single power supply by centertapped transformer and bridge rectifier.

I am constructing a fixed power supply of 5v and 12v,I have a centertapped transformer with following rating 230v 50hz at primary and 24v-0v-24v ,12v-0v-12v at secondery .I gave 230v at primary of the transformer and used only 24v-0v-24v rating supply at o/p.
out of the three wires of 24v-0v-24v i connected 24v to one i/p of DB107 bridge rectifier and center tap wire(0v) to second i/p of bridge rectifier and i kept the third wire(24v) hanging (not connected to anything).
I checked o/p voltage of bridge rectifier by multimeter in dc mode by connecting +ve probe to +ve of bridge and -ve probe to -ve of bridge,i got 24v dc and when reversed polarity i got -24v dc ,until this ok but when i change multimeter on AC mode I am getting 50v AC and 0v with reverse polarity.
I am not understanding why am i getting 50v ac at bridge rectifier o/p ?

If there is no smoothing on the DC (no capacitor fitted) then the output from the bridge will be a series of pulses, all going to +24V, but returning to zero in between. The fact that the voltage at any instant can be going + or minus (as it returns from + 24V to 0V), can be interpreted by the meter as AC. The fact that it appears as 50 V is due to the sensitivity of the meter being different between DC and AC.
Frank

The result can be expected by a "poor mans" AC meter made with a half-wave rectifier.

The measurement is more or less meaningless anyway. For a real power supply design, you'll firstly add a filter capacitor, raising the DC voltage to Vpeak (about 33 V). Then select suitable voltage regulators.

With 33 V input, you'll burn a lot of power in linear voltage regulators.

If you use a center-tapped transformer then for a positive DC output you use two rectifier diodes, not a bridge rectifier module that has 4 rectifier diodes.
For 12VDC output the transformer should be about 18V center-tapped then the filtered output is about 9V x 1.414= 12.7V - 1V= 11.7V when loaded since the peak voltage of the sinewave is 1.414 times higher than its RMS voltage and a rectifier diode produces about 1V drop when it charges the filter capacitor..

You did not tell us what load current you need. For 5VDC and 12 VDC it may be easier for you to find another suitable transformer with say 10-15 V AC output which can be easily regulated to 5 and 12 V DC.
Using your higher voltage from the rectifier, you will lose a lot of power by dropping the voltage through regulators to 5 and 12 V.

If you connect a bridge to the 12V-12V windings you will get about 15V DC filtered from the bridge.

The voltage at the center-tap will look like a full-wave rectifier output (from the two rectifiers going to the bridge common terminal) and will be over 7V DC filtered.
You can get a little more voltage if you use Schottky rectifiers.

That should give you regulated +12V and +5V if you use low-dropout regulators.

I miscalculated on my previous post.
With a 12V-0-12V winding the bridge will give a filtered output of about 29V and the center tap filtered output will be about 14V.