12V – 4A Power Supply
This is a circuit for 12V – 4A Power Supply. This circuit uses BD705 and L200. To regulate the output voltage this circuit use the 3.3 k resistor and the 1K potentiometer, PT1. Here is the circuit :
From the second figure, we can see the current limitation of this circuit. The trimmer PT2 works on strech AB of characteristic. The current from 3 to 4 A can be limited by using the given values (R=3k, PT2=1k, PT3=470 ohm). connecting one terminal of PT to the power transistor’s base or increase the Rsc’s values will increase the field of variation. But, the stable limitation will decrease. The section BC will be move, if section AB is moved.
The PT3 can be used to vary the slope BC. The zener diode’s voltage is used to fix the voltage level at point B. Together with capacitor in parallel connection, it will ensure correct switch-on with full load.
The last figure shows the addition protection for the external power transistor. The prevention of the Tj of the power transistor from reaching its maximum value is done by the PTC resistor that should be fixed to the dissipator near the power transistor. Selecting PTC will affect dimensioning of RB and RA. [STMicroelectronics Application Note]
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August 5th, 2010 at 8:02 am
I have a 24V 8A power supply. Using an LM7812, I want the output to 12V but retain the 8A from the supply, or atleast 4A. How can i achieve this?
Thanks.
August 5th, 2010 at 1:04 pm
You can use the circuit in http://www.eleccircuit.com/12-volt-30-amp-by-7812/ . You’ll see 6 transistors for boosting the