All You Need To Know
I'm working through The ALL YOU NEED TO KNOW series. These are my notes.
There is a Learn Electronics Repair Discord server.
For later viewing:
- The Art Of Methodical Fault Finding - A Practical Example
- Learn electronics Repair #14 - Circuits and Components for Beginners 1. What Is Electricity?
- Convert Your MULTIMETER into an accurate SHORT CIRCUIT TRACER Finder for less than $1 Build This DIY
In circuits resistors are marked with 'R'.
- 1R0 = 1.0 Ohm
- 1K0 = 1.0 K Ohm
- 1M0 = 1.0 M Ohm
Three digit code: value value multiplier Four digit code: value value value multiplier
The reason for connecting a bunch of lower value resistors in series is to provide an aggregate resistance which can handle a higher current.
With SMD components:
- if they look black they are almost certainly resistors (but could be capacitors)
- if they look brown they are almost certainly capacitors (probably not resistors)
- if they look white they are usually capacitors
- if they look green they are usually resistors
In circuits capacitors are marked with 'C'.
Be careful with capacitors! Use a resistor to discharge them before working on them.
- capacitance (Farads)
- voltage (volts, this is a limit)
- effective series resistance (ESR) (Ohms)
1,000 pF = 1 nF 1,000 nF = 1 μF
Three main uses for capacitors:
- filtering (coupling)
- smoothing (decoupling)
- capacitors in parallel: double (100 + 100 = 200)
- capacitors in series: half (100 + 100 = 50)
Ripple current can cause a capacitor to fail, it can literally boil the electrolyte.
Three failure modes of caps:
- short (burns, explodes), usually happens on surface mount capacitors, Multi Layer Ceramic Capacitors (MLCC), the changing temperature can cause them to crack, also mechanical stress (being dropped) can cause fracture
- leaky (passes DC current), usually happens to through-hole ceramic caps
- low capacitance, high ESR, usually happens to electrolytic caps, they age, dry out (it's possible to recondition them, but he didn't go into how)
In circuits inductors are marked with 'L'.
Explained Back EMF with water wheel analogy.
An inductor will pass DC and block AC.
Uses for inductors:
- as a filter to block AC current
- in Voltage Regulator Modules (VRMs)
- in buck/boost regulators
- buck decreases voltage
- boost increases voltage
- power factor correction
Anything with a coil inside it is effectively an inductor:
He noted that 'I' (for current) is from the word "intensity".
The inside of the inductor around which the wire is wound is called the "former", and it can be ferrite, cardboard, etc.
Inductors are simple and rarely fail. Failures can be from:
- physical damage (you can usually see this)
- it can lose enamel and short
- it can vibrate and the joints go bad
If you see a smoking inductor the problem isn't the inductor but too much current being passed through it. This will be a short circuit caused by something else.
If your inductors are getting hot it's not the inductor the problem is the capacitor on the low side (the output side).
Test an inductor with continuity test, if it's open circuit that's a problem, should be short circuit.
You can use an LCR meter for larger inductors.
Use an ESR meter for smaller inductors.
The symbol for reactance is XL.
XL = R = 2πfL = 628,000L.
You can decrease the inductance by wrapping another wire around the coil.
In circuits diodes are marked with 'D'. 'ZD' for Zener diodes.
Electricity (conventional electricity) flows from anode to cathode (black band is the cathode, like the bar on the diode symbol).
The property of a diode whereby AC is converted to DC is called "rectification".
There are all different types of diodes but this video is about rectifier diodes.
Three types of diodes:
- signal diodes (for very low currents)
- rectifier diodes (for power, high current)
- Schottky diode (a special type of rectifier)
When a diode is forward biased it is passing current. When a diode is reverse biased it is blocking current.
Voltage drop over a silicon signal/rectifier diodes from 0.5V to 0.7V. Germanium diodes around 0.2V. Schottky diode drops around 0.15V.
Before germanium diodes we had selenium diodes (50's, 60's) and prior to that thermionic diodes (vacuum tubes). There were also crystal diodes, a particular type of which were cats whistle diodes.
Two main ratings for diodes are:
- the voltage rating is how many volts it can block in reverse before it breaks down and passes current
- silicon diodes ~100 mA
- rectifier diodes 1-10 A
A diode also has a "recovery time" which is how fast it can turn on or off. You can get diodes called "fast rectifiers" (FR). Schottky diode are fast.
Typical applications for diodes include:
- ensuring correct polarity of power supply
- to drop voltage (~0.6V each)
- isolating input voltages
- joining 5V ATX and 3.3V CMOS battery
- for Back EMF on an inductor
He also discussed how a bridge rectifier works.
Three ways diodes can fail:
- goes short circuit (no voltage drop)
- goes open circuit (e.g. burn out)
- less common, with signal diodes, can become "leaky", like a resistor
You can get a diode matrix, with common cathode or common anode.
- if you have a faulty bridge rectifier and don't have suitable spare parts, you can always replace it with four separate diodes wired as a bridge circuit -- if they will physically fit
- likewise, you can replace four individual diodes in a bridge circuit with a bridge rectifier
- you can even use a bridge, and connect between one of the AC terminals and +ve to replace a single high current diode if you are desperate
- BAT54 (single diode, one pin unused)
- BAT54A (common anode)
- BAT54C (common cathode)
- BAT54S (serial)
They are not compatible. They are fast Schottky diodes. They come in three pin packages.
In circuits Zener diodes are marked with 'ZD'.
Also we will look at TVS diodes.
A diode passes current in one direction from Anode (A) to Cathode (K).
With a normal diode you usually pick a large break down voltage so that it never happens. With a Zener diode you rely on the break down voltage.
The main ratings for Zener diodes are:
- the Zener voltage (the break down voltage)
- the wattage (e.g. 400mW, 1W, 5W, etc)
Zener diodes are very inefficient voltage regulators so not often used for that purpose any more.
Surface mount Zener diodes look similar to the through-hole versions with metal caps on the end.
There are two ways to figure out if a diode is a Zener diode:
- you can look at the circuit and if the cathode is connected to positive it's most likely a Zener diode
- the marking 'ZD' printed on the circuit board
The only time you are likely to see a normal diode connected "backwards" in a circuit is when it is there to prevent back EMF from an inductor, or in buck and PFC circuits.
Zener diodes are used in two main ways:
- as a voltage reference
- as a voltage clamp (a safety feature)
Zener diodes fail in one of two ways:
- they go short
- they go open
They don't usually fail in a way where the Zener voltage changes.
You can use a multimeter to test for short circuit and open circuit. The slightly higher than normal forward voltage drop (~0.7V) may also help you identify a Zener diode front a rectifier diode.
To your multimeter a Zener diode looks like a normal diode and there's no way to measure the Zener voltage. To do that you need to connect the Zener diode to a voltage higher than the Zener voltage you want to test (say ~30V) in series with a resistor (390R and 470R are good choices) and then use your multimeter to measure the voltage across the Zener diode. That's the Zener voltage.
A TVS (Transient Voltage Suppressor) diode is a type of electronic component that is designed to protect sensitive electronics from transient voltage spikes or surges. These spikes can be caused by a variety of factors, such as lightning strikes, electrostatic discharge, or inductive switching.
TVS diodes work by providing a low impedance path to ground for voltage surges above a certain threshold. This threshold is called the breakdown voltage or clamping voltage of the TVS diode. When a voltage surge occurs, the TVS diode begins to conduct current, effectively shorting out the surge and protecting the downstream electronics from damage.
TVS diodes are typically used in circuit protection applications, such as in power supplies, data communication lines, and automotive electronics. They are available in a range of voltages and current ratings, and can be packaged in a variety of formats, such as surface mount, axial lead, or through-hole.
One important thing to note is that TVS diodes are designed to handle transient voltage spikes, but are not intended to provide protection against continuous overvoltage conditions. For this reason, it is important to select the appropriate TVS diode for the specific application and voltage requirements.
In circuits transistors are marked with 'Q'. In some older stuff marked as 'TR'.
Talking about bipolar transistors: NPN and PNP.
- B: Base
- C: Collector
- E: Emitter (with the arrow)
To a multimeter a transistor looks like two diodes: base-collector, base-emitter. The one with a higher voltage reading is the base-emitter junction.
If you think of NPN and negative-postive-negative and your red meter lead is positive you won't go far wrong.