⚡ electrical engineering

A Low Cost Discrete Digital Isolator Circuit

This paper presents a low-cost, fully discrete digital isolator built with general-purpose transistors and an air-core transformer that achieves over 1 kV isolation, ~200 ns propagation delay, and 1 Mbps data rates while eliminating reliance on specialized ICs to mitigate obsolescence and vendor lock-in risks.

Original authors: Thomas Conway

Published 2026-03-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Thomas Conway

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you need to send a secret message from one room to another, but there's a massive, electrified moat between them. You can't throw a rope (wires) across because it would get shocked, and you can't shout (sound waves) because the walls are too thick.

This paper presents a clever, low-tech solution to cross that moat: a "wireless" bridge built right into the floorboards (the circuit board) using nothing but common, cheap parts.

Here is the breakdown of how this "Low Cost Discrete Digital Isolator" works, using everyday analogies.

1. The Problem: The "Obsolescence" Trap

Usually, when engineers need to cross that electrified moat, they buy a special, expensive "magic box" (a specialized chip) from a specific company.

The Risk: What if that company goes out of business? Or stops making that specific box? Suddenly, your product is dead because you can't find the part.
The Solution: This paper says, "Let's stop buying magic boxes." Instead, let's build the bridge ourselves using generic transistors (the electronic equivalent of standard screws or nails) that have been around for 40 years and will be around for 40 more. No one can stop making these.

2. The Bridge: A "Ghost" Transformer

The core of this design is a transformer (a device that transfers energy without wires).

The Old Way: Transformers usually need a heavy iron core and copper wire wrapped around it.
The New Way: The author built the transformer using spiral tracks drawn directly onto the circuit board (like drawing a spiral on a piece of paper).
How it works: Imagine two people standing on opposite sides of a glass wall. One person shakes a spring (the transmitter). The vibration travels through the glass (the air-core transformer) and shakes a second spring on the other side (the receiver).
The Safety: The "glass wall" here is the circuit board itself. It's thick enough to stop high voltage (over 1,000 volts) from jumping across, keeping the two sides safe from each other.

3. The Message: A Light Switch Game

How do we send data (0s and 1s) across this bridge?

The Transmitter (The Sender): Think of this as a light switch.
- Logic 1 (ON): You flip the switch. A tiny, super-fast oscillator (a buzzing bee) starts humming at 15 million times a second. This vibration travels through the "glass wall."
- Logic 0 (OFF): You turn the switch off. The buzzing stops. Silence.
The Receiver (The Listener): On the other side, there is a microphone (the receiver circuit) listening for that buzz.
- If it hears the buzz, it knows "1" is coming.
- If it hears silence, it knows "0" is coming.
The Magic Filter: The receiver is designed so that it ignores the tiny "static" when the buzz starts or stops. It only cares about the steady hum. This ensures the message is clear and doesn't get garbled.

4. The Special Trick: The "Dead Man's Switch"

The paper also describes a super-cool application for controlling power switches (like in electric cars or solar inverters).

The Scenario: You have two switches, a "High Side" and a "Low Side." If you turn both on at the same time, it causes a massive short circuit (a boom!). You need a system that physically cannot let both be on together.
The Solution: The author built a "tug-of-war" transmitter.
- If you pull the rope to the Left, the High Side turns on, and the Low Side is physically locked off.
- If you pull the rope to the Right, the Low Side turns on, and the High Side is locked off.
- If you let go (or pull both ways equally), neither turns on.
Why it's great: It's a hardware "safety lock." Even if the computer controlling it crashes, the circuit physically prevents the explosion.

5. The Results: Fast, Cheap, and Reliable

Speed: It can send data at 1 million bits per second. That's fast enough for most communication needs (like talking to a computer or a sensor).
Cost: It costs about 50 cents to build, including the board.
Safety: It can handle over 1,000 volts of isolation.
Simplicity: It uses no fancy, expensive chips. Just standard transistors and a board with some squiggly lines drawn on it.

The Bottom Line

This paper is like a DIY guide for building a bulletproof, future-proof communication bridge. Instead of relying on a single supplier for a complex part that might disappear, it shows how to build a robust, high-voltage isolator using the electronic equivalent of "duct tape and baling wire"—but in this case, it's actually high-tech, reliable, and incredibly cheap.

It proves that sometimes, the best way to solve a high-tech problem is to go back to basics and build it yourself with parts that will never go out of style.

Based on the paper "A Low Cost Discrete Digital Isolator Circuit" by Thomas Conway, here is a detailed technical summary covering the problem, methodology, contributions, results, and significance.

1. Problem Statement

The paper addresses the challenges associated with component obsolescence and vendor lock-in in digital isolation applications.

