Salvaged transformer

[solis365]

Second post, but it's for a different part. The casette recorder/player box that I salvaged had a transformer. I tried to figure out exactly what it did.

two input leads for 110V.
5 output leads
(1)
(2)
(3)
(4)
(5)

now I know working with AC mains is a no-no but i insulated all of the dangerous contacts before plugging it in and (carefully) checking:

the voltage between pin 1 and pin 2 was: 5.3V
between 3 and 4: 13.8V
between 4 and 5: 13.8V
between 5 and 3: 27.7V
between 4 and 2: 0V

weird, non-standard voltages between other pins (6.8, 3.2, etc. must be just the result of coupling). leads me to believe i have a center-tapped winding for +/- 13.8V and a secondary winding for 5.3V. This makes sense from a system standpoint; the 5v for logic and signal processing, the 13.8V for analog circuitry and motors.

question: i used my multimeter in VAC setting, and I believe it would thus be an RMS voltage. Since the 5.3v did not quite make sense to me; a typical diode drops at BEST only .5V so you would be left right on the 4.8V supply threshhold for most 5V digital stuff. If the 5.3V were an RMS voltage, that means the peak is a bit higher than that, allowing for the .7V or so drop that comes with rectification and allowing the output DC voltage to be closer to 5V.


do my assumptions seem sound?

 

[colin55]

The voltages you are measuring will increase by 35% when converted to DC.
To see if you have a "real voltage" between two leads, use a 12v car globe such as for the interior light. It will only glow when a "real voltage" is present.
It will also show what voltage is present.

 

[solis365]

excellent tips, thanks. i have lots of transformers that I salvage and figuring out what the actual taps are for without plugging the things into 220V or 440V mains is a pain.

wheres your 35% figure coming from? changing RMS into peak voltage and then subtracting diode losses?

 

[colin55]

You need to plug the transformer into the mains to get an accurate output voltage.
The 135% is approx DC value after rectification.

 

[Sceadwian]

Working with mains isn't a nono... if you know what you're doing.

 

[mneary]

A cheap transformer is often 10% to 20% higher when unloaded, so the loaded voltages are probably 22-25Vrms Center Tapped (I will call this 24VCT), and 4.3-4.8Vrms (I will call this 4.5). Assuming a bridge on the 4.5V, the peaks after the rectifier will be (1.414 * 4.5) - (2 * .7) ~ 5V. Assuming a bridge on the 24VCT winding, it will generate (+) and (-) voltages with only one diode drop each. All other assumptions the same, +/- 16 to 17V.

The finished estimates assume that each voltage is loaded to its ratings. It will be 10-20% higher with no load. We haven't discussed what those ratings might be.

Pin

 

[solis365]

thanks, that bit about the 10% to 20% while unloaded thing had not crossed my mind. I suppose i've got myself a start for a benchtop supply now.