2x24 Transformer

[snape]

Hi,

I am new to electronics. I have a question regarding a transformer with secondary voltage as specified in the subject. Full wave bridge rectifier is connected to its output and voltage measured after being rectified is... 88 - 89V dc. This does not seem right since by my simple logic it should be around 48V. I used 1n4001 diodes to build the bridge. Also put 63V 100uF smoothing capacitor. Any ideas why this voltage is so high?

[Pauldf]

Have you wired the primary correctly?
What AC voltage do you get without the rectifier connected?

[nicholasdark]

Hi Paul,
It sounds like your problem is mainly due to lack of a load. I believe there are two issues at play here:

1/ Rectification of a sine wave produces a peak voltage of root 2 (around 1.41) times the RMS voltage of the sine wave. So in your case, the no load, the voltage would be at least 67.88V DC.

2/ Generally, a transformer only produces it's rated secondary voltage at it's rated load. The typical 12V 300mA un-regulated wall wart type power supply from say maplin will happily produce say 18-20V without a load and potentially even higher. ~1.66 times higher.

To summarise the above. Assuming that you have a transformer with 2 times 24V 300mA wired in series, then the following sounds very plausible.

24V * 2 = 48V to account for them being in series.
48V * 1.66 = 79.68V to account for the no load voltage
79.68V * 1.41 = 112.3V to account for the peak voltage produced by rectification.

Now evidently, 112V is higher than you are seeing, however the 1.66 is based purely on reading I took a long time ago. So I would suggest that is it most likely deviance between my example and the results you are seeing.

Are you able to supply more details of both the transformer and the load (if any) present during your testing.

Nick