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Practically Speaking Hands-on techniques – by Jake Rothman A shocking experience! I had a bad electric shock the other day. I hadn’t suffered one for years, so complacency had obviously set in. A friend had brought in his Yamaha electric motorbike charger and battery. The bike can be used on motorways, so the battery is huge, along with its associated switch-mode power supply unit/charger. The SMPSU was giving no output, so I took it apart. My friend assured me he hadn’t plugged it in for a week. Looking inside, I found that it was a common SMPSU problem: bad lead-free soldered joints around the enamelled wire from the ferrite transformer. This usually happens when the enamel hasn’t been fully removed before soldering. Also, lead-free solder has inferior wetting properties and its brittleness results in cracking from the high-frequency vibration due to magnetostriction in operation. I soon set to work, scraping off the burnt flux with a scalpel around the joints to get a good look. BANG! I got a massive DC belt from one arm to the other, very nasty, like an old Fender valve amp HT rail but worse. The scalpel was nowhere to be seen; luckily it wasn’t embedded in my friend’s head! I shouted “that felt like bloody 350 volts”. I got my meter out and shakily measured between the pin I was scraping and the chassis, and it was. I thought (belatedly) that I had better discharge the main smoothing capacitors, of which there were three in parallel. So, stupidly, I got my nice insulated Bahco Ergo pliers out and shorted the pins. BANG! It blew one of the tips off. 4500µF of capacitance charged to 350V is a lot of energy (E=CV² so 550J)! Thank god I had urinated not long before I undertook this procedure! Still shaking a bit, I continued and fixed the joints. I wasn’t going to be beaten by this modern ‘disposable’ electronics. Having fixed the bad joints, I soldered a bleeder resistor of 39kΩ 5W across the caps and switched it on. The LEDs lit up and it gave the correct 80V DC output at 10A; perfect. My friend thought the whole thing was most entertaining! It’s a good job it worked. He’s now enjoying his bike and I found the scalpel stuck in the skirting board a week later. Fig.1: Discharge high-voltage capacitors with a bleeder resistor assembly or a purpose-built discharger. A 7W 10kΩ wirewound resistor is used here. 16 Morals of the story • Don’t assume something is discharged, even if the client says it hasn’t been switched on for a week! • Don’t use a metal Swann Morton scalpel for repair work; use a plastic-handled one instead. • Don’t assume all SMPSUs have bleeder resistors wired across the main smoothing capacitor bank. Even if it has, the resistor may have become open circuit. This circuit didn’t have one because the continuous dissipation would be high and reduce its efficiency. Editor’s note: clever designers can add bleeders that are out of circuit when mains power is applied. • Measure the voltage across big capacitors as soon as you open the case and before you start working on the PCB. If they are charged, discharge them slowly with a bleeder resistor attached to insulated test probes, as shown in Fig.1. I use the retractable probes with an internal spring with extra rubber sleeving on all accessible parts (Fig.2). The retractable probes can clip directly onto the capacitor leads protruding from the underside of the PCB, as shown in Fig.3. Alternatively, the capacitor can be discharged above the board as illustrated in Fig.4. • Remember the ‘left hand in pocket’ rule; don’t make an easy current route through your heart. If you must hold the metal enclosure or chassis while working on it, physically clamp it or insulate your hand. Even relatively mild shocks involving the heart can lead to cardiac arrhythmias. • Have a pee before working on highvoltage circuits. There’s nothing more embarrassing than getting a small shock and wetting yourself, then getting a massive one because you are standing in a pool of urine. I know because I once did it in front of a load of students! It became a standing joke; I defused it by having a spare pair of M&S underpants in the first aid box. I said if anybody needed them, they would be £12.00 PE plus a clean-up fee. Practical Electronics | November | 2024 Fig.2: Some retractable test probe assemblies use a small screw to attach the wire. These are some old gripping ones from RS. Fig.3: The gripping test probes give time for the capacitor to discharge while leaving your hands free for checking the voltage. GET T LATES HE T CO OF OU PY R TEACH -IN SE RIES A Fig.4: on this Brother printer power supply, the probes can be clipped onto the capacitor leads directly. 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In PDF format This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a singlestage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! 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ORDER YOUR COPY TODAY JUST CALL 01202 880299 OR VISIT www.electronpublishing.com Practical Electronics | November | 2024 17