# Linear Power supply with centre tap AC GND Transformer and adjustable LM338 regulator

#### Milo777

Joined Apr 5, 2017
5
I am building a Linear power supply for a project at school and the transformer I am using is centre tapped so that there is a split voltage of two 16 volt AC supplies if needed. I need a higher voltage of 28 Volts to supply an audio Amp IC (TDA7262) so I will not be using the split supply rather I will use the 32 Volt potential difference across the two 16 volt AC outputs.

I have found a rather simple circuit that looks like it will suffice. The difference is that the circuit i have found does not use a centre tapped transformer. So on the diagram i have attached i have drawn in where i believe the AC ground should go.

My question is:

Is that the correct way to ground my circuit, using the ac ground from the centre Tap? A rather simple question I know but I am just in two minds about it. I feel that there could be noise coming from the ground source, or is that non-true?

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#### AlbertHall

Joined Jun 4, 2014
11,689
Don't connect the centre tap at all. As you say, just treat it as a single winding.
There is a possible problem with the LM338. If the supply was operated at an output voltage of 1.2V and an output current of 5A then the power dissipated in the LM338 would be large.
(24V x 1.414 - 2V - 1.2V) x 5A = 154W
The datasheet doesn't give a maximum power rating, just says it is internally limited, but it is definitely going to need considerable heat-sinking to cope with that. Even with an 'infinite' heatsink, the junction temperature will be over 100°C above ambient.

#### Colin55

Joined Aug 27, 2015
519
The losses will not be 154watts.
You are using 28 volts of the output. So only the voltage above this will become heat.

#### AlbertHall

Joined Jun 4, 2014
11,689
The losses will not be 154watts.
You are using 28 volts of the output. So only the voltage above this will become heat.
Yes, I was going by the posted schematic.