EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: electr_peter on November 01, 2014, 03:42:07 pm
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Hi,
I have always wondered if a typical DMM could read it's own battery voltage via test leads (accurately and safely).
I read somewhere on this forum that cheap multimeter like 830 series cannot measure it's own battery voltage via test leads because +/- test leads are voltage level shifted (to something like +5V/-3V) and thus measurement would create short circuit condition.
Does this inability to measure battery voltage directly apply to all standard configuration multimeters (single battery powered, one channel input for voltage/current/resistance)?
Can someone provide a link to schematic that proves/disproves above statements?
If above is true, then how does DMM sense low battery condition? Does it use off the shelf IC with a reference and comparator?
I am not interested too much in actually measuring some battery voltages on DMMs, but a possibility to have more options to kill dangerous cheap multimeters would be welcome :D.
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I believe someone reported killing an expensive (Fluke?) multimeter by trying to measure the installed battery voltage with the meter test leads.
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The only way to answer this definitively is to review all the data sheets of all the ICs used in multimeters. Having said that, many multimeters have been damaged from people trying to measure the battery.
I would guess that a multimeter measures its own battery by using an internal reference and a comparator that is separate from the measurement input section for the functions.
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I believe someone reported killing an expensive (Fluke?) multimeter by trying to measure the installed battery voltage with the meter test leads.
Forum topic fluke-87v-_bricked_-when-probing-own-battery (https://www.eevblog.com/forum/testgear/fluke-87v-_bricked_-when-probing-own-battery/) mentions how poor 87V died :-BROKE.
I see the DMM battery measurement as problem of non-floating measurements (everything in a DMM is referenced to some point) and requirement to measure +/- voltages. Thus actual leads should be level shifted (depending on the design). Battery measurement is implemented via some tricks with additional circuitry or with isolated power supply if such exists.
Safest bet is not to measure DMM internals with the same DMM (because DMM is not isolated/floating in this case).
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I believe someone reported killing an expensive (Fluke?) multimeter by trying to measure the installed battery voltage with the meter test leads.
Forum topic https://www.eevblog.com/forum/testgear/fluke-87v-%27bricked%27-when-probing-own-battery/ (https://www.eevblog.com/forum/testgear/fluke-87v-%27bricked%27-when-probing-own-battery/) mentions how poor 87V died :-BROKE.
I see the DMM battery measurement as problem of non-floating measurements (everything in a DMM is referenced to some point) and requirement to measure +/- voltages. Thus actual leads should be level shifted (depending on the design). Battery measurement is implemented via some tricks with additional circuitry or with isolated power supply if such exists.
Safest bet is not to measure DMM internals with the same DMM (because DMM is not isolated/floating in this case).
That link is about shorting "the positive terminal of the 9V Battery, with the COM lead ". That's never good in any situation.
I don't see why probing the battery ONE TERMINAL AT A TIME with the correct mode and the + lead only can hurt anything. I've probed the internals of a vacuum tube scope with itself, no problem. But I didn't clip the ground lead to anything!!!!! Because that would be insane.
I just tried with my own DMM because I'm 100% sure... And it reads some split voltage, +3 on the + of the battery, and -5.7 on the - of the battery.
It's pretty obvious the battery somehow floats wrt the DMM "ground", so using both leads means instant death.
One voltage at a time with the + lead only. Meter still works.
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I don't see why probing the battery ONE TERMINAL AT A TIME with the correct mode and the + lead only can hurt anything. I've probed the internals of a vacuum tube scope with itself, no problem. But I didn't clip the ground lead to anything!!!!! Because that would be insane.
I just tried with my own DMM because I'm 100% sure... And it reads some split voltage, +3 on the + of the battery, and -5.7 on the - of the battery.
It's pretty obvious the battery somehow floats wrt the DMM "ground", so using both leads means instant death.
One voltage at a time with the + lead only. Meter still works.
This description seems like a complete analogy to oscilloscope ground terminal and the fact that scopes are not floating/isolated (typically).
"+" lead is input terminal of scope, "-" lead is ground of scope input (which is mains earthed). Just as with the scope, you cannot just clip ground to any point on mains referenced circuit (with :-BROKE result), multimeter cannot connect it's "-" terminal to DMM internals at any point.
"+" terminal of DMM and oscilloscope input terminal are high impedance (in almost all cases), thus these can be connected almost anywhere (high impedance ensures safe conditions).
"-" terminal of DMM and oscilloscope ground are low impedance and can create problems if you are not careful.
Still, I would be very careful before I try probing with only "+" lead in DMM internals. I am not sure about exact DMM schematics - maybe "+" and "-" leads are reversed for a design reasons. Just want to be sure to avoid stupid :-BROKE event.
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Still, I would be very careful before I try probing with only "+" lead in DMM internals.
Good point. The high impedance of a scope probe input comes in part from the probe itself. A DMM probe is just a straight piece of wire.
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Since generally all modern DMMs are capable of measuring negative as well as positive voltage with respect to the COM jack (analog ground), the A/D converter must have a bipolar power supply. To do that, most late-model designs will bias analog ground to some point between the battery negative and battery positive. For example, on the venerable ICL7106, the analog ground is regulated at 2.8V below the battery positive.
Connecting the COM lead (analog ground) to either of the battery terminals will effectively short out the regulator circuitry that maintains the bias. This could easily lead to destructive currents. In voltage-measuring mode, since the positive input jack leads to the ADC input and presents a very high impedance to analog ground, it is generally safe to connect to either battery lead (as noted by Alex above.) Although I would not recommend this as a trouble-shooting technique. :)
Some older Fluke designs like the 8060A connect the battery negative straight to analog ground, and use a charge pump to generate a negative power supply rail. These units can indeed measure their own battery voltage.
Detecting a low battery condition can be done in all manner of ways, but it isn't done with the DMM's own A/D converter. A comparator referenced to the regulated supply, or sometimes on cheap meters, you may find it's just a voltage divider between raw battery voltage the regulated supply, biasing the B-E junction of a BJT.
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The Tektronix Tx1 and TX3 (which became the Fluke183 and 185) can measure their own battery voltage; its a power-up option.
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Another sad case of internal battery measurement with Fluke 177 Fluke-177-failed-and-need-support-to-repair (https://www.eevblog.com/forum/testgear/fluke-177-failed-and-need-support-to-repair/)
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The Tektronix Tx1 and TX3 (which became the Fluke183 and 185) can measure their own battery voltage; its a power-up option.
This option is not listed on the back of the tx3.
You press softkey 4 while turning the meter on, keep pressing softkey 4 until the display goes blank.
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The EEVblog 121GW displays it's own battery voltage, in the upper right corner of the display, as soon as you turn it on,
(it has 4 AA batts). :-DMM