(Update) Negative Resistance Archived!

Or my UNI-T guent grong...

Trusty things those! Which is wrong, physics or the DMM?

JS

joaquins said:

Or my UNI-T guent grong...

Trusty things those! Which is wrong, physics or the DMM?

JS

Click to expand...

Albert Einstein apologized many times in history, for making wrong calculations, or wrong claims...
so, it could be both  8)

ps:  good to be back from my vacation..... groupdiy.com on iphone scuks... time for ipad for travelling

Nice! send me some of those coils. I guess that means they are making current! A couple of them should power my tv remote...

joaquins said:

Or my UNI-T guent grong...

Trusty things those! Which is wrong, physics or the DMM?

JS

Click to expand...

An artifact of how resistance is measured by the VOM. A reference current is supplied and the voltmeter measures the voltage developed across the resistance due to the known current.

Of course if the component is in a powered up circuit, stray currents or voltages can corrupt the measurement.

Or your meter is busted. 

JR

joaquins said:

Or my UNI-T guent grong...

Click to expand...

Did you discharge the inductor before you measured it?

Since that meter reads L and C, it would put out an AC tone. It probably also measures R with the same tone (cost-cutting design?).  An L, perhaps on the side of some resonant slope at that freq, apparently "tricks" it. A BK Precision 875 LCR meter is like this, a 1K tone in R mode, no DC component.

Measure the R of a set of headphones with that meter set as you have it, and then put them on. Careful, might be loud, my BK was very faint. Sounds like when getting your hearing tested.

Gene

Shorted leads also did the same thing, so no charging or ac problem. The generator on this varies with value measured and anly present if activated by the LC key.

It slowly keeps going more negative, with shorter leads seems faster than with the coil. A 10k 1% resistor (or many of them)  started measuring let's say 10k3 an slowly decayed, if left a few minutes to 9k6 and keeps going down slower. Similar things in other R ranges. No much more relevant testing. Swapped the bat, nothing changed. It was sitting in a HIGH humidity environment for a few month, wouldn't surprise me if it had something to do with that.

I guess is time for a new one, my older one seemed to be out of specs as well. Is the tiny Fluke 106 worth it? It's about $130 here, same price range as my wonky one. Or I could go for the cheapest one for now... I'll take the bad one(s) to the university and see if I can get any help with them.

JS

Is battery good?

JR

JohnRoberts said:

Is battery good?

Click to expand...

My first thought too, but he says "Swapped the bat, nothing changed."  Could be humidity problem, check pcb for corrosion or contamination.  Wash with isopropyl alcohol.

you need some kind of feedback to have negative resistance isn´t? it is your meter calibrated? if it was zeroed with a longer cable maybe the cause of negative readings. still -15 ohms is a bit high(or low)

I didn't split the 2 boards wich have the switch builded between, i did cleaned with 2-ethanol 99.6%  and inspected the board.

It doesn't seems an offset or cal issue since it keeps drifting with time, few minutes and keeps going. Same thing with shorted terminals or a 10k resistor which drift down 9k something

It has around 20 trim pots inside, olschool calib...

JS

You obviously have problems with Vref voltages.
The free schematic is here
http://elektrotanya.com/multimeter_ut70a_sch.pdf/download.html

From -V to gnd I measured ~6V, higher potential at GND. Seems a bit too much and diode D1 is biased to about 0.5V

I couldn't see the resistor wich connects V- to GND, I can't read the name or the value in the schematic but is probably underneath the lcd assembly I didn't took apart.

The resiator and the IC are only 2 common points between the battery -V and the analog side GND. I measure about 1k with it switched off, about 6V with it switched on.

JS

> you need some kind of feedback to have negative resistance isn't?

No. An electric arc (or neon lamp) is high positive resistance up to some voltage, then a low NEGative resistance as the gas breaks-down avalanche style. In this range the current wants to go to infinity. We have to add series impedance (resistor, reactive transformer) to set a reasonable current.

In some sense, this could be seen as positive feedback on the energy stored up in gas molecules in the pre-breakdown phase.

In a coil meter, it could be the voltage has got so out of phase with current that the meter brain gets confused.

Though meter fault is sounding likely.

So.... should we perhaps call this "negative resistance" "insistence"?

are you testing the resistor in circuit?

pucho812 said:

are you testing the resistor in circuit?

Click to expand...

I said I read that value between those two points, I didn't said I measured the resistor.

JS

  An update... I took the bastard to a lab at the university, drift was much much slower, barely noticeable, and all the ranges where close enough to specs, I wouldn't trust the thing with any critical thing right now but with another check in a few month it may hit the road again.

  Cheers and thanks for the help.

JS

Some devices, like Gunn diodes, do exhibit negative resistance and can be used for ascoilators.

Cheers

Ian