[sdiy] Transformers

[Karl Ekdahl]

You're like the book i never had, thanks a lot!
/Karl

 --- John L Marshall <j.l.marshall at comcast.net> skrev:

> There have been several questions about transformers
> recently.
> 
> Transformer primer:
> 
> V-out = V-in (N-out/N-in)
> 
> The voltage ratio (V) corresponds directly to the
> turns (N) ratio in a perfect transformer.
> 
> I-in = I-out (N-out/N-in)
> 
> The current ratio( I) corresponds to the inverse of
> the turns (N) ratio in a perfect transformer.
> 
> P-out = P-in*eff.
> 
> The efficiency (eff.) for small transformers may
> vary from 70% to 95%.
> 
> Z-out = Z-in (N-out/N-in)^2
> 
> The Impedance (Z) ratio is the square of the turns
> (N) ratio in a perfect transformer.
> 
> 
> Many small transformers are wound approximately 4 (3
> to 5) turns per volt. If you can accurately measure
> the DC resistance of the wire and determine the
> gauge of the wire in the winding then you can tell
> how long the wire is by using a wire table. From
> there the number of turns can be determined  by
> approximating the circumfrence of each turn for the
> windings.
> 
> A good approximation rule to remember: #10 wire has
> a diameter of approximate 0.1 inches and has a
> resistance of approximately 1 milliohm per foot.
> Each three gauges up will double the resistance and
> halve the cross sectional area (not the diameter).
> Each three gauges down will halve the resistance and
> double the cross sectional area. Approximately.
> 
> Use a variable voltage transformer (Variac) for
> testing unknown transformers. A common light dimmer
> will generate harmonics that may contribute to
> additional transformer heating.
> 
> If you measure the primary current while increasing
> the primary voltage the current will approximately
> linearly increase with the voltage. However, if the
> current sharply increases then the transformer has
> reached saturation.
> 
> It is a good idea to let an unknown used transformer
> "cook" with no load. The transformer should get warm
> but not hot. If the transformer gets hot there may
> be shorted turns. Time to recycle the transformer.
> 
> Measure the size of the secondary winding wire.
> Allow about 1 Ampere load for each 700 (500 to 1000)
> circular mils of wire cross sectional area.  
> 
> The Volt*Ampere rating of a transformer is related
> to the transformer core cross sectional area: 
> 0.4sq.in. = 6VA, 1sq.in. = 45VA, 2sq.in. = 120VA.
> The cross sectional area is the iron area directly
> inside the winding only. I have not seen a simple
> formula for iron area versus VA rating.
> 
> Transformers are rated in Volt-Amperes not Watts.
> The transformer must supply full current when the
> load is reactive.
> 
> Avoid transformers with primary and secondary
> bobbins on separate paths of the core. These
> transformers are probably, self-regulating,
> ferroresonant type. If you look carefully you will
> find an air gap in the magnetic path. There is a
> special secondary winding that requires an AC
> capacitor.
> 
> I think the question was asked if a transformer
> designed for 230 VAC will work on 120 VAC. The
> answer is yes, but. The copper losses will be
> greater. The primary of seconday windings may not
> support additional current. So, a transformer
> designed for 230 VAC to 24 VAC if used on 120 VAC
> will output 12 VAC but will not supply double the
> current.
> 
> Universal transformers have a split primary that is
> used in series for 230 VAC or in parallel for 120
> VAC.
> 
> I have successfully misused transformers. I have
> used telephone hybrid transformers and repeating
> transformers as power transformers. This is not
> efficient but it works.
> 
> I arranged for the utility company to supply
> instrument transformers to power a cellular
> telephone base station directly from 110kV high
> tension lines. An unusual approach but it did solve
> the power problem.
> 
> Take care,
> John
> 
>