On 02/05/2015 10:27 AM, Noel Chiappa wrote: Because the equipment under test is connected to the mains. A bridge rectifier is between the AC input and the rest of the circuitry, therefore there is no part of the EUT that is at neutral potential, and if you hook up a grounded scope, you will have fireworks. Floating a scope above ground should be reserved for the most special cases where you are testing line-powered gear that MUST not run on an isolation transformer, for some reason. Jon

With most 'scopes one side of the input is grounded -- you have a coaxial cable with the shield connected to the 'scope case and true ground. In most, if not all, countries, one side of the mains is connected to ground too. So if you have something directly connected to mains (the common example on this list would be the the chopper circuit of an SMPSU) then you can't just clip the 'scope ground lead on it. If the input is a bridge rectifier (as it is in most SMPSUs and in the DEC regulator bricks that started this), then clipping the 'scope ground to the obvious point -- the -ve output of that rectifier -- will effectively put a diode across the mains. This is a good way to let lots of magic smoke out. The correct way round this is to run the item under test off an isolating transformer. Then no point on the secondary side of that transformer is necessarily grounded (that is the whole point of it being an _isolating_ transformer), so you can arbitrarily connect any single point in the circuit to ground. So you can clip your 'scope ground lead where you like. The dangerous way is to 'float the scope' -- that is to disconnect the mains earth from the 'scope (in the mains plug, for example. You can now connect the 'scope ground lead where you want, but since that point is not necessarily the same voltage as true ground, you can end up with the metal 'scope case at a high voltage wrt true ground. Very nasty!. The official manual for the Philips PSU in my P854 computer gives all the waveforms in the chopper circuit. It actually states that you should use an isolating transformer on the mains input of the PSU to look at these, but if one is not available to disconnect the mains earth lead of the 'scope. Rather you than me. Variacs, BTW, are variable autotransformers and do not provide isolation. So the same problems exist if you are running something off a mains-powered variac. Of course you can use a variac together with an isolating transformer. Over here bulbs above a certain power (is it 60W now?) can't be sold for 'domestic lighting', but can AFAIK, be sold for any other purpose. And there is no restriction on what you use the bulb for after buying it, although I am told some insurance companies won't pay out if they discover a fire was caused by such bulbs used for domestic lighting. More usefully, you can still get normal incandescent bulbs for special applications (e.g. lighting the inside of a cooking oven), 'rough service' bulbs, and so on. All of which make perfectly good ballast resistors. Am I the only person to have a cable with an IEC 'kettle socket' on one end, a similar plug on the other (think of the plug used to power the IBM5151 monitor) with a lampholder wired in series with the live wire? I connect it between the power cable and input socket of something I suspect of having a mains short, and put a suitable bulb in the holder. For larger appliances (the iron somebody mentioned), I have read that those 1kW halogen security lamps make quite good ballasts. -tony

On Thu, 5 Feb 2015, Noel Chiappa wrote: They already are. It took effect in 2014.