I have a center-tapped transformer. 120V input. Output shown as 12Volts at 1.2 amps.
I checked the voltage. From either leg to center, on the secondary, it gives me approx 17.5 volts. From leg-to-leg it gives me approx 25 volts.
I have calculated the following:
Vdc = 2Vmax/pi or 51.28/pi or 16.32 volts
0.637Vmax = 25.64 X 0.637 or 16.33 volts
0.9Vrms = .9(0.7071 X Vmax) or .9(0.7071 X 25.64) or .9(18.13)= 16.32
Where Vdc = 2Vmax/pi = 0.637Vmax = 0.9Vrms
My question is: do I need a diode rated at 1.2 amps and what else? Is “forward voltage” the voltage applied to the diode or the voltage drop across the diode. Should max voltage of the diode be 18 or more volts?
A simpler question: which diode should I use for the rated 12 volts and rated current of 1.2 amps?
From leg-to-leg: VAC=25 V (approx).
From leg to center: VAC=17.5 V (approx).
After rectification and filtering, the DC output will be roughly:
VDC=1.414VAC(Center)-Vf
Vf =0.7 for silicon diodes, 0.3 for schottky diodes.
VDC=1.41417.5-0.7=22.4V. If you need 12V, you will need a voltage regulator or buck converter. You can use LM7812: -LM7812 Voltage Regulator - Share Project - PCBWay
The transformer’s secondary is rated at 1.2A, so your rectifier diodes must handle this current.
For diode, you can use 1N5408 or 1N5822.
Hello @selmafrog,
People often forget about the reverse breakdown voltage:
While the diode may have a forward (conducting) voltage of about 0.7 VDC, it must also withstand the transformers reverse (non-conducting).
You application is known as a half-bridge center-tapped design. Each diode will see a V_{RMS}\sqrt2 peak voltage. We must also include a safety margin. For your design, the minimally acceptable reverse voltage is 2 * 17.5 * \sqrt2 which is about 50 VDC.
Best wishes,
APDahlen
Hello selmafrog,
Click Here for some Diode options.
These are axial diodes from 50~100V, and 2A~3A rated.
Please check the datasheets for compatibility to your application.
Thank you to all who have taken the time to contrubute valuable input.