calibrationmultimeter
Anyone have experience adjusting the Voltage reading on these? My less expensive meters have a single pot for calibrating, but this one has three. Labeled on the circuit board as V1, V2, and V3. I'd be interested to find out what each of them does, but mainly, I need to know which adjusts the Voltage reading.
Thanks!
Small parts like transistors don't stop the 230V. It's very probable that most of the 230V went into the controller and that that is fried as well. This would explain why the meter doesn't do nothing anymore. Having the controller replaced may cost you more than a new DMM (if they replace it at all, you'll probably get a replacement DMM). This is a custom IC which you won't find to do it yourself.
It's impossible to identify the fried parts without a schematic if they don't have readable markings anymore. (It's even impossible to identify good parts without marking with absolute certainty.)
TP1 and TP2 probably refer to test points rather than components; TP is normally not used as a prefix for components.
The 8/S shaped pattern is used to calibrate magnetometers in mobile phones and other devices.
Background
Typical mobile phone era magnetometers measure the magnetic field strength along three orthogonal axes, e.g.:
\$\textbf{m} = m_x\boldsymbol{\hat{\imath}} + m_y\boldsymbol{\hat{\jmath}} + m_z\boldsymbol{\hat{k}}\$
With the magnitude of the field given by,
\$\|\textbf{m}\| = \sqrt{m_x^2 +m_y^2 + m_z^2}\$
and the angle of rotation from each axis as
\$ \theta_k = \cos^{-1} \frac{m_k}{ \| \textbf{m} \| }, \text{ where } k \in [x,y,z] \$
Calibration
Since the magenetic field of the earth is relatively constant, the magnitude of the as field measured by the magnetometer should also be constant, regardless of the orientation of the sensor. i.e. if one were to rotate the sensor around and plot \$m_x\$, \$m_y\$, and \$m_z\$ in 3D, the paths should plot out the surface of a sphere with constant radius.
Ideally it should look somthing like this:
However due to hard and soft iron effects and other distortions, it ends up looking like a deformed sphere:
This is because the magnitude of the magnetic field as measured by the sensor is changing with orientation. The result being that the direction of the magnetic field when calculated according to the formulas above is different from the true direction.
Calibration must be performed to adjust each of the three axis readings so that the magnitude is constant regardless of orientation - you can think of it as the deformed sphere must be warped into a perfect sphere. The LSM303 application note has lots of detailed instructions on how to perform this.
So what about the figure 8 pattern!?
Performing the figure 8 pattern 'traces out' part of the deformed sphere above. From the coordinates obtained, the deformation of the sphere can be estimated, and the calibration coefficients obtained. A good pattern is one that traces through the greatest range of orientations and therefore estimates the greatest deviation from the true constant magnitude.
To estimate the shape of the deformed sphere, least squares ellipse fitting can be used. The LSM303 application note also has information on this.
A simple method for a basic calibration
According to the app note if you assume no soft-iron distortion, the deformed sphere will not be tilted. Therefore a simple method for a basic calibration may be possible:
\$r_k = \tfrac{1}{2} (\max(m_k) - \min(m_k))\$
\$z_k = \max(m_k) - r_k\$
\$m_k' = \frac{m_k - z_k}{r_k}\$
This is based off the code found here.
Solving using least squares
MATLAB code to solve using least squares is shown below. The code assumes a variable mag where the columns are the x y z values.
H = [mag(:,1), mag(:,2), mag(:,3), - mag(:,2).^2, - mag(:,3).^2, ones(size(mag(:,1)))];
w = mag(:,1).^2;
X = (H'*H)\H'*w;
offX = X(1)/2;
offY = X(2)/(2*X(4));
offZ = X(3)/(2*X(5));
temp = X(6) + offX^2 + X(4)*offY^2 + X(5)*offZ^2;
scaleX = sqrt(temp);
scaleY = sqrt(temp / X(4));
scaleZ= sqrt(temp / X(5));
To do a dynamic figure 8 calibration, you could run the least squares routine with every new reading and terminate when the offset and scale factors have stabilised.
Earth's Magnetic Field
Note, the Earth's magnetic field is usually not parallel to the surface and there may be a large down component.
Best Answer
The other answer is right, adjust VR1 with a tiny screwdriver. Clockwise is lower voltage and counter is increase. Tested using DCV function. You should use a voltage reference module, but I just used the readings from 2 other meters that seemed accurate and adjusted it likewise. The other 2 meters have a thousandths place, this 3320 only has 2 digits. And we don't know what the other 2 VR2 and VR3 are.