Dual Rail Power Supply Design Center Tap Connection

[D3K]

Hi All,

I am attempting to design my first linear dual rail power supply, and I came up with the following problem:

I have seen a lot of circuits on the internet and most of them connect the center tap connection of the transformer to the ground connection between the two filter capacitors at the input of the power supply ( figure attached). My question is that if the other two terminals (NOT centre tap) are being fed to the full bridge rectifier, what is the purpose of this connection?

The typical capacitances used as filters are very large, I have found one circuit with 10,000 uF capacitaces. Are these capacitances not large enough to guarantee a stable virtual ground for the power supply?

Thank you.

[PA0PBZ]

And how is the charge of the capacitors going to be divided between the 2 if your positive and negative load isn't exactly the same?
Or, why would you want a virtual earth if you can have a real one?

[madires]

I have seen a lot of circuits on the internet and most of them connect the center tap connection of the transformer to the ground connection between the two filter capacitors at the input of the power supply ( figure attached). My question is that if the other two terminals (NOT centre tap) are being fed to the full bridge rectifier, what is the purpose of this connection?

The center tab is your ground connection. Imagine V1 and V3 are alternating around V2.

The typical capacitances used as filters are very large, I have found one circuit with 10,000 uF capacitaces. Are these capacitances not large enough to guarantee a stable virtual ground for the power supply?

The capacitance is based on the current the prower supply has to provide and the ripple which would be acceptable for your application. Higher current and lower ripple equals to larger caps. Rule of thumb: about 2200 µF for 1 Ampere.

[T4P]

Center tap-grounding is a method used in full wave bridge rectifiers to attain positive and negative voltages

[D3K]

Thank you for your replies.

As regards to:

The center tab is your ground connection. Imagine V1 and V3 are alternating around V2.

My initial query was that if V1 and V3 are fed to the full bridge rectifier, what is the purpose of having the power supply ground to the common connection?

[madires]

My initial query was that if V1 and V3 are fed to the full bridge rectifier, what is the purpose of having the power supply ground to the common connection?

I thought I answered that question :-) Ok , let's try another way. Since you want to have a dual rail power supply you need at least 3 terminals, one for +V, one for -V and a common ground which is related to +V and -V. The filtering caps are put between +/-V and the common ground. Actually that third terminal is missing at the right side in your picture. If you leave the common ground (V2) away you got a single rail power supply. But then you would only use a single filter cap or put both caps in parallel.

[shebu18]

I may be rude but if you don't know why they do that with the transformer than it would be better to read theory before starting to design. Gather more knowledge.

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