Lethal Voltages in Electronic Equipment

Electrical Shock Hazards established that current, driven by voltage through your body's resistance, is what actually injures or kills. This lesson applies that framework to specific places inside real electronic 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 modified by hobbyists.

High-voltage sections inside a valve amplifier. The filter capacitors can hold a lethal charge long after the power is switched off; bleeder resistors and lid interlocks are there to protect you.

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High-Voltage Supplies in Valve Amplifiers and Microwave Ovens

Valve (vacuum tube) amplifiers, still popular for guitar and hi-fi use, need a high DC voltage on the valve's plate (anode) to accelerate electrons across the vacuum gap. Typical plate voltages are 300 V to 500 V DC in guitar and hi-fi amplifiers, supplied by a high-voltage transformer winding, a rectifier, and a bank of filter capacitors. The high-voltage circuit of a microwave oven goes much further: a transformer, a high-voltage capacitor and a diode produce around 2,000 V or more for the magnetron, from a transformer able to deliver a large current. These supplies are not incidental to the design — they are the most dangerous parts of equipment a hobbyist is likely to open, and every experienced repairer treats them with a level of caution well beyond anything else on the bench.

Because these supplies can easily deliver a fatal shock through dry skin alone (the worked example below makes this explicit), well-designed equipment includes safety interlocks: a microwave oven's door interlocks cut the high-voltage circuit the instant the door opens, and some high-voltage equipment has a cover interlock wired in series with the mains input. These are genuine, load-bearing safety features, not regulatory formalities.

CRT Anode Voltages in Older Test Equipment

Older analog oscilloscopes, and any vintage television or monitor you 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 valve 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. These capacitor banks store real, significant electrical energy — easily tens to hundreds of joules in a large amplifier or power supply — and that energy does not vanish when the power switch is turned off. Capacitor Discharge Danger 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.

Switching Power Supplies: Line Potential on Internal Parts

Modern switching power supplies (covered in the Power Supplies module) 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 the Electrical Shock Hazards lesson 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 the Electrical Shock Hazards lesson.