Version 1.40 (2-Feb-10)
Copyright © 1994-2013 Samuel M. Goldwasser — All Rights Reserved — For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.
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Table of Contents
- Safety Guidelines
- Safety Tests for Leakage Current on Repaired Equipment
Consumer electronic equipment like TVs, computer monitors, microwave ovens, and electronic flash units, use voltages at power levels that are potentially lethal therefore are expected to be handle with care. Even more so for industrial equipment like lasers and anything else that is either connected to the power line or uses or generates high voltage.
Normally, these devices are safely enclosed to prevent accidental contact. However, when troubleshooting, testing, making adjustments, and during repair procedures, the cabinet will likely be open and/or safety interlocks may be defeated. Home-built or modified equipment, despite all warnings and recommendations to the contrary – could exist in this state for extended periods of time – or indefinitely.
Depending on overall conditions and your general state of health, there is a wide variation of voltage, current, and total energy levels that can kill.
Microwave ovens in particular are probably THE most dangerous household appliance to service. There is high voltage – up to 5,000 V or more – at high current – more than an amp may be available momentarily. This is an instantly lethal combination.
TVs and monitors may have up to 35 kV on the CRT but the current is low – a couple of milliamps. However, the CRT capacitance can hold a painful charge for a long time. In addition, portions of the circuitry of TVs and monitors as well as all other devices that plug into the wall socket are line connected. This is actually more dangerous than the high voltage due to the greater current available – and a few hundred volts can make you just as dead as 35 kV!
Electronic flash units and strobe lights, and pulsed lasers have large energy storage capacitors which alone can deliver a lethal charge – long after the power has been removed. This applies to some extent even to those little disposable pocket cameras with flash which look so innocent being powered from a single 1.5 V AA battery. Don’t be fooled – they are designed without any bleeder so the flash can be ready for use without draining the battery!
Even some portions of apparently harmless devices like VCRs and CD players – or vacuum cleaners and toasters – can be hazardous (though the live parts may be insulated or protected – but don’t count on it!
This information also applies when working on other high voltage or line connected devices like Tesla Coils, Jacobs Ladders, plasma spheres, giga watt lasers, hot and cold fusion generators, cyclotrons and other particle accelerators, as well as other popular hobby type projects. 🙂
In addition, read the relevant sections of the document for your particular equipment for additional electrical safety considerations as well as non-electrical hazards like microwave radiation or laser light. Only the most common types of equipment are discussed in the safety guidelines, below.
These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage.
Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in may cause collateral damage. There are likely to be many sharp edges and points inside from various things like stamped sheet metal shields and the cut ends of component leads on the solder side of printed wiring boards in this type of equipment. In addition, the reflex may result in contact with other electrically live parts and further unfortunate consequences.
The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe!
- Don’t work alone – in the event of an emergency another person’s presence may be essential.
- Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system.
- Wear rubber bottom shoes or sneakers. An insulated floor is better than metal or bare concrete but this may be outside of your control. A rubber mat should be an acceptable substitute but a carpet, not matter how thick, may not be a particularly good insulator.
- Wear eye protection – large plastic lenses eyeglasses or safety goggles.
- Don’t wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts.
- Set up your work area away from possible grounds that you may accidentally contact.
- Have a fire extinguisher rated for electrical fires readily accessible in a location that won’t get blocked should something burst into flames.
- Use a dust mask when cleaning inside electronic equipment and appliances, particularly TVs, monitors, vacuum cleaners, and other dust collectors.
- Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Microwave ovens use the chassis as ground return for the high voltage. In addition, do not assume that the chassis is a suitable ground for your test equipment!
- If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in place with string or electrical tape. Prop them up with insulation sticks – plastic or wood.
- If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 2 W or greater resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor, use a 20K to 100K ohm resistor). Monitor while discharging and/or verify that there is no residual charge with a suitable voltmeter. In a TV or monitor, if you are removing the high voltage connection to the CRT (to replace the fly back transformer for example) first discharge the CRT contact (under the insulating cup at the end of the fat red wire). Use a 1M to 10M ohm 1W or greater wattage resistor on the end of an insulating stick or the probe of a high voltage meter. Discharge to the metal frame which is connected to the outside of the CRT.
