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Stop those zaps double insulated If you have experienced mild tingles or shocks from double insulated equipment, you will want to know how to prevent them. Here’s our “Tingle Terminator”: a simple solution to this vexing problem. There is no need to gain access inside the equipment and it will not void the warranty. M odern double insulated equipment such as DVD players and digital set top boxes (STBs) can give you a mild electric shock. The experience is not unlike the static zap you sometimes get when alighting from a car in dry weather or from the metal hose of a vacuum cleaner. Such a shock may well reduce your confidence in the safety of the appliance even though there is no inherent fault. Whether or not you experience such a shock will depend on the installation. But perhaps we need to describe the problem first. These days most home entertainment equipment is double insulated. This includes most TV sets (CRT, plasma, LCD, projection, etc), VCRs, STBs, DVD players and amplifiers. This means they are powered from the mains using a 2-pin mains plug – one that does not include a mains earth connection. Normally though, such equipment has a metal case or some exposed metal in its construction. When double insulated, the case is left floating and its safety is reliant upon the insulation between the mains and the metal case. Double insulation does not necessarily mean there are only two layers of insulation. Often there are more than two insulation layers between the mains wiring and any exposed parts. For example, the mains lead is double insulated with a sheath of tubing over the two active and neutral wires. This lead passes through into the chassis using a plastic cord grip grommet that adds further isolation between the mains and the case. The connections inside are also made to ensure insulation is up to standard. Any appliance with Compare the difference in conventional (older) style power supplies (Fig.1, above) with the “switch mode” supplies found in the vast majority of modern electronics (Fig.2, right). In the older style supply, the entire power supply is isolated by the mains transformer but in the switch-mode supply, there are components on the primary side of the high frequency transformer. These can have small leakage currents which can give you a tingle. They’re not (in normal circumstances) dangerous. But they are certainly annoying! 92  Silicon Chip siliconchip.com.au from equipment! By JOHN CLARKE double insulation wiring is marked on the outside of the box with a double rectangular symbol. Older style double insulated items like VCRs and CD players used a 50Hz mains transformer. The transformer is wound on an insulating bobbin with complete isolation between the primary and secondary windings. This supply arrangement is shown in Fig.1. Thus there is no connection between the 240VAC mains supply and the low voltage power supply and so there is complete isolation. More recent double insulated equipment uses a switchmode power supply which is much more efficient and typically consumes much less power If it has this symbol on it (usually on the back) it is double insulated! when on standby. This dispenses with the 50Hz transformer in favour of a smaller, lighter and cheaper high frequency transformer. These highfrequency transformers have a core made of ferrite material rather than the iron used for the 50Hz transformers. This supply can then power a highfrequency driver circuit that switches Inside a modern DVD player. The switch-mode supply is on the brown PC board at the right of the unit – note the size of the high-frequency transformer compared to older conventional power supplies. Also note the interference suppression components close to where the mains voltage is applied. While essential, it’s these which cause the tingle problems. siliconchip.com.au July 2006  93 Here’s how we solved the tingle problem: a single mains (X2) rated capacitor, mounted inside a plastic box stuck on the back of the player. Fig.3 (below) shows the simple wiring necessary. It’s important to note that you are NOT interfering with the Active and Neutral mains wiring – in fact, you don’t even have to open up the DVD player (or other device) case. the primary of a high-frequency transformer at a very fast rate. The switching is typically several hundred kilohertz, much higher than the 50Hz mains frequency. The secondary output of the transformer is isolated from the primary via insulation between the primary and secondary windings. One problem with this type of supply is that it produces electromagnetic interference (EMI), caused by fast switching at the high frequencies. The switching produces harmonics that extend well into the radio spectrum. These must be attenuated in order to comply with EMI standards. This involves using filter circuits with capacitors connected from the switching supply to the