Fault Detection
Fault detection! That's the latest addition to my ongoing QRP transceiver project. After too much time spent replacing fried transistors, I decided to devise a circuit that would protect the radio from less than ideal operating conditions.
This circuit is essentially a series of comparators, fed into a latch, which in turn switches the power to the transmit amplifier. The latch circuit can be reset manually, or on power cycling.
The fault conditions being monitored are excessive input voltage, PA drain current, and antenna SWR. The SWR input comes from the directional coupler bridge at the antenna input. The voltage monitor is a simple buffered voltage divider. The drain current sensor is a shunt resistor amplified by an op-amp. Each parameter can be set with trimmer resistors.
The latch is a basic CMOS logic IC with multiple set and reset inputs. The output of this latch IC drives a P-channel FET in-line with the PA supply, as well as an indicator LED.
A very basic circuit, but a welcome one in a rig likely to be exposed to non-laboratory conditions on a regular basis. Sure, this could all be digitized, but doing it with basic analog parts and some simple transistor logic was pleasant - no writing code, no hunting for the right microcontroller... etc.
That's all, just thought I'd share the fun. Dit Dit.
2026-05-25