thank you It's "pulser", "a thing that pulses", not "pulsar", "pulsating star". :-) Yes, $299.00 is outrageous. I once got a logic probe at Radio Shack for about $9.95; I don't know if they still have them, but it wouldn't be hard to build one. A pulser is just an oscillator, maybe with selectable output levels/logic families/etc - this is also easy to build. A current probe is just an inductive pickup - I envision a small ferrite core with a tiny gap and a bunch of turns of wire, much like a tape head. But if you don't want to build, just keep shopping around. Good Luck! Rich >> I am looking to get a set of logic probes and need some >> recomendations. The only set I see with the current tracer is the old [quoted text clipped - 20 lines] >Good Luck! >Rich B&K makes logic probes for about $40. If you realy want a logic probe. John >> I am looking to get a set of logic probes and need some >> recomendations. The only set I see with the current tracer is the old [quoted text clipped - 15 lines] > A current probe is just an inductive pickup - I envision a small ferrite > core with a tiny gap and a bunch of turns of wire, much like a tape head. Take the head from the drum of a junk VCR, and add a handle and BNC cable: one current probe good to well past 5MHz Bye. Jasen |
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