HD trouble?
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From: Pegasus [MVP] on 17 Dec 2009 05:14 "Paul" <nospam(a)needed.com> schrieb im Newsbeitrag news:hgcvqo$msq$1(a)aioe.org... > Pegasus [MVP] wrote: >> >> >> The multimeter test might destroy the RAM chip, like the tap on the head >> with the hammer. I find it difficult to think of a more inappropriate >> test for a RAM chip. Remember - it contains a hundred million or more >> transistor gates operating at extremely low currents, less than the most >> sensitive multimeter can detect! > > You can safely test silicon devices, if you use a multimeter with > "low power ohms" setting. It applies a voltage not intended to > cause forward conduction in the silicon chips. > > If you lost the manual for your multimeter, you can verify this by using > two multimeters. One multimeter set to "ohms". The second multimeter > set to "volts". The "volts" multimeter will show you a rough idea > of the open circuit voltage that the "ohms" meter is putting out. > (Connect red to red, black to black.) > > Test all the "ohms" ranges on the meter, to understand which > ranges are "high power" and "low power". If an ohms range doesn't > put out more than 1 volt open circuit, then it should be relatively > safe to use on just about anything. For the highest resistance > ranges, the results need some interpretation (since the voltmeter > has a finite input impedance of its own) > > The $100 The $20 > meter meter > > Range Ohmmeter Voltmeter Assumed Confidence > Reads Reads Power type > > Beep Infinity 2.66V High > Diode infinity 2.67V High High on purpose, for diode > test > 200 Infinity 1.01V Low > 2K infinity 1.01V Low > 20K Infinity 0.48V Low > 200K infinity 0.43V Low > 2M "0.995" 0.23V Low OK, see interpretation > 20M "0.99" 0.04V Low > 2000M "001" 0.27V ??? suspicious > > (Note - a separate set of tests were done, and the "ohms" multimeter > never applied more than 1 milliamp of current to the test leads. Typical > silicon clamp diodes are rated for about 10 milliamps. So there is > no danger from the level of current flow either.) > > In the 2 megohm test case, the "volts" multimeter appears to > have only a 1 megohm input impedance. Half of the open circuit > voltage is across the "volts" multimeter, and half is across > the constant current source inside the meter. We could conclude > from that, that the actual open circuit voltage applied > by the multimeter, is 2 * 0.23 volts or 0.46V. And that is suitable > for low power ohms. So you actually have to stare at the display > on both devices to understand what is going on. > > My suspicion is, the "insulation test" range on my multimeter, is > actually high power ohms. It is pretty hard to test insulation, > with a low voltage. Insulation testing would normally be done > with a "megger". Since I am suspicious of what I see for the > 2000M range, I likely would not switch to it while working on > the average PCB. > > There are quality voltmeters, with much higher input impedance > than my $20 "volts" multimeter used in this test. For such a > device (perhaps a $1000 unit), I likely would not need interpretation > except for the last test case. And the voltmeter in that case, could > tell me what the open circuit voltage is, correctly, for the > rest of the ranges. > > The $20 meter is the one I lend to friends :-) > > So there are six ranges on my $100 multimeter, that I'd use on a DIMM. > Without being concerned about any side effects to the silicon. > > Some older meters, like my analog Simpson, are a bit meaner. > They apply nine volts open circuit, and would be unsuited > for this purpose. I could have run the same set of tests for > my Simpson, but there is no point, because I simply wouldn't > use it. My Simpson is good for volts "trend analysis", as digital > meters aren't perfect for all purposes. There are still > occasions for reaching for my oldest multimeter. "Ohms" would > not be one of those reasons. > > If a person had concerns about any silicon device they were > working on, they could look in the datasheet for inspiration. > I haven't done that in this case for the memory chip. I'm reasonably > confident, that the six tested ranges on my multimeter, would > be safe enough. > > HTH, > Paul Regardless of the characteristics of any multimeter, it is a totally unsuitable device for checking out a RAM chip.
