Advanced DMM Tips & Tricks - Workbench Wednesday

Summary

In this Workbench Wednesdays video, James explores advanced tips and tricks for using digital multimeters (DMMs) beyond basic measurements like voltage, current, and resistance. He covers troubleshooting current measurement issues, using DMMs to test transistors, MOSFETs, and LEDs, as well as measuring the breakdown voltage of Zener diodes. Additionally, James shares useful accessories like grabbers and probe holders, enhancing the DMM's versatility. With helpful insights from the Element14 community, this video aims to improve your DMM skills, making your measurements faster and more accurate in your electronics workbench.

Takeaways

• 😀 Always return the current measurement lead to the voltage terminal after use to avoid blowing the fuse.
• 😀 Use resistance mode to check the functionality of a DMM's fuse without needing to disassemble it.
• 😀 Test BJTs by treating them as two diodes in series and checking for expected voltage drops in diode mode.
• 😀 For PNP transistors, reverse the leads to test the junctions in diode mode.
• 😀 MOSFETs can be tested by briefly shorting the terminals and then checking resistance between the drain and source.
• 😀 Diode mode can help identify LED colors based on their forward voltage when biased correctly.
• 😀 Verify the orientation and color of surface-mount LEDs using the DMM's diode mode to test the forward voltage.
• 😀 To measure a Zener diode's breakdown voltage, use a power supply and a 1k resistor to reverse bias the diode.
• 😀 Use grabbers with banana plugs for hands-free measurements, particularly for ground connections.
• 😀 The probe holder on the back of most DMMs can help keep the probes in place, preventing accidental contact during measurements.
• 😀 DMMs alone may not be sufficient for dynamic measurements of BJTs or FETs, but they provide useful first-order tests for component health.

Q & A

What is the main purpose of a Digital Multimeter (DMM)?

A Digital Multimeter (DMM) is a versatile electronic tool used to measure various electrical parameters like voltage, current, and resistance, among others.

Why should you always move the lead back to the voltage terminal after measuring current?

When measuring current, the DMM's fuse protects the internal shunt resistor from overcurrent. Leaving the lead in the current terminal can lead to a blown fuse, which could damage the meter.

How can you test if the fuse in a DMM is working?

To test the fuse, switch the DMM to resistance mode and measure the resistance across the current terminals. If the fuse is intact, the 10 amp terminal should show near zero ohms, while the 10 milliamp terminal should show high resistance.

What are the key steps to checking a transistor with a DMM in diode mode?

For an NPN transistor, place the positive lead on the base pin, then measure the voltage drop between the base and emitter, and base and collector, which should each show about 0.7V. Check for short circuits by measuring the resistance between the emitter and collector.

What should you do if you suspect a transistor is faulty?

If a transistor is suspected to be faulty, apply a test voltage and current, then measure the junctions with the DMM. A working transistor should show a proper voltage drop, while a faulty one will have significantly lower resistance across its junctions.

How does a DMM help in measuring the functionality of an in-channel MOSFET?

A DMM can be used to measure an in-channel MOSFET by first charging its gate with a small current to make the channel conductive. Afterward, the resistance between the drain and source will show a low value if the MOSFET is functional.

What is the significance of the gate terminal in a MOSFET when using a DMM?

The gate terminal in a MOSFET is a tiny capacitor. By charging it with a small current, the DMM can measure if the channel between the drain and source becomes conductive, helping to determine whether the MOSFET is functioning properly.

What is a common mistake when identifying an LED's polarity?

A common mistake is relying on the length of the LED legs to determine the anode and cathode. This is not a reliable method, as someone might have clipped the leads. The DMM's diode mode is a better method for identifying the LED's polarity.

How can a DMM help in determining the color of an LED?

By placing the LED in the correct biasing direction and using the DMM in diode mode, the LED will light up, and the DMM will display the forward voltage. This can be used to identify the LED's color, especially for clear-lensed LEDs.

How can you verify the breakdown voltage of a Zener diode using a DMM?

To measure a Zener diode's breakdown voltage, reverse bias the diode with a power supply, use a 1k resistor to limit the current, and increase the voltage. The DMM will show the voltage at which the Zener diode clamps, which is its breakdown voltage.