by Electro-Tech-Online forum member Crutschow
The schematic to the right shows a simple way to get two full-wave rectified voltages from one center-tapped transformer winding and one bridge rectifier. Adding a capacitor filter at the center-tap gives roughly half the DC voltage from the center-tap output to ground as compared to the bridge output to ground.
This can be useful for example, if you want to build a power supply with two different output voltages, such as 5v, along with a higher voltage, such as 10-12V. (The LTspice simulation is shown for a 12Vrms center-tapped transformer). This minimizes dissipation in the linear regulators and reduces the transformer current requirements.
This works because all four diodes are acting as a full-wave bridge for the outer transformer connections while the center-tap is using the two bridge diodes to common as a full-wave rectifier. The two bridge rectifiers connected to common are, in effect, doing double duty.
It may look a little confusing since normally a full-wave rectifier is shown with the diodes in the outer paths to the output with the transformer center-tap grounded, but it works just as well with the diodes in the ground path and the output taken from the center-tap.
Of course, it should be apparent that the transformer and bridge rectifier must be rated for the sum of the currents from the two outputs. Typically the transformer should be rated at ≈2.4 times the bridge DC current plus ≈1.6 times the center-tap DC current. This is due to the high RMS value for the current pulses charging the output capacitors (true of all rectifier-capacitor supplies).
For example, the LTspice simulation gives an RMS current in the transformer of 702mA for a full-wave bridge DC output of 298mA (not shown below). Note the increased current in the ground diodes of the bridge (ID4 shown) due to it having to carry the sum of the two output currents.
Just a little hint: if you want anyone to like this you need to at least make that “schematic to the right” readable, because now it’s not. Maybe I need a microscope to look at it, but, sadly, I don’t have one. Also, adding downloadable LTSpice file helps.
For now this article is completely useless.
Hi, Alex,
Thank you for your feedback. Take another look at the article and try double clicking the image. It should open up much larger in a new tab. You can also go to the original article on Electro-Tech-Online.com: The spice files should be there available for download. You can also contact the author for more information.
cheers,
Aimee
I’m pretty sure I tried clicking it before with little luck 🙂
In any case, thanks!
Try a bit harder
interesting
This is of great help
.Unlike some comments the schematic cant open larger i beg to differ. I double click on it and it opened larger
It is good to recap some well known methods.
This is a good one. I will try it definitely.
very interesting but a couple of 78 regs would probably be cheaper
I think you miss the point.
This is essentially just the standard circuit for creating a split-rail power supply. The only difference being that you have chosen to ground the negative side of the bridge rather than the centre tap of the transformer & the second load has been moved from one of the outer rails of course. It’s a good idea though as the currents are still balanced in the transformer and it’s efficient due to this component functioning as the voltage divider element.
You’re flying pretty close to the PIV ratings of those 5819s though! Using 4001s would probably be a better choice despite there higher forward voltage drop, especially if higher rail voltages were to be used.
Great Idea, worth trying. I had no problem enlarging the image and text using the normal finger control on an iPad. I was able to read the text while viewing the image as I was tracing the current path from the C.T. Via the 2 diodes to common.