Lethal Voltages in Radio Equipment

M22A established that current, driven by voltage through your body's resistance, is what actually injures or kills. This lesson applies that framework to four specific places inside real ham radio equipment where the voltage is high enough that body resistance — wet or dry — provides nowhere near enough protection. These are not theoretical hazards: every one of them appears in equipment that is commonly owned, repaired, and homebrewed by radio amateurs.

A cabinet interlock switch cuts primary power the instant the cover is removed — never defeat or bypass this switch.

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High-Voltage Supplies in Tube Amplifiers

Vacuum tube linear amplifiers, still widely used by hams for HF and VHF high-power operation, require a high DC voltage on the tube's plate (anode) to accelerate electrons across the vacuum gap efficiently. Typical plate voltages for amateur HF amplifiers range from roughly 1,500 V to over 3,000 V DC, supplied by a dedicated high-voltage power transformer, a rectifier (commonly a full-wave bridge using high-voltage diodes), and a substantial bank of filter capacitors. This supply is not incidental to the design — it is the single most dangerous part of the entire radio station, full stop, and every experienced amplifier builder and repairer treats it with a level of caution well beyond anything else on the bench.

Because this current is easily capable of delivering a fatal shock through dry skin alone (the worked example below makes this explicit), commercial and well-designed homebrew amplifiers include a cabinet interlock switch: a mechanical switch, wired in series with the primary AC input, that is forced open the instant the top cover is removed, cutting power to the high-voltage transformer before the internal compartment becomes accessible. This is a genuine, load-bearing safety feature, not a regulatory formality.

CRT Anode Voltages in Older Test Equipment

Older analog oscilloscopes, and any vintage television or monitor a ham might still have on the bench, use a cathode ray tube (CRT) to display a trace. A CRT requires a very high DC voltage — commonly in the range of 10,000 to 25,000 V — applied to its anode to accelerate the electron beam toward the phosphor screen with enough energy to produce a visible trace. This voltage is generated internally from the line-voltage-derived supply by a flyback transformer and a voltage multiplier stage, meaning the CRT anode voltage is present even though the equipment plugs into an ordinary wall outlet, and it is generated and stored inside the chassis regardless of the supply voltage at the wall.

The CRT itself behaves like a capacitor: its conductive aquadag coating on the outside of the bulb and the internal anode form the two plates of a genuine capacitor, with the glass envelope as the dielectric. This means a CRT can retain a dangerous, lethal-level charge for hours, days, or even longer after the equipment has been unplugged and left untouched — a hazard that surprises many people who reasonably assume "unplugged means safe." Anyone servicing equipment with a CRT must safely discharge the anode (using an insulated high-voltage discharge probe with one end clipped to chassis ground) before working anywhere near the tube, every single time, regardless of how long the unit has been sitting unused.

Capacitor Bank Energy Storage

Both the tube amplifier supply above and many other high-power circuits rely on a substantial bank of filter capacitors to smooth the rectified DC and to supply brief surges of current during transmission. These capacitor banks store real, significant electrical energy — easily hundreds of joules in a large amplifier supply — and that energy does not vanish when the power switch is turned off. M22C covers the physics of stored capacitor energy and safe discharge procedures in full detail; the point to internalize here is that "the power is off" and "the equipment is safe to touch" are two completely different statements, and treating them as the same thing is one of the most common ways experienced technicians are injured.

Switching Power Supplies: Line Potential on Internal Parts

Modern switching power supplies (covered in Module 8) are compact and efficient, but many designs — particularly non-isolated or partially isolated topologies sometimes found in low-cost equipment — have internal components, including the primary-side heatsink, that sit at a DC potential derived directly from rectified line voltage relative to the chassis or earth ground. In a well-designed, fully enclosed commercial product this is not a hazard during normal use because the enclosure prevents any contact, but the moment you open the case for repair or modification, those internal points must be treated as fully energized whenever the unit is plugged in — and, per the capacitor discussion above, potentially energized for some time after unplugging as well, since SMPS designs also use bulk filter capacitors on their input stage.

A specific, common mistake is touching what looks like a "low voltage" heatsink on a switching supply board while it is plugged in, on the assumption that heatsinks are grounded chassis parts — on the primary side of a non-isolated design, that heatsink can be sitting at a significant fraction of the rectified line voltage. Always verify with a meter, referenced to true earth ground, before assuming any internal metal part of a switching supply is at a safe potential.

Reference: Lethal Voltage Points by Equipment Type

Worked Example: Why "Dry Skin" Does Not Save You at 2,500 V

Safe Approach and Emergency Response

If a shock accident occurs around this kind of equipment, the same emergency procedure from M22A applies, with one addition: do not assume the area is now safe simply because the person has been freed from contact. A capacitor bank or CRT in the same compartment may still be charged and may still pose a hazard to a rescuer reaching back in to assist, so cut all power at the source (the wall outlet or breaker) before anyone re-approaches the equipment itself, then proceed with calling 911 and administering first aid to the victim as described in M22A.