output signal earth and the metal case. As you can see in Fig.2, some of this bypassing is on the 240VAC mains side of the supply. These include capacitors C1 and C2. The total amount of capacitive coupling is only relatively small and amounts to about 3.3nF and so the current flow is only a mere 260mA. However, the small current coupled with the high voltages will mean that the metal case of the double insulated item can be sitting as high as 170VAC. If you make a connection between this 94  Silicon Chip charged case and mains earth then you will experience a small zap while the voltage on the case passes through you to ground. In some cases the shock repeats itself once every second or so if the person keeps making contact. The shock is a harmless one and is more a nuisance than a safety concern. Even so, it can be an unnerving experience from a mains-powered product and can raise doubts about the safety of the appliance. Some equipment that has a brushed finish metal case will give nasty tingles even without a direct connection to earth. In normal circumstances you may not notice the floating voltage, for one of two reasons. Firstly, all of your equipment could be floating at this high voltage and with no earthed equipment nearby, there will not be any means to conduct the current. This is typically the case where people have double insulated equipment such as a TV set, DVD player and a VCR all connected together in a timber cabinet. Secondly, when you connect a double-insulated DVD player to an earthed stereo amplifier, for example, the signal earth (and case) of the player will be earthed via the amplifier. This means that any hazard is avoided. However, there can be a shock hazard if you have a bank of double-insulated equipment connected together and earthed equipment nearby. In that case you might get a tingle every time you touch both sets of equipment. The problem can be worse where siliconchip.com.au Fig.4 (right): the wiring is very simple – just make absolutely sure that you attach the earth wire from the Tingle Terminator to the earth pin (and only the earth pin) on the 3-pin plug. The earth pin is always labelled “E”. A side-entry type is used because these have a clamp which holds the wire in place. a sound system is connected to a micro-phone for public address or for karaoke use. It is important that the shell of the microphone is not sitting at around 170VAC with respect to earth! Another problem arises when disconnecting double insulated equipment from equipment that earths the signal. Often a shock will be experienced if the signal leads are disconnected before the mains plug is removed, even if the equipment has been switched off! This is a common situation with a public address system that comprises a mixer, amplifier and a CD/DVD player. This happens often when the DVD player is borrowed and the person using the equipment is not aware of the hazard. The obvious solution to this last problem is to disconnect power before removing the signal leads. However, this is not practical since non-technical people will not necessarily follow this procedure or even be aware that siliconchip.com.au they should disconnect it this way Solving the zap problem A simple solution to all the above problems is to connect the case of a double-insulated appliance to mains earth. This will very effectively remove the shock hazard. However, we do not recommend it, for a number of reasons. First, it could be argued that earthing a double-insulated case decreases safety because an incorrectly wired plug or mains socket can produce a situation where the metal case is directly connected to mains Active. We don’t accept this argument because earthed mains appliances such as fridges, washing machines and microwave ovens are still very prevalent and quite safe. Second, if you modify the mains wiring to a double-insulated appliance such as a DVD player, you will probably void the warranty and pos- sibly cause other circuit hazards – if the job is not done to normal safety standards. Third, earthing the case will definitely cause a hum loop where the equipment is connected to another piece of the earthed gear such as a large stereo amplifier. This will produce a nasty hum in the audio signal and is caused by currents circulating around the loop created by the signal connections and multiple connections to mains earth. Ultimately, the only practical solution is to couple the case of the DVD player (or whatever) to mains earth via a small capacitor. The capacitor value needs to be small enough to cause minimal AC current into the earth but large enough to reduce the case voltage to a low value. We found a 22nF 250VAC class X2 capacitor to be a good compromise. For example, a DVD player which had its case floating at 170VAC had the July 2006  95 Connecting the Tingle Terminator requires only a powerboard or double adaptor. The powerboard itself is unaltered so the other