a). The resistance value of thermistors is by design affected by temperature so this would have to be taken into account when measuring their resistance.
They can either be NTC (negative temp coefficient) or PTC (positive temp coefficient) types.
With NTC as the temp increases their resistance value decreases. With PTC as the temp increases their resistance value increases.
This is usually at a known rate although it does vary as to the actual thermistor model number used.
-
Here's an [https://www.farnell.com/datasheets/33552.pdf|example] showing the resistance values of a some NTC thermistors -see p.26, to show what readings to expect at what temperature for different thermistor models.
+
Here's an [link|https://www.farnell.com/datasheets/33552.pdf|example] showing the resistance values of a some NTC thermistors -see p.26, to show what readings to expect at what temperature for different thermistor models.
Most do have a specified reference resistance at 25°C. but you would need to find their datasheet to know what it is.
basically, you would need to know the specific model number of the thermistor and find its datasheet, to know if it was OK or not except if it tested open circuit or short circuit.
b). With fuses it may pay to include a reference to the fact that you need to check if it is a standard fast acting fuse or a slow blow fuse (aka timed fuse).
A lot of devices e.g. TVs use slow blow fuses in lieu of standard fuses as the main supply fuse.
This is due to the high inrush currents that flow in the circuit when the power is first connected to the device. This current can be several times the value of the rating of the fuse. It is only there for approx 2-10mS (depends on circuit design).
A standard fuse would blow immediately the power was connected but a slow blow fuse will hold until the inrush current has subsided. A slow blow fuse is denoted by a T or S in the fuse rating e.g. T5A 250V (or S5A 250V) as opposed to a standard fuse with the same rating i.e. 5A 250V. This information is stamped on the end caps of the fuse (if axial type) or printed on the fuse itself if a different type of fuse, and is also usually printed on the board next to where the fuse is mounted.
c). The resistance value of heating elements can depend on its application.
Heating elements in heaters, electric stoves, dryers, toasters, kettles etc can have a high wattage output so using the Ohm's Law formula of R=V²/ P you can work out the expected resistance value and then test it with an Ohmmeter to see if it near this value or not e.g. shorted turns in an element will still show continuity, though the resistance will be less but there will be an increase in the current flow which may affect how the circuit behaves.
(R = resistance in Ohms, V = Volts, P = Power in Watts)
d). Testing capacitors with a digital multimeter, unless it has a specific capacitance test is not very useful.
-
Unless it is short circuit you have no idea whether it is any good or not. Even if the meter registers that it is charging/discharging (when meter connected and then when meter leads are reversed and connected again) you don't know if it is leaky or not.
+
Unless it is short circuit you have no idea whether it is any good or not. Even if the meter registers that it is charging/discharging i.e. not open circuit even though capacitors are effectively open circuit to DC (when meter connected and then when meter leads are reversed and connected again) you don't know if it is leaky or not.
The only way to really test a capacitor is OK or not is with an ESR meter. This tests the internal equivalent series resistance of the capacitor.
e). Testing diodes can also be tricky because although they only conduct in one direction, their resistance values in the conducting and non-conducting directions are dependent on the specific type of diode and it should not be assumed that they are all the same and have the same resistance value except that the conducting direction resistance is lower than the non conducting direction e.g. zener diode versus standard diode. Having a diode test in the DMM is better than just relying on the resistance test alone
Hi @ngurney,
a). The resistance value of thermistors is by design affected by temperature so this would have to be taken into account when measuring their resistance.
They can either be NTC (negative temp coefficient) or PTC (positive temp coefficient) types.
With NTC as the temp increases their resistance value decreases. With PTC as the temp increases their resistance value increases.
This is usually at a known rate although it does vary as to the actual thermistor model number used.
Here's an [https://www.farnell.com/datasheets/33552.pdf|example] showing the resistance values of a some NTC thermistors -see p.26, to show what readings to expect at what temperature for different thermistor models.
Most do have a specified reference resistance at 25°C. but you would need to find their datasheet to know what it is.
basically, you would need to know the specific model number of the thermistor and find its datasheet, to know if it was OK or not except if it tested open circuit or short circuit.
b). With fuses it may pay to include a reference to the fact that you need to check if it is a standard fast acting fuse or a slow blow fuse (aka timed fuse).
A lot of devices e.g. TVs use slow blow fuses in lieu of standard fuses as the main supply fuse.
This is due to the high inrush currents that flow in the circuit when the power is first connected to the device. This current can be several times the value of the rating of the fuse. It is only there for approx 2-10mS (depends on circuit design).
A standard fuse would blow immediately the power was connected but a slow blow fuse will hold until the inrush current has subsided. A slow blow fuse is denoted by a T or S in the fuse rating e.g. T5A 250V (or S5A 250V) as opposed to a standard fuse with the same rating i.e. 5A 250V. This information is stamped on the end caps of the fuse (if axial type) or printed on the fuse itself if a different type of fuse, and is also usually printed on the board next to where the fuse is mounted.
c). The resistance value of heating elements can depend on its application.
Heating elements in heaters, electric stoves, dryers, toasters, kettles etc can have a high wattage output so using the Ohm's Law formula of R=V²/ P you can work out the expected resistance value and then test it with an Ohmmeter to see if it near this value or not e.g. shorted turns in an element will still show continuity, though the resistance will be less but there will be an increase in the current flow which may affect how the circuit behaves.
(R = resistance in Ohms, V = Volts, P = Power in Watts)
d). Testing capacitors with a digital multimeter, unless it has a specific capacitance test is not very useful.
Unless it is short circuit you have no idea whether it is any good or not. Even if the meter registers that it is charging/discharging (when meter connected and then when meter leads are reversed and connected again) you don't know if it is leaky or not.
The only way to really test a capacitor is OK or not is with an ESR meter. This tests the internal equivalent series resistance of the capacitor.
e). Testing diodes can also be tricky because although they only conduct in one direction, their resistance values in the conducting and non-conducting directions are dependent on the specific type of diode and it should not be assumed that they are all the same and have the same resistance value except that the conducting direction resistance is lower than the non conducting direction e.g. zener diode versus standard diode. Having a diode test in the DMM is better than just relying on the resistance test alone