Re: current transformer winding wire help please


"Winfield Hill" <hill_a@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
message news:d4ddep0dl7@xxxxxxxxxxxxxxxxxx
> Gordon W wrote...
> >
> > Genome wrote ...
> >>
> >> Gordon W wrote ...
> >>> I am thinking of using 0.125mm wire for the secondary of a current
> >>> transformer but don't know its current rating. DAGS and wire
> >>> tables give me other specs but not current.
> >>
> >> A guess for iron transformers is 4A/mm^2 or 4E6A/m^2 and that's for a
> >> guess of a 30C rise above ambient based on a guess based on another
> >> guess. Isn't it strange that if you use a word repeatedly it begins
> >> to lose its sense?
> >
> > Thank you Genome and John Fields for your replies, I guess I should have
> > given more information. Hope this is not too long and I don't show my
> > ignorance too much.
> > I've got my computer turning on/off the booster to my solar hot water
> > system but I don't have any feedback (closed loop to monitor).
> > The thought was to put a current transformer in series with the heating
> > element, rectify etc and feed +5V to the computer when the heater was
on.
> > I've done an experiment with a small iron cored transformer, the bobbin
> > is 20x10x10mm, a layer of 10A wire (the heater is 240VAC at 7.5A) well
> > insulated from the secondary of lots of turns (my rev counter gave up
> > part way through) of the 0.125mm wire.
> > Rectified/filtered output came out at 71V and 11mA with a 6K resistive
> > load and 47V and 30mA with a 1.5K load.
> > I can easily reduce the turns to lower the voltage but what current
> > should I aim for so that this 'thing' will live happily ever after?
> > The transformer will be in a ventilated plastic box under the house.
> > Ambient sometimes gets to 35 °C.
>
> Whoa, making a mountain out of a mole hill. Look at those miniscule
> currents you're getting on the secondary, 11mA, 30mA - you can employ
> virtually any wire size you like without worry. Secondary already
> wound? Fine, keep it. Generally in commercial current transformers
> the primary is simply the current-carrying wire to be monitored going
> through the hole in the current transformer (which may be a toroid).
> In your case, that'd be one turn, which gives you higher voltages on
> the secondary, for any given mandatory resistive load, but less current.
> If you want to power something serious on the secondary, then add to
> the primary turns and reduce that load resistor. Whatever you do, it's
> not likely the secondary wire size will be chosen by anything other
> than convenience.
>
> One thing that is very important, the secondary load resistor. Keep
> in mind the current transformer's primary resistance will look like
> the load resistor divided by the square of the turns-ratio. You don't
> want the primary to have significant voltage drop, affecting the load
> you're monitoring, so keep the ratio high, and the load resistor small.

Well, almost. The limiting factor in a current transformer is the flux
capability of the core. Generally, commercial current transformers are
proportioned so that the core saturates long before the primary voltage drop
becomes an issue. The lower the secondary circuit resistance, the smaller
the core may be. A secondary with lots of turns of small wire may have so
much resistance that it is unable to develop the secondary ampere-turns
needed to keep the core out of saturation!

Having said that, the OP may not care if the core saturates or not if all he
is doing is detecting the presence of a substantial current. However, if the
core is into saturation the secondary voltage is more difficult to
calculate. The saturated core will have a "squarish wave" change of flux
over time, and of course the secondary voltage will be the derivative of
this waveform. Too, the core heating will increase at high flux levels.


.

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