two outlets can be used for other 240V devices (TV, etc) in the normal way. case voltage reduced to 27VAC while not causing hum loop problems. The Magnavox DVD player pictured in this article had a floating case voltage of 82VAC when it was off and 120VAC when it was on. When the capacitor earthing modification was added, these case voltages were reduced to 2.8VAC and 4.5VAC, respectively. Warranty OK, that’s the solution but how do you it without voiding the warranty? You must not modify the existing mains wiring or modify the existing 2-core mains cord and moulded plug. With our approach, you do not have to open the case, which may in itself cause the warranty to be voided. We mounted the 22nF 275VAC capacitor inside a small plastic box which was attached to the rear of the case with double-sided foam adhesive tape. One side of the capacitor is connected to the case via a short lead and eyelet secured by one of the case screws. The other side of the capacitor is connected to mains earth via a single lead terminated (only) to the Earth pin of a side entry 3-pin moulded plug. The side entry plug is used because it has extra clamping that enables thinner cables to be securely clamped. The earth wire is sheathed with tubing in the region of the cord clamp. The earth wire with its 3-pin plug is wrapped around the existing 2-core mains flex and 2-core plug and both are plugged into a 240VAC GPO or mains power board. This arrangement is shown in Fig.4. Fig.3 shows the mounting arrangement for the 22nF class X2 capacitor. It is terminated to a 2-way insulated terminal block with one lead con96  Silicon Chip nected to the earth wire that connects to an existing screw on the case and the other wire to the 3-pin mains plug Earth terminal, as described above. Safety first: wiring the plug While you do not make any changes to the DVD (etc) mains wiring, you do have to connect to the earth pin on a mains plug. If you have never opened up a mains plug before, you need to carefully read and understand the following. Even if you have, read it anyway! There are three pins in a standard (Australian) mains plug. The Active and Neutral are both angled, while the Earth is always vertical and between and below the active/neutral. All three pins are also always la- Parts List – Tingle Terminator 1 3-pin mains side entry plug 1 double adaptor or mains power board 1 length of 250VAC-rated earth (green/yellow) wire (to match length of appliance power cord) 1 short length of earth wire (to suit distance to crimp eyelet) 1 plastic bulkhead box (Jaycar HB-6065 or equivalent) 1 2-way mains terminal block 1 22nF 275VAC X2 capacitor 1 M3 x 15mm screw 1 M3 nut 1 4mm crimp eyelet Length of black spaghetti tubing insulation or heatshrink (optional – for appearance only) Double-sided foam tape or adhesive foam pad belled A (Active), N (Neutral) and E (Earth) – usually inside the plug. It is absolutely vital that you do not connect anything to the A or N pins – and just as essential that you don’t have any strands of wire floating inside the plug which could conceivably short to other pins (the first time you plug it in there would be a brief flash, a bang and a blown fuse – but you want to avoid even that!). Inside the side-entry plug specified you will also find a clamp which helps to hold the wire in place. Normally it clamps three wires in a mains cable but when assembled, the clamp does help to hold the wire in place. Finally, the wire itself should be mains-rated. There is almost no current flow in the wire – that’s not the reason mains-rated wire is specified. It is because the insulation is rated at 240V AC (or better) which means that if for some reason the insulated wire did come into contact with the Active or Neutral pins inside the mains plug, there would be no danger of the insulation breaking down. Ordinary hookup wire insulation is usually rated at 100V or less, so don’t take the chance. We covered the green/yellow earth wire in black heatshrink so it didn’t look so obvious against the black mains lead. Finishing it off Once you have finished wiring the Tingle Terminator and checked the AC voltage with your digital multimeter (both before and after!) to ensure it is working, you should glue the plastic case closed. While there are of course no dangerous voltages inside the case, this does prevent inquisitive fingers poking around. A drop of super glue on the case lid to body join will do this perfectly – and it will also mean that if necessary later, you can “crack” the case open. That’s it! OK, so this approach should solve problems with being zapped by doubleinsulated audio & video equipment. And if you do have to make a warranty claim, remember to completely remove the capacitor box and earth wire and its 3-pin plug from around the mains lead. If they don’t understand it, they’ll blame your Tingle Terminator for any ills your DVD player might have! SC siliconchip.com.au