AC Mains Voltages

[emeritus]

It was mentioned on another thread a few years ago that small electrical appliances in the USA were often provided with crowbar circuitry to prevent them from being damaged by overvoltage due to lost neutrals. It is common for US domestic premises to be supplied with a 240V single phase supply and a transformer with a centre-tapped 240V secondary that provides two 120V circuits for their mains sockets and lighting. If the centre tap goes open circuit, the two 120V circuits end up in series, and with a high current appliance on one circuit and a low current one on the other, most of the 240V will appear across the low current appliance. The crowbar detects the overvoltage condition and creates a short circuit to blow the appliance's internal mains fuse.

[Jez1234]

Originally Posted by GrimJosef

The name's just a forum convention and no more stupid than calling a coupling capacitor feeding an output valve "that capacitor". It does confuse people who aren't part of the 'in crowd', but there you go ... To be honest I've been to quite a few bake-offs and they do seem to involve a wholly disproportionate amount of cake, and sometimes there's a hi-fi system playing in the background .

Back on topic, the poor quality of the mains, both in terms of RMS voltage and waveform purity, can be a trial when you're trying to carry out careful measurements. I believe I'm not the only one who's used a stand-alone synthetic mains supply (mine's based on a modified computer UPS) when I want repeatable results from my valve tester. The voltage here varies from about 234V to 249V (extreme values seen in checks carried out perhaps a few times a year over a decade). Slightly off-topic (again), given the thread title, but I also went through a phase of having a small DC offset (less than a volt if I remember rightly) which would appear around 19:00 and last through the evening. I was working at the time on some US-manufactured equipment with hard-working toroidal mains transformers and the DC caused them to buzz irritatingly. I never confirmed the source of the problem but I was aware that a nearby large supermarket started its overnight bakery operations at around that time.

Cheers,

GJ

We'll have to agree to disagree on that one then Graeme .... and also on "that capacitor"..... and also on mains quality. One of my permanent bete noires on the hi fi forums was with the obsession with mains quality when in fact it makes no difference whatsoever to properly designed equipment beyond the possibility of a buzzing transformer. I can though see your problem in the case of the valve tester but I would probably add regulated supplies for LT and HT as a solution.

[broadgage]

I recently had several power cuts, presumably due to flooding. Mains voltage was only about 200/210 volts at one point. I suspect that power normally supplied by the nearest substation, was in fact "back fed" from a slightly more distant substation. This reduces the voltage both due to losses in the longer cable route and by increasing the load on the assisting substation.