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GCA08 High efficiency universal power supply for 1 or 3 Amp


By Peter Giling


On many places under the railroad table, we need all kind of DC power supplies.
Buying adapters is one option, nowadays also many of them are high efficiency, meaning not evaporating a lot of energy into heat.
Unfortunately many of these adaptors do not give what they promise.
Furthermore, the very annoying thing about having these adaptors under your table is the fact that you also need 230V there.
And exactly that, specially where infants like to help there parents with this nice hobby, is not much to encourage.
The better option is to run a low 16V ac line under the table and use that.
This power supply will give you that option and can be set-up in various ways and voltages, either adjustable or solid.
For ac input, it needs at least the same voltage as the desired output.
If you are running dc line under the table, it should be at least 2 Volts higher than output.
Adjustable versions could be as low as 1,25 V, up to the maximum you put in.
The switching regulator is very energy efficient.
On average the efficiency will be more than 80%.

There are two units available:

  • GCA08-1 1 Amp output, size 40x22 mm
  • GCA08-3 3 Amp output, size 40x62 mm

The final results

GCA08-3 the 3 Amp unit GCA08-1 the 1 Amp unit
For both units a professional board is available

GCA08-1, 1 Amp output

This simple supply can be used in different ways.
If you install a standard LM78xx regulator, it can supply a full 1 Amp when the regulator is cooled sufficiently.
Without cooling, it is very depending on the incoming voltage and the used current, how far you can.
If it goes too far (regulater gets too hot) the regulator will stop.
Output voltage can be between 3,3 and 24 volt
This is the cheapest way.
If you use a R-78Ex.x regulator, no cooling is required, and the full load is available.\\
No external settings are needed, since this very efficient chip is option in most common fixed voltages.
The maximum output is 0.5 or 1 Amp, input from transformer can be up to 20V ac or 24V dc.
A total of 6 parts are located on this board.
Efficiency with this power chip is better than 90% which makes cooling totally unnecessary.

GCA08-3, 3 Amp output adjustable

This board is able to supply 3 Amp in a chosen voltage.
Please refer to tables below for selection of the right output.


Selecting the output.

This board can be equipped in 1 Amp output max, or 3 Amp, depending on the chosen chip.
Maximum input ratings for all configurations:
dc 30 Volt
ac 24 Volt
For correct calculation of needed transformer, see which transformer below

Selection table for various voltages and current.
Output (V) VR1 for 1 Amp VR1 for 3 Amp R1 R2 P1
1,24 LM2575T-adj LM2576T-adj x short* no
3,3 LM2575T-3,3 LM2576T-3,3 x short* no
5 LM2575T-5 LM2576T-5 x short* no
12 LM2575T-12 LM2576T-12 x short* no

* *short means to insert a short bridge (piece of wire).

Other voltages can be achieved by installing resistor R1 and a small adjust potmeter 20K.
Output (V) VR1 for 1 Amp VR1 for 3 Amp R1 R2 P1
1,24-12 V LM2575T-adj LM2576T-adj 1 KΩ no 20 KΩ

With this setup output will be adjustable from 1,24 V to max 26 V depending on input voltage.

Using a potmeter inserts the risk of output changing after a while.
Potmeters are never stable on the long run.

This table shows various , not adjustable voltages.
Vout (V) VR1 for 1 Amp VR1 for 3 Amp R1 R2 P1
6 LM2575T-adj LM2576T-adj 1 kΩ 3,9 kΩ* no
8 LM2575T-3,3 LM2576T-3,3 1 kΩ 5,6 kΩ no
9 LM2575T-3,3 LM2576T-3,3 1,8 kΩ 12 kΩ no
10 LM2575T-5 LM2576T-5 1,2 kΩ 8,6 kΩ no
16 LM2575T-12 LM2576T-12 1 kΩ 12 kΩ no

Vales in this table are a close approach.
Far more exact values can be achieved by using 1% metal resistor from 96 series.

Resistors are calculated by the following formula.

R2 = R1 x ( ( Vout/1,24) -1) Ω
N.B. Potmeter is not installed.

Fuse F1


Do not think it will be unnecessary, because little mistakes can cause injuries or fire hazard.
With PTC fuse assembled, you are always safe, and never have to worry about ever having to replace it.

Also the fuse need to be adapted to the the desired output.
In general the values for the multifuse should be:
For 1 Amp out use 0,5 - 0,8 Amp fuse.
For 3 Amp out use 1,5 - 1,85 Amp fuse.

Which transformer ?

Basically the transformer should have the following parameters:
Secondary voltage : minimal equal to output.
Power (in VA) Minimal (V x A x 1.2), where :

  • V = Volt output
  • A = Ampere output.

It is possible to use a DC power supply.
The input voltage should be at least 3V higher than output.

gca08-en.txt · Last modified: 2021/04/07 16:58 by smitt48