An item such as this
How can this be a double-insulated class II applience if the output has (by definition) a metalic connection to the mains?
Is it any more dangerous than a 1-way mains extension cable?
Do powerstrips carry double-insulated symbols?
Class 2 protection relates to the insulation of the enclosure, basically there has to be 2 separate layers of insulation enclosing live parts. That doesn't preclude the enclosure having a socket outlet.
Explained in great detail by JW at
UK let me rephrase the question without reference to appliance class.
What makes it OK to tie some foreign mains receptacle to the incoming supply but not (say) a 5v USB receptical?
Graham. snipped-for-privacy@mail.com wrote in news: snipped-for-privacy@4ax.com:
My view is that these devices cannot comply to UK safety standards so the CE marking will not (cannot) be supported by legitimate safety testing/test certs.
Reason being, as you imply, that US 2 pin plugs are unshrouded making finger contact possible and a single fault in the device (open circuit winding) or a supply reversal at the outlet in the 13A outlet (another a single fault) would result in poss contact to mains voltages.
From the full item listing:
pretty much ... plus fusing, plus it must be screwed together, plus some testing methods.
Well I guess if there is no exposed part which is live other than a socket that complies with the relevant regs. TVs of the 60s used to all run with a live chassis as they used autotransformers to tap off the voltages they needed.This was why you never got a headphone socket or if you did it was completely isolated by some other means, The aerial sockets were only insulated by having capacitors in series with them, and if one went short, you probably fried. Brian
A tapped resistor (or in the trade, "dropper") - not any sort of transformer - was the overwhelming case with "universal" TVs of those days, Brian.
PA
And the heaters all in series...
You got it! All those 300mA valves. Now, which one has blown???
And the dropper mounted horizontally in a punctured extension to the fibre-board rear cover, Great idea until the owner put a cloth on top of the tele and covered up that part of the rear cover !
PA
Yes, it was all about price. If they had had the money for an autotransformer it would not have cost them much more to put an isolating transformer in.
Cossor 918.
The one with 400v across the heater?
haha.
Better than tumble driers for catching fire, back in the day
I think that is exactly right.
valve heaters in series with series dropper. Cant remember how the HT was generated - Mmm. Looking it up it seems that the earliest sets were a single diode giving half wave rectification. Valves were either U prefix (e.g. UL84) (100mA heaters) or P prefix (300ma). The set operated on 300 odd HT volts for the valves - that is well within normal operating range for common valves (200-400V)
loudspeaker output did use a pathetic little transformer
I think in the 70s silicon rectifiers started to be used. Hybrid sets with transistors in the front end and valves driving the CRT came next, and finally when high voltage high speed transistors became available all solid state bar the tube.
Trying to make a colour set with valves...ugh!
The one with the mains voltage across the heater pins - but which?
Fortunately it was smouldering rather than blazing. One of the dropper sections would then die, cutting off the heating. Then either a repair from stock or an order to Radiospares for the needed "dropper section"
P for televisions, U for Radiograms.
PA
Thing with a TV is there are very high voltages inside, so a live chassis may be the least of your worries if an idiot decides to take the back off it while it is switched on.
The point is that you could access what might be the line side of the mains without taking the back off.
Universal TVs were designed to work on both AC and DC mains - hence the term "universal". As such any design using mains transformers - Auto or not - was out of the question. As the universal design was so cheap - it carried on well after the end of DC mains.
PA
In fact in may ways the SMPS is a direct descendent - no big mains transformer, and some circuitry at mains potential.
And, of course, equally happy on either AC or DC supplies !
What goes around, comes around.
PA
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