From: Paul on 17 Dec 2009 08:05 Pegasus [MVP] wrote: > > > "Paul" <nospam(a)needed.com> schrieb im Newsbeitrag > news:hgcvqo$msq$1(a)aioe.org... >> Pegasus [MVP] wrote: >>> >>> >>> The multimeter test might destroy the RAM chip, like the tap on the >>> head with the hammer. I find it difficult to think of a more >>> inappropriate test for a RAM chip. Remember - it contains a hundred >>> million or more transistor gates operating at extremely low currents, >>> less than the most sensitive multimeter can detect! >> >> You can safely test silicon devices, if you use a multimeter with >> "low power ohms" setting. It applies a voltage not intended to >> cause forward conduction in the silicon chips. >> >> If you lost the manual for your multimeter, you can verify this by using >> two multimeters. One multimeter set to "ohms". The second multimeter >> set to "volts". The "volts" multimeter will show you a rough idea >> of the open circuit voltage that the "ohms" meter is putting out. >> (Connect red to red, black to black.) >> >> Test all the "ohms" ranges on the meter, to understand which >> ranges are "high power" and "low power". If an ohms range doesn't >> put out more than 1 volt open circuit, then it should be relatively >> safe to use on just about anything. For the highest resistance >> ranges, the results need some interpretation (since the voltmeter >> has a finite input impedance of its own) >> >> The $100 The $20 >> meter meter >> >> Range Ohmmeter Voltmeter Assumed Confidence >> Reads Reads Power type >> >> Beep Infinity 2.66V High >> Diode infinity 2.67V High High on purpose, for >> diode test >> 200 Infinity 1.01V Low >> 2K infinity 1.01V Low >> 20K Infinity 0.48V Low >> 200K infinity 0.43V Low >> 2M "0.995" 0.23V Low OK, see interpretation >> 20M "0.99" 0.04V Low >> 2000M "001" 0.27V ??? suspicious >> >> (Note - a separate set of tests were done, and the "ohms" multimeter >> never applied more than 1 milliamp of current to the test leads. Typical >> silicon clamp diodes are rated for about 10 milliamps. So there is >> no danger from the level of current flow either.) >> >> In the 2 megohm test case, the "volts" multimeter appears to >> have only a 1 megohm input impedance. Half of the open circuit >> voltage is across the "volts" multimeter, and half is across >> the constant current source inside the meter. We could conclude >> from that, that the actual open circuit voltage applied >> by the multimeter, is 2 * 0.23 volts or 0.46V. And that is suitable >> for low power ohms. So you actually have to stare at the display >> on both devices to understand what is going on. >> >> My suspicion is, the "insulation test" range on my multimeter, is >> actually high power ohms. It is pretty hard to test insulation, >> with a low voltage. Insulation testing would normally be done >> with a "megger". Since I am suspicious of what I see for the >> 2000M range, I likely would not switch to it while working on >> the average PCB. >> >> There are quality voltmeters, with much higher input impedance >> than my $20 "volts" multimeter used in this test. For such a >> device (perhaps a $1000 unit), I likely would not need interpretation >> except for the last test case. And the voltmeter in that case, could >> tell me what the open circuit voltage is, correctly, for the >> rest of the ranges. >> >> The $20 meter is the one I lend to friends :-) >> >> So there are six ranges on my $100 multimeter, that I'd use on a DIMM. >> Without being concerned about any side effects to the silicon. >> >> Some older meters, like my analog Simpson, are a bit meaner. >> They apply nine volts open circuit, and would be unsuited >> for this purpose. I could have run the same set of tests for >> my Simpson, but there is no point, because I simply wouldn't >> use it. My Simpson is good for volts "trend analysis", as digital >> meters aren't perfect for all purposes. There are still >> occasions for reaching for my oldest multimeter. "Ohms" would >> not be one of those reasons. >> >> If a person had concerns about any silicon device they were >> working on, they could look in the datasheet for inspiration. >> I haven't done that in this case for the memory chip. I'm reasonably >> confident, that the six tested ranges on my multimeter, would >> be safe enough. >> >> HTH, >> Paul > > Regardless of the characteristics of any multimeter, it is a totally > unsuitable device for checking out a RAM chip. Doing an ohms test on the chip itself has no meaning. (Like measuring the resistance from DQ0 to DQ7. That wouldn't mean anything in particular.) If you're checking continuity on a series damping resistor, that could be used as a check that the wiring is OK. For example, doing this, shows the resistor pack is soldered down OK. contact ---- resistor ----- Memory_chip ^ ^ | | +- ohm from here to here -+ There aren't a lot of other tests you can do. If you buy "generic" DIMMs by the barrel full, one good test to run, is measure the rail to rail resistance. Some failed cheap bypass caps are a dead short, and can cause the motherboard socket to get burned. (I've seen reports of this on Newegg.) If you were doing incoming inspection, a quick check between rails might be a good test. (We actually used to do that for large circuit packs as well, as an optional test before applying power. I was surprised, while recording the results for this test, how well correlated it was. There was little unit to unit variation, in the ones I recorded. If something was out of the ordinary, I wouldn't plug in the unit, until determining why.) But I can't see a lot of reasons to be using a multimeter on a DIMM. If memtest shows it is bad, just throw it away (or return it under the warranty terms). I can't remember the last time I combined "multimeter" with "DIMM". It would not be high on my to-do list, if I was just trying to get something running again. If you want to play around, as long as the meter uses low power ohms ranges, I don't see the harm in it. But you could just as easily spend your time probing a rock with the multimeter. Paul
From: Pegasus [MVP] on 17 Dec 2009 10:46 "Paul" <nospam(a)needed.com> screv in news:hgda8n$4l8$1(a)aioe.org... > But you could just as easily spend your time probing a rock with the > multimeter. > > Paul Exactly. We finally appear to agree.
From: Bill Cunningham on 17 Dec 2009 13:42 "Paul" <nospam(a)needed.com> wrote in message news:hgda8n$4l8$1(a)aioe.org... > If you want to play around, as long as the meter uses > low power ohms ranges, I don't see the harm in it. But > you could just as easily spend your time probing a > rock with the multimeter. Mine is a small digital multimeter. I think it sends out 1.5 to 2 volts or so. I don't think that would damage digital equipment would it? That's about the voltage in my DDRAM that is a "bit turned on". Bill
From: Bill in Co. on 17 Dec 2009 14:08
Paul wrote: > Pegasus [MVP] wrote: >> >> >> The multimeter test might destroy the RAM chip, like the tap on the head >> with the hammer. I find it difficult to think of a more inappropriate >> test for a RAM chip. Remember - it contains a hundred million or more >> transistor gates operating at extremely low currents, less than the most >> sensitive multimeter can detect! > > You can safely test silicon devices, if you use a multimeter with > "low power ohms" setting. It applies a voltage not intended to > cause forward conduction in the silicon chips. Paul, this only applies to checking *discrete* devices, like diodes and transistors, it does not apply to integrated circuits. |