making voltmeter

Aamir Ali said:

if  use MCU, it needs resistor divider which reduces its ADC bits.

??? A resistor divider does not reduce any ADC bits (I guess you mean resolution).

If you have a 12 bit ADC, it measn it can any voltage range with a resolution of 1/4095. If the range is 0V-2.5V (defined by the reference), resolution is 2.5/4095 = 0.6105mV. If you have 10V input signal, you'll of course need a voltage divider to bring the signal down to the allowed input range (below the reference), however, maximum achievable resolution is still 10V/4095 = 2,552mV. No 'bits reduced'.

Of course, if you divide the signal range to a value that is below the used reference, then you limit the resolution, but then that's due to chosign an improper voltage divier or picking the wrong reference.

Aamir Ali said:

Then how does voltmeter measures voltages high, so accurately

Voltmeters may use an ADC with a higher resolution. teh SD16, for example, has a maximum resolution of 16 bits, resulting in 1/65535 resolution, or, for the typical SD16 reference of 1.2V, 18.31µV. Multiplied with the scaling factor of the voltage divider. So for a 10:1 divider, a signal with 1.2*10 = 13V (or below) would be sampled with 0.1831mV.
However, ADCs with higher resolution are usually either much slower (for a multimeter, a few samples per second are enough anyway) or much more expensive and energy hungry than their lower-resolution counterparts.

Jens-Michael Gross said:

If you have a 12 bit ADC, it measn it can any voltage range with a resolution of 1/4095. If the range is 0V-2.5V (defined by the reference), resolution is 2.5/4095 = 0.6105mV. If you have 10V input signal, you'll of course need a voltage divider to bring the signal down to the allowed input range (below the reference), however, maximum achievable resolution is still 10V/4095 = 2,552mV. No 'bits reduced'.

Note that you get exactly the same effect on a "traditional" analogue meter with a moving pointer:

• If the scale is divided into 10 divisions, and your full-scale reading is 2V, that is 0.2V/division;
• If the scale is divided into 10 divisions, and your full-scale reading is 20V, that is 2V/division.

 

BTW, what is the simplest way to measure AC and negative DC voltages? I'm sure there is a hardware trick for that.

Simplest but without additional special components :)

Well, one hardware trick to transform negative DC or AC to positive DC is to use a full bridge rectifier. 

The problem is, of course, the voltage fall on the diodes. To overcome this problem, you can use diodes combined with opamps, forming a precision rectifier where the voltage drop of the equivalent diodes is almost zero.

http://en.wikipedia.org/wiki/Precision_rectifier

Thank you for reply. This thread inspired me to make (mainly for fun but also as usable gadget) a pocket multimeter. I have found old MSP430F425 with 16-bit sigma- delta ADC in my wardrobe so I' m ready to make this. :)

Best regards,

Mikolaj

Ok, suppose we have precision rectifier and we want to measure voltage up to 400 V. We can give big resistor between input and this rectifier (>1M) and lower the voltage using voltage divider. There is a question how we can check input DC voltage is positive or negative? We can use diode and MSP430 ADC pin (any voltage => positive, no voltage => negative) but is it the correct solution (for small values there is additional voltage drop on diode)? We can't use any transistor or other switching element because we don't know the current would be enough to open/ close this element.

PS I don't know I should create my own topic or I can write here.

edit:

Now I am reading about Sigma- Delta ADC in MSP430 and it simplify a lot. Cool :)

Mikolaj Filar said:

There is a question how we can check input DC voltage is positive or negative? We can use diode and MSP430 ADC pin (any voltage => positive, no voltage => negative) but is it the correct solution (for small values there is additional voltage drop on diode)?

Basically the same OpAmp solution as for the precision rectifier, just with an open-loop gain, so any positive voltage latches to V+ and any negative to GND.