- For TVs and monitors in particular, there is the additional danger of CRT implosion – take care not to bang the CRT envelope with your tools. An implosion will scatter shards of glass at high velocity in every direction. There is several tons of force attempting to crush the typical CRT. Always wear eye protection. While the actual chance of a violent implosion is relatively small, why take chances? (However, breaking the relatively fragile neck off the CRT WILL be embarrassing at the very least.)
- Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations.
- If you must probe live, put electrical tape over all but the last 1/16″ of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand.
- Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter.
- Provide a reliable means of warning that power is applied and that high voltage filter capacitor(s) still hold a charge during servicing. For example, solder a neon indicator lamp (e.g., an NE2 in series with a 100K ohm resistor) across the line input and a super high brightness LEDs in series with 100K, 1 W resistor across the main filter capacitor(s).
- Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac ™ (variable autotransformer) is not an isolation transformer! However, the combination of a Variac and isolation transformer maintains the safety benefits and is a very versatile device. See the document “Repair Briefs, An Introduction”, available at this site, for more details.
- The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but may not protect you from shock from many points in a line connected TV or monitor, or the high voltage side of a microwave oven, for example. (Note however, that, a GFCI may nuisance trip at power-on or at other random times due to leakage paths (like your scope probe ground) or the highly capacitive or inductive input characteristics of line powered equipment.) A GFCI is also a relatively complex active device which may not be designed for repeated tripping – you are depending on some action to be taken (and bad things happen if it doesn’t!) – unlike the passive nature of an isolation transformer. A fuse or circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment. However, these devices may save your scope probe ground wire should you accidentally connect it to a live chassis.
- When handling static sensitive components, an anti-static wrist strap is recommended. However, it should be constructed of high resistance materials with a high resistance path between you and the chassis (greater than 100K ohms). Never use metallic conductors as you would then become an excellent path to ground for line current or risk amputating your hand at the wrist when you accidentally contacted that 1000 A welder supply!
- Don’t attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool – deductive reasoning – will not be operating at full capacity.
- Finally, never assume anything without checking it out for yourself! Don’t take shortcuts!
Safety Tests for Leakage Current on Repaired Equipment
It is always essential to test AFTER any repairs to assure that no accessible parts of the equipment have inadvertently been shorted to a Hot wire or live point in the power supply. In addition to incorrect rewiring, this could result from a faulty part, solder splash, or kinked wire insulation.
There are two sets of tests:
- DC leakage: Use a multi-meter on the highest OHMS range to measure the resistance between the Hot/Neutral prongs of the wall plug (shorted together and with the power switch on where one exists) to ALL exposed metal parts of the equipment including metallic trim, knobs, connector shells and shields, VHF and UHF antenna connections, etc.
This resistance must not be less than 1 M ohm.
- AC leakage: Connect a 1.5K ohm, 10 Watt resistor in parallel with a 0.15 uF, 150V capacitor to act as a load. Attach this combination between the probes of your multi-meter. With the equipment powered up, check between a known earth ground and each exposed metal part of the equipment as above.
WARNING: Take care not to touch anything until you have confirmed that the leakage is acceptable – you could have a shocking experience!
The potential measured for any exposed metal surface must not exceed 0.75 V. This corresponds to a maximum leakage current of 0.5 mA.
Note: A true RMS reading multi-meter should be used for this test, especially where the equipment uses a switch mode power supply which may result in very non-sinusoidal leakage current.
If the equipment fails either of these tests, the fault MUST be found and corrected before putting it back in service (even if you are doing this for your in-laws!).
Checking for correct hookup of the Hot, Neutral, and Ground wires to the AC plug should also be standard procedure. There’s no telling how it may have been scrambled during a previous attempt at repair by someone who didn’t know any better or by accident. Unlike logic circuits, black is NOT the standard color for ground in electric wiring! 🙂