Context: While commercial integrated circuits (ICs) like opto-isolators, capacitive couplers, and micro-transformers are standard, they pose risks for long-lifetime products or low-volume production due to potential discontinuation and long lead times.
Need: There is a demand for a discrete digital isolation solution that relies on ubiquitous, general-purpose components (specifically transistors) which have multiple sources and are unlikely to be discontinued, ensuring supply chain stability for critical applications.

2. Methodology

The proposed solution is a fully discrete, low-cost digital isolator that eliminates the need for specialized isolation ICs. The design relies on magnetic coupling via a custom PCB transformer and standard logic-level transistors.

Core Architecture:
- Transmitter (Primary Side): Utilizes a cross-coupled MOSFET pair (2N7002) to create a negative resistance oscillator. It employs an LC resonant circuit where the inductor ( $L$ ) is a PCB trace, and the capacitance ( $C$ ) is derived from the parasitic capacitance of the MOSFETs. The circuit uses On-Off Keying (OOK) modulation: a logic '1' powers the oscillator (generating high-frequency AC), while a logic '0' leaves it unpowered.
- Receiver (Secondary Side): Uses a full-wave rectification and amplification stage built from general-purpose NPN transistors (MMBT3904). The induced voltage from the coupled inductor is rectified. Crucially, the slow recovery from saturation of the bipolar transistors (approx. 100ns) acts as a built-in low-pass filter, smoothing the rectified signal into a stable logic output without requiring additional filtering components.
- Transformer Implementation: The galvanic isolation is achieved using air-core transformers formed by spiral copper traces on opposite sides of a standard 1.6mm FR4 PCB. The FR4 dielectric strength (>20 kV/mm) provides the necessary voltage isolation (>1 kV) without high-voltage components.
- PCB Layout: The design uses Surface Mount Device (SMD) components (SOT23 transistors, 2010/1206 resistors) placed on the top and bottom layers. The layout is optimized to bridge connections across the coil without vias, maintaining isolation integrity.
Advanced Application (Half-Bridge Driver):
- The paper extends the concept to a half-bridge gate driver application. By combining two transmitter oscillators into a single circuit with specific transistor polarities, the design creates an inherent hardware lockout.
- The circuit ensures that the high-side and low-side drivers cannot be active simultaneously. If both inputs are high or both are low, the circuit remains unpowered, preventing shoot-through conditions.

3. Key Contributions

Discrete Implementation: A novel circuit topology that achieves digital isolation using only generic transistors and PCB traces, removing reliance on proprietary isolation ICs.
PCB Air-Core Transformer: Demonstrates the feasibility of using standard FR4 PCBs with spiral trace coils as effective high-frequency transformers for galvanic isolation, leveraging the board material for voltage breakdown strength.
Intrinsic Hardware Lockout: The half-bridge configuration provides a fail-safe mechanism to prevent simultaneous conduction of high/low-side switches without complex control logic.
Supply Chain Resilience: The design explicitly targets components with 40+ years of production history, mitigating obsolescence risks for critical, long-life systems.

4. Results

The prototype was built and tested on a 2-layer FR4 PCB with the following performance metrics:

Isolation Voltage: > 1 kV (validated by the breakdown strength of the 1.6mm FR4).
Data Rate: Validated Non-Return-to-Zero (NRZ) data rates of 1 Mbps.
Propagation Delay: Approximately 200 ns, which is sufficient for UART, SPI, and modest power electronics switching frequencies.
Power Consumption:
- Transmitter: Max 25 mW.
- Receiver: Max 5 mW.
Eye Diagram: Clear eye diagrams confirmed robust signal integrity at 1 Mbps.
Half-Bridge Performance: Measured waveforms showed clean logic transitions with a natural dead time of ~300 ns, suitable for IGBT half-bridge applications.
Cost: Estimated at approximately 50 cents per unit (excluding PCB fabrication costs), making it highly cost-effective.

5. Significance

This work is significant for the electronics industry, particularly in sectors requiring long-term reliability and supply chain security (e.g., industrial automation, medical devices, and aerospace).

Obsolescence Mitigation: It offers a viable alternative to single-source ICs, ensuring that critical systems can be maintained for decades without redesigning the isolation stage due to part discontinuation.
Design Flexibility: By using standard components and PCB processes, engineers can customize isolation parameters (frequency, size) without being constrained by the fixed specifications of commercial isolator ICs.
Cost and Complexity: It reduces the Bill of Materials (BOM) cost and complexity by eliminating specialized chips, relying instead on the PCB layout itself to perform the isolation function.

In conclusion, the paper successfully demonstrates that high-performance digital isolation can be achieved through simple, discrete, and widely available components, providing a robust solution for applications where component longevity and supply chain independence are paramount.