[sdiy] Strange transformer behaviour

[harrybissell]

Sounds like a Signal Transformer "flathead" PCB or similar from
just about every other manufacturer, including Tamura, etc...

The primary coils should be wired in series for 220VAC, parallel for
120VAC.   The secondaries can be put in series or parallel, and if in
series... the center point 'is' the center tap.

Small transformers have poor regulation... they have less iron than they
need so they are designed to run hot, and with greater voltage drop
than large ones. So a transformer rated at (let's say) 24V @ 80mA might
actually deliver maybe 35V at very light loads.  The line voltage can be another
kick up...and the capacitor charges to the peak of the applied voltage... so its
another 1.4x the actual AC volts...

this could easily get you into that 40V range.

watch out... almost every 78xx and 79xx regulator cannot stand more that 35VDC
at the input (7824, 7924 are exception - 40V input).   They will get hot as a pistol
with too much voltage drop as well.

I usually 'try' the transformer first.  I had some 36V at 170mA I expected to use for a
+/-12VDC... the voltage was too high. the 24V at 80mA was too low. I had to buy 30V
units to get the right voltage.  Ouch !

I cannot explain why the coils metered different... that IS unusual.  However...resistance
measurements of a transformer coil are not real accurate because they don't take into
account the reactance.  Go figure ???

H^) harry

Karl Ekdahl wrote:

> I've found a couple of pcb transformers that has four
> windings, two primary and two secondary. The two
> primary windings has 3kohm / 2kohm resistance and the
> secondaries has 70ohm/100ohm resistance (no center
> tap).
>
> I connected my 220v to one of the primaries (i think
> the 2kohm) and measured the outputs on the
> secondaries, i got ~20v on each. I thought this was a
> little bit strange behaviour since the secondaries has
> different resistance...
>
> ..Well, to the problem, after rectifying (single wave
> rectifier) and adding a 1000uF cap, the ~20v turned
> into a 40v (!) output! I suspected i would get
> something like 23v and i need something around 20v but
> i only get 40v and that is too much. Is this correct
> behaviour?
>
> Happy for any thoughts...
>
> /Karl
>
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