troubles with multimeter

[yeeshkul]

I measured a weird numbers on this simple circuit - N-FET, two 10M resistors gate->drain / gate->ground and additional 5.1k drain->9V. My multimeter keeps showing something like 0.8V on the gate, which is nonsense.

To check it out i just connected two 10M resistors in series on breadboard and put them in between 9V and the ground. And got the same result in the middle ...  0.8V instead of 4.5V.

Anyone has idea what can be wrong? Do have these cheapo multimeters difficulties to measure right numbers on large resistance?

thanks
Jan

[Auke Haarsma]

I had the same issue. I has to do with your DMM. Its resistance is too low to measure correctly. In fact, instead of a neutral observer, it becomes part of the circuit, that is why the reading is strange.

[petemoore]

Do have these cheapo multimeters difficulties to measure right numbers on large resistance?
  You betcha, when reading huge R's, your body circuit may throw the reading to lower than, make sure you aren't touching one side of the DDM probe contacts.
  Sometimes a workaround...for measureing really high resistances, I use a smaller resistance [say 1 meg] and see what adding the huge R parallel to that reads [drops the built value a bit lower than the 1 meg alone], then figure I'm in the ballpark for anything over 2meg and can trust the color code, 2mega-ohm being the biggest resistance my DMM measures.

[yeeshkul]

Guys thanks! Do the more expensive DMM do the same thing? I just dont want to waste money

[petemoore]

Do the more expensive DMM do the same thing?
  That depends on what the specifications say, and how closely the unit adheres to them.
  IOW, no, or, there is test equipment which is more accurate for reading high R values than a cheep DMM.

[Auke Haarsma]

I beg to differ. Yeah, a better DMM has often a higher internal resistance.

I upgraded mine after experiencing what you experience. My new DMM (10 times the price of the old one, if price says anything) is more accurate.

[petemoore]

  I don't see a differ, really. "Do the expensive ones do the same thing?" [No].
  DDM "quality" and internal resistance varies, sort of according to price-point, I guess to think the spec sheets show these values.

[George Giblet]

> Guys thanks! Do the more expensive DMM do the same thing? I

All multimeters independent of price will have some loading effect.  Most reasonable quality *digital* multimeters have a 10MEG impedance, this is like the defacto standard.   *Some* really cheap multimeters have impedances of 1MEG  (one of my meters is this type, and it cost $10)  but many other cheapish meters will have 10MEG,  like a "good" meter.  The impedance is not something which varies in accordance with quality it is something which is specified in the manual - as peter said.   Note one reason why meters are 1MEG is because it allows them to get more accuracy on the AC range at higher frequencies - wide bandwidth AC performance is usually costly at 10MEG impedance.

These days "high quality" multimeters aren't always accurate.    However, if you want high accuracy meters then you have to buy high end meters and will cost you $$$ - 20 years ago many cheap meters were fairly accurate.    So what do you get for your money with high wiality meters?  Stronger case, longer lasting switches,  a guarantee that the meter will be within specifications for a given time (that is not to say the cheap meters will not do this too, it is just you can't be sure they will!).

You have to buy a meter which suits you needs.  If high impedance is what you need then just look for meter that specs high impedance, the same goes for accuracy.

Getting back to your meter:  Let's assume it is a 1MEG ohm input impedance then the meter will create a voltage divider with 10MEG on the top of the divider and 10MEG in parallel with 1MEG at the bottom of the divider.  Now 10MEG in parallel with 1MEG is about 909k ohm.  If you calculate the voltage divider output 9 * 909k / (909k + 10MEG) = 0.75V.   So it looks like you meter is a bit over 1MEG.  (We could work out the exact value but it won't help you.)

*** The 1MEG/10MEG input impedance has nothing to do with the ability of a meter to measure 10MEG resistance.  The resistance measuring and voltage measuring parts of the circuit are completely separate!!!

[Auke Haarsma]

Do the more expensive DMM do the same thing?
  That depends on what the specifications say, and how closely the unit adheres to them.
  IOW, no, or, there is test equipment which is more accurate for reading high R values than a cheep DMM.

You are right, I misread you apologies.

[yeeshkul]

Getting back to your meter:  Let's assume it is a 1MEG ohm input impedance then the meter will create a voltage divider with 10MEG on the top of the divider and 10MEG in parallel with 1MEG at the bottom of the divider.  Now 10MEG in parallel with 1MEG is about 909k ohm.  If you calculate the voltage divider output 9 * 909k / (909k + 10MEG) = 0.75V.   So it looks like you meter is a bit over 1MEG.  (We could work out the exact value but it won't help you.)

This explains everything! Thank you George.