This is the second in a two-part series looking at safety when experimenting with mains-voltage electronic equipment, including the voltages you might find derived from a mains supply but not extending to multi-kilovolt EHT except in passing. In the first part we looked at the safety aspects of your bench, protecting yourself from the mains supply, ensuring your tools and instruments are adequate for the voltages in hand, and finally with your mental approach to a piece of high-voltage equipment.
The mental part is the hard part, because that involves knowing a lot about the inner life of the mains-voltage design. So in this second article on mains voltages, we’ll look into where the higher voltages live inside consumer electronics.
The Power Supply
When the mains supply enters a piece of equipment, unless it is a very simple device such as an electric kettle that uses the mains-voltage AC as-is, it’s going to do something to that AC to turn it into more useful voltages. We’ll now consider the most common types of mains power supply you may encounter when you open a piece of equipment, to aid you in identifying where the hazardous circuits will lie.
Look at where the power enters the device. If it passes straight into a transformer from which a low voltage is derived, then you have little to worry about except for any fuses or switch components on the mains side of the transformer which you will have to avoid when powered up. If the low-voltage side is at a battery-equivalent voltage, then the rest of the electronics that derive their power from it can be treated as though they were battery-powered. They are at a low voltage and isolated from the mains, you can safely work on them while they are powered up.
It is however not always safe to assume that equipment with a transformer power supply will contain safe low voltages in the rest of its circuit. If for example your tastes run to vintage tube-based equipment then you’ll find several-hundred-volt HT supplies which though they will be isolated from the mains by the transformer should still be considered as just as dangerous as the mains supply.
In previous decades it was the norm for mains power to be converted to low voltage in an iron-cored transformer. More recent equipment though is more likely to use a switch-mode power supply in which the AC mains is rectified to a high DC voltage and sent through a much smaller and lighter ferrite-cored transformer at a much higher frequency. Switch-mode power supplies are lightweight and efficient, but from the point of view of this article they introduce more hazards as they contain significant mains-derived DC voltages.
The output side of a switch-mode power supply is isolated from the mains by the transformer and at a low voltage and thus safe to work on. The mains side will contain both a reservoir capacitor with a high DC voltage and a heatsink for the switching transistor which will be at a high voltage. If you look at the PCB of a well-designed switch-mode power supply it should have a clear demarkation between its high and low voltage sides, with a safe boundary distance between the two sides bridged only by the transformer and an optocoupler or small transformer for feedback. Unfortunately though not all such power supplies are so well designed and you may even find ones that would fail the most basic safety tests.
The good news is that switch-mode supplies are often off-the-shelf items that occupy their own circuit board, so any low-voltage logic boards they supply will be isolated from the mains and thus safe to work on while powered up. In the mid-2000s set-top-box picture above it is only the brown board that contains mains voltage, the green logic board is entirely low-voltage and safe to work on.
There is a third type of mains-derived power supply that you may occasionally encounter, one in which the mains is rectified directly to high-voltage DC and any lower voltages are derived without a transformer either through a regulator circuit or a potential divider. Some LED light bulbs, older CRT TV sets, and some older tube radios get their power this way, for example.
You especially need to be aware if you are working on this type of equipment, because every part of its circuit has to be considered to be at mains potential when it is powered up. You will see TV sets powered in this way referred to as “Live chassis”, and you will often find warning labels on their back covers or on the chassis itself. Any equipment that uses this power supply set-up will have extra insulation, and probably very few, well-insulated, external connections.
Now that you’ve identified what parts of your piece of equipment are live, you can tailor how you approach working on it with respect to high voltage safety.
If your interest lies only with low voltage parts of an item in which the live parts of the power supply are clearly separate and the power supply is an isolating one, then your job is easy, simply take extreme care to avoid touching those high voltage parts, exercise caution while it is powered up, and work on the low voltage parts as you normally would. The fuses next to the toroidal transformer or the brown switch-mode power supply PCB in the set-top-boxes pictured here should be avoided but you can prod away at will on their the low-voltage sides with your multimeter.
If however the piece of equipment you are working on is live at the point you are working on it, perhaps a switch-mode power supply or a live-chassis TV set, you will have to take a different tack. It is best to describe the approach you should be taking as “exaggerated caution”. Try to keep the device unplugged as much of the time as possible. Set up your probes for a measurement with the device unplugged, and only power it up to make a measurement or observation before powering it down again. It was not uncommon for TV repair engineers to work on live chassis sets with one hand behind their back, to avoid the possibility of a hand-to-hand shock.
Even unplugging the device is not foolproof. It is worth remembering that significant charge may remain in any large electrolytic capacitors in a piece of high-voltage equipment after its power has been removed. If you are examining these circuits when you have powered them down it is important to discharge these capacitors safely beforehand to avoid a shock. It is suggested that you always check for any charge remaining in a capacitor with your multimeter, and though you can spectacularly discharge them with a screwdriver across their terminals it is probably a bit safer to do so through a resistor connected to a set of test probes. Measure the voltage to ensure that the capacitor has safely discharged, and be prepared for a surprise as with some capacitors the voltage will return after discharge due to an unwanted property of large electrolytics. Repeat the discharge until the voltage is gone.
Really High Voltages
We mentioned earlier in this series that we’d look at multi-kilovolt EHT in passing, though it is a separate subject with an entirely fresh set of safety concerns that demands another article. Suffice to say that you should not be powering up a microwave oven or a radar set with the lid off. But still, it is likely that you may encounter such voltages in a CRT TV set or monitor so we should talk about that application here.
It is important to state immediately that the final anode voltage on a CRT is likely to be somewhere over 20 KV depending on the tube, and can be considered not merely dangerous but lethal. The glass funnel behind the screen of a CRT forms the reservoir capacitor for this EHT power supply, and can hold that 20 KV charge for long after the TV or monitor has been powered down. So you have to treat these devices with a lot of respect, for they can kill you.
That said, this should not stop you working on a device with a CRT in it. The designers of CRT circuits were aware of the hazards, and made sure that those EHT voltages were safely locked away where they were very difficult to touch. Take a look at the picture above showing the monitor components of a Mac Plus, the EHT circuit is the red cable and big round red sucker on the side of the CRT in the foreground. All the high voltage is safely contained behind that red insulation, and if you do not attempt to remove that connector from the side of the CRT it will remain there in a perfectly safe condition while you look at the other parts of the device.
In the unlikely event that you find yourself needing to remove a monitor or TV chassis from its CRT it is quite straightforward to discharge the CRT anode capacitor, however it is a procedure that must be performed with some care. Your aim is to discharge that anode connector to the black outer coating of the tube, known as the aquadag. To do this you will need to find a large flat-blade screwdriver with the thickest plastic handle you can. That handle is going to protect you from over 20KV for a short time.
You will need to connect the metal part of the screwdriver with a piece of insulated wire to the earth connection to the tube which is normally somewhere on the metal band that goes round the edge of its face. This is probably best achieved with crocodile clips on the end of your piece of wire. Then you simply hold the screwdriver by its plastic handle, and gently slide its tip under the rubber sucker covering the anode connector until you hear the “crack!” sound of it discharging. Then to ensure it’s really discharged, wait a few minutes and repeat the procedure, repeating it again after you’ve removed the connector. (Images from [Ax0n]’ CRT tutorial.)
The aim of these articles has not been to cover every possible electrical hazard in minute detail, but to equip you with some of the caution required and some of the skills to assess what risks lie in front of you when you open up a piece of high voltage equipment. However it is important to bear in mind that the decisions lie with you There is no dishonour in deciding that the risk in front of you is too great and backing away. Indeed it is healthy caution that keeps engineers alive to collect their pensions. We hope though that when you do decide to proceed with a piece of mains-powered kit, at least you too will now have a better chance of making it that far. Good luck!