DMM phantom reading

[throbscottle]

I have a cheapish multimeter I bought about ten years ago, which I have recalibrated recently.(this is how I did it) but I'm getting phantom readings on the highest resistance ranges (2M and 20M) and low readings on the 200K range. It looks as though something is shunting the input, but don't know much about how a DMM measures resistance so this is just a guess. I tried cleaning the switch, which made a bit of difference, but still have the fault.

Anybody have any suggestions as to what I should be looking for?

TIA
throbscottle

 

[Mikebits]

Can you explain what you mean by phantom readings?

 

[ssundar.shan]

Do u mean your DMM shows different resistance values for the same resistor?

 

[throbscottle]

Oh sorry I forgot the rest of the world doesn't speak the language I use inside my head!

On the 2M and 20M ranges the meter shows unstable readings of 0.65 and 0.539 respectively when nothing is connected, and on the 200K range it reads 87K for a 100K resistor and 175K for a 270K resistor. It only went off the scale slowly when I applied a 330K resistor. On the open circuit readings for the 20M and the 2M range the LSD and middle digit "cycle" up and down through a range of values between around 0.630 and 0.660.

I think the readings on the lower ranges are probably off too, but not enough to make them useless.

 

[ssundar.shan]

Hi throbscottle,
I suspect,that Ur DMM Battery may slowly discharging because as your measure the low to High(100K to 370K) value resistor, the observed resistor values are less compared to the actual value.

 

[Mikebits]

There is capacitance and inductance in the meter leads as well as the measuring device, in the meg ohm range this capacitance will have a large effect and will cause meter drift. For lower range readings, keep in mind resistor tolerances, also be aware with a cheap meter, your meter leads may have less than adequate connections.
If you want accurate reading, invest in a good meter and leads.

 

[unclejed613]

if you've exceeded the voltage limits of the meter, or you live in a moist climate, it may be leakage on the PCB. if you've measured voltages with the meter on the ohms scale a big no-no), you may have damaged the constant current source. a DMM uses a constant current source for resistance measurements with a constant current of 1mA, it will develop 1V across a 1k resistor (so you get a reading of "1.00k). each ohm scale uses a different current. if the current source is defective, you get faulty readings.

these may also be symptoms of almost dead batteries.

 

[alec_t]

I suggest try a known fresh battery (even if you've already replaced it!).

 

[throbscottle]

I've accidentally connected to a voltage source on the ohms range a few times, so seems likely to be that. Thanks for the pointer. It's done the same with the last couple of batteries, and the leads are soldered permanently to the board, so no problem there, I've never exceeded the meter's voltage limit, and Britain is damp, but not /that/ damp!