Electrical transformers probably weren’t on the minds of the comic artists and TV producers in 1984. It’s likely pure coincidence that Transformers actually have a lot in common with electrical transformers. Yes, the Transformers are cartoon robots in disguise, but consider; they come in all shapes and sizes, and can perform all kinds of functions. Real-life electrical transformers are also powerful, multi-purpose devices. And as everyone who works in reverse engineering knows, there’s a lot more than meets the eye.
Before getting into reverse engineering though, let’s start with an overview of these electrical devices.
When people think of transformers, what might come to mind are the big metal boxes out on the street or beside large buildings. But there are transformers much closer than that, for example, the ones that are found in wall chargers for our phones and laptops.
There are many types of transformers, of all different shapes and sizes, and visually it can be difficult to see why they all get grouped together. The common thread is that transformers all use the fundamental relation between electricity and magnetism. A changing electric field on the primary coil creates a time-varying magnetic field. That magnetic field gets conducted through the core to the secondary side, causing an electric field to be created on the secondary coil. It allows electricity on one side to be passed over to the other side without any direct electrical connection between the two. No matter what type of transformer it is, this same basic principle applies.
Learning how to harness time-varying fields was a critical turning point in electrical engineering and quite literally reshaped the modern world, giving rise to the AC electric grid system around us all today. Those first transformers were what we would consider today to be power transformers. That’s one of the major types: power transformers that step up or step down AC voltage so it can be used by devices requiring a different voltage level. On PCBs, these power transformers are normally found in the power supply section of the circuit board.
Another type of transformer used frequently in electronic devices is a pulse transformer. These can be used in VFDs to create the isolated supplies for gating circuits, or in low power electronics to convey digital signals and provide electrical isolation. In practical reverse engineering terms, these pulse transformers can appear all over the board, but are usually grouped for input phases, output phases, or isolating signals passing from one power section of a PCB to another.
The third common type of transformer seen commonly in reverse engineering and PCB repair is the current transformer. These transformers are shaped to have a hole in the middle, and the primary AC current going through the center creates a secondary current on the output of the current transformer. These transformers are generally used in high power electronics, such as motor drives, to monitor input and output currents. Note that even though current transformers are toroidal in shape, they are not the same thing as a toroidal transformer (that designation refers to the shape of the magnetic core of the transformer, and is more about the construction than the application). It can also be difficult to differentiate between a true current transformer and a current sensor that looks similar but has different operation (most commonly a hall effect sensor).
In real-world PCB reverse engineering, transformers aren’t all clearly spelled out on the board. But with some knowledge of transformer applications, and a little hands-on experience, it’s easy to identify common topologies. Let’s go through an example identifying transformers in a circuit.
This is a power board and gate driver board from a 3-phase input / 3-phase output AC motor drive. Three areas in the photo have been labelled, to make it easier to follow along.
Area 1 has a classic power supply transformer; these yellow-wrapped, laminated-core, often multi-coil transformers are extremely common to see. Notice that there are lots of capacitors nearby and that it is located close to the 3-phase power input. These are also signs that the transformer is being used for low voltage power supply. Many AC drives require only high voltage AC inputs: 120VAC, 240VAC, 480VAC or 600VAC are the most common in North America. But inside these drives there are electronics which require DC voltages at much lower levels: +/- 15V, 24V, 5V, and 3.3V, again to name some of the most common for ICs. One way to generate these is to use one of the input phases on the primary of a small step-down transformer, like this one in the photo. It may have multiple output coils, transforming that single phase high voltage AC into several low AC voltages. Those low AC voltages are then rectified using diodes and smoothed using capacitors to create the DC voltages needed for the rest of the control electronics.
Now, moving on to the device labelled 2, it would be very easy to mistake this for a toroidal transformer. Yes, it is a wire-wrapped magnetic core, but no, it is not functioning as a transformer. It is an inductive filtering device for the 3-phase high voltage lines.
Finally, the area of the board labelled 3. It would also be pretty easy to miss the three small yellow transformers in this section of the board, but they play a crucial role. These transformers are being used for low voltage power supplies, but a very specific application: these are the isolated power supplies for the low voltage electronics which control the gates for the output of the drive. They function similarly to transformer 1, where the resulting AC voltage is rectified and filtered across some capacitors to generate a smooth DC voltage. However, their main purpose is not to step voltage up or down, but to pass it through and in doing so provide isolation from the low voltages used in the control section of the board to the floating output phases of the drive.
Why is the isolation necessary? Consider the AC waveforms on the outputs phases of the drive oscillate up and down hundreds of volts as they drive a motor. The voltages on the pins of the ICs that control the gates must also be free to travel up and down hundreds of volts along with the outputs (think of these ICs as floating at a fixed voltage, e.g. +15V, above the output voltage wave). They also must be independent from each of the other output phases, which are going up and down 120 degrees out of phase.
If the gating ICs were fixed with respect to a common ground, the high-voltage AC drive outputs would blow up the gating circuitry. For this reason they cannot share any electrical connections with the rest of the board: no connection to 5V, or Gnd, or any of the other main control voltages generated by transformer 1. That’s why these three little transformers in section 3 are needed to provide each of the phases with its own independent, isolated power. Despite their small size, they’re performing a very mighty task.
Reverse engineering the transformers used in PCBs doesn’t stop with identifying their function in the circuit: in addition, they have to be analyzed based on number of coils, topology, polarity, turns ratios, power transference, maximum ratings, and physical footprint. But in reverse engineering, the most important step is the first one: understanding the original components and their functions. And when it comes to transformers, there’s certainly more than meets the eye.
Transformers can be found in all kinds of electronics, and we can be found at firstname.lastname@example.org . ENA specializes in reverse engineering and electronic circuit board repair, please contact us directly for your electronics needs in industrial, renewable energy, or reverse engineering applications.