[Techtalk] Re: Electrical hacking

Conor Daly conor.daly at oceanfree.net
Mon Apr 14 12:02:39 EST 2003

On Mon, Apr 14, 2003 at 12:44:50AM +0100 or so it is rumoured hereabouts, 
Maria Blackmore thought:
> On Sun, 13 Apr 2003, Conor Daly wrote:
> > On Sun, Apr 13, 2003 at 01:55:46PM +0100 or so it is rumoured hereabouts, 
> > Maria Blackmore thought:
> for reproducing the suspension from a Citroen Xanti with lego.  (it's very
> funky, all sorts of hydraulics and everything)

Scared hell out of me when I tried to bleed the brakes on an old Citroen.
With the engine off, the brake pedal was rock hard and wouldn't bleed.
Start the engine and the brakes started to work again.
> > My current car is 11 years old and I plan to keep it for another 15 or
> > more.
> hmm, I think the only problem with that is likely to be rust.

Or being unable to get replacements for the throw away modules...  Ireland
isn't as bad as the UK for car rust since our roads don't freeze nearly
as often as UK ones and so don't get salted so much.
> Ah, shame, I've been looking for one recently.
> Actually, I'd really like a nice shiney HP spectrum analyser for work, one
> of the ones that goes up to about 5 GHz, but i don't think i'll be able to
> get one, somehow.

A colleague of mine has a 50MHz one that he never uses.  Dunno if he'd be
willing to part with it but you never know.  Shout offlist if you like.
> > exactly _how_ they do it.  The transformers you see on building sites and
> > used by trades people work similarly:  They have a transformer with a
> > centre tap (which AFAIK is connected to ground) and 55VAC available off
> > either end of the transformer.  This supplies 110VAC to the tool but
> > nowhere is there more than 55V above ground so shock hazard is very low.
> meep, no, this isn't how it works at all
> Voltages are relative, you always measure the voltage *between* two
> things.

[snip details of 1:1 isolating trafo]
> I may be wrong, since I have never actually used one, but I believe that
> the yellow transformers on building sites work on the exact same
> principle and isolate the user of the power tool from mains.  I believe
> the reason that they have a centre tap is because the mains input is at
> 230V, but the output is at 110V.  Therefor the most efficient way to do it
> is to, for example, have 200 turns on the mains side, and 200 turns on the
> load side, but have an extra connection at 100 turns, as illustrated:

I'm not positive either but as I understand it, they have a 2:1 step down
to produce 110V total with 55V either side of the centre tap.  I said I
thought the centre tap was also connected to ground but I may be making
assumptions here.  I do recall being shown around inside one many years
ago and hearing a suggestion of 55V as a sop to shock hazard.  I've
modified your diagram to reflect what _I_ understood about them.  BTW,
AIUI, neutral is generally _not_ connected to local ground though it is
usually at 0V wrt ground.  This allows the centre tap to be grounded while
still being isolated from the mains and ensures that the load side is
nowhere more than 55V away from ground.
> Mains                            Load
>              2  : 1
>    Live -----\ || /------- Hot
>              & || &        /\
>    /\        & || &        55V (ish)
>              & || &        \/
>  230V        & || +------- Cold
>              & || &        /\                 /\
>    \/        & || &        55V (ish)           |
>              & || &        \/                  |
> Neutral --+--/ || \------- Hot                0V      
>    /\                      /\                  |
>                                                |
>    0V (but isolated)       Isolated           \/
>    \/                      \/
>  Ground --+--------------- Ground ----------------------
> Cool, yes?

> Now, 55V AC is still a shock hazard.  You'll find that your telephone line
> has about 50V between the two wires, well, it should be around 48V, but
> it'll depend on your distance from the exchange, etc.  In any case, if you
> accidently connect yourself across it, you'll find it tingles a little
> bit, perhaps sting somewhat.  I seem to recall relating a story about a
> friend who had seen me stripping insulation from wire with my teeth, and

Copper plated teeth eh?

> > though the various motor workshops I've worked in/visited did tend to
> > have the ubiquitous girlie pinup calender hanging up somewhere...
> I think it's probably something about the men that work in them

Or about the perceptions of the car marketing types who produce the
calenders.  Interestingly, recently when my wife and I were out buying her
new car, the salesman remained focussed on her throughout the deal.  Maybe
they are learning...
Conor Daly <conor.daly at oceanfree.net>

Domestic Sysadmin :-)
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