Editing Category:Common Issues (section)

=== Basic Electronic Measurements ===
Most measurements you will need to make can be done with a DMM (digital multimeter). A DMM allows you to measure a variety of signals, which vary from meter to meter. However, a basic understanding of how the meter is making its measurements is necessary in order to properly use it and get useful readings. Another generally useful tool is an oscilloscope, which you can use to look at dynamic signals beyond the bandwidth of the multimeter. 

==== Multimeter Probes ====
You need 2 probes to do any measurements, in any mode, period. One is for the positive side of the signal, and the other is for the COM ("common") side of the signal. The reason that it is COM and not ground is that the DMM is a floating reference{{---}}voltage measurements always need to be taken with respect to some point.  The two probes set the point that you are trying to measure against.

This is a really important thing to understand. Suppose that you want to measure a 6{{nbsp}}V signal. If you probe with the red probe on a 6{{nbsp}}V test point and the black one on ground, you will find a reading of 6{{nbsp}}V. However, if you were to measure with the red probe on a 18{{nbsp}}V signal and the black on a 12{{nbsp}}V signal, you would find that this also reads 6{{nbsp}}V (18{{nbsp}}V {{minus}} 12{{nbsp}}V = 6{{nbsp}}V). So, the meter is not always referenced to ground, and this is why you need to pay attention to where you are probing. The probes will also have different functions based on what type of thing you are trying to measure. For voltage measurements, the probes are in parallel with the circuit under test. For current measurements, the probes are '''in series'''. So, if you don't pay much attention when setting up your measurements, it's very easy to end up with useless information about what you are looking at, and even damage your circuit beyond the state it came to you in.

==== AC and DC Volts Mode (ACV/DCV) ====
This mode is how you would measure voltage with your multimeter. It is an easy mode to understand and use. In this mode, the circuit basically appears in parallel with your meter. You can just probe the area of interest with the red probe (inserted in the V socket) and set the black probe on ground or another point of reference. Generally, the multimeter will have an impedance of >10{{nbsp}}M{{ohm}}, meaning that for measurements smaller than this (less than {{~}}2{{nbsp}}M{{ohm}}) your measurement should not effect the circuit or your reading. If you are having a hard time finding a ground point, trying grounding the black probe on the chassis or a screw on the device{{---}}usually this is a grounded point and can be used just as well as a test point.

==== Resistance Mode (Ohms) ====
This mode injects a current through the two probes (small, typically 1{{--}}10{{nbsp}}mA) and measures the voltage developed across the load. for resistive loads (like resistors!) this follows Ohm's law: V = IR. Because the DMM is good at measuring voltage, and we know the output current (or at least the meter does) this gives us the resistance of the component under testing. Of course, there are some traps here that you should know about before you are complaining that your measurements don't work correctly. First, because it is outputting a current, this mode will tend to charge up capacitors. This is why sometimes when you are trying to do a reading, the value will drift up as the capacitor soaks up the current and develops a larger voltage across it. Another side product that can happen from this mode is that the current being injected into the circuit can leak into diode junctions and (very slightly) turn on diodes/transistors, causing the voltage to discharge away. So, if you want to take good resistance measurements, just keep these things in mind and try to probe mindfully to make the most of your measurements. The most accurate way to make resistance measurements is to remove the component and test it out of circuit.

==== Diode Mode ====
Diode mode is basically resistance mode, but sources a voltage on the positive probe and measures the forward voltage drop on the negative probe. For a healthy diode, this is somewhere in the ballpark of 0.6{{nbsp}}V{{--}}0.7{{nbsp}}V  but depends on the diode (see the specific datasheet for more information). By measuring the voltage drop, you can infer if the diode is healthy or not. once you do this, you should also swap the probes and do the measurement backwards to make sure there is not leaking in the reverse bias direction{{---}}this is a likely fault for diodes. 

==== Capacitance Mode ====
This mode outputs a frequency on the probes and tries to measure the voltage change relative to frequency, and uses a bit of math to figure out the capacitance. This is generally a pretty useless mode{{---}}it's unusable in circuit, so it only works on parts that have been removed and generally capacitors don't fail by drifting in value{{---}}you only need to know if it's shorted or not, so you are better off doing resistance measurements in circuit.

==== Current Mode ====
In this mode, you need to move the probes over to the other socket. The reason for this is that while in voltage and resistance measurements, the meter is in parallel with the circuit{{---}}in current measurements, the meter ''becomes'' part of the circuit{{---}}it exists in series. When the meter is in current mode, the two leads are basically shorted together. This means that you have the very real possibility to damage your circuit if you try to probe something like you would a voltage signal, and instead end up dumping the power supply onto a sensitive chip or straight into ground. 

The way the measurement works when in current mode is similar to the resistance measurement. There is an internal resistor with a small known value (to not limit the normal flow of the circuit). As current flows across the resistor, a voltage is developed across the two sides, and this is measured by the voltage part of the meter. Since we know the voltage and the resistance, this gives us the current through the device (again, refer to Ohm's law). This is likely the least useful mode and most likely to damage your device, so make sure you know what you are doing when you are setting up experiments to measure current. 

Once a fault has been located, verified and isolated, the next step is the actual repair, which of course is a procedure in itself.

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[[Category: General Repair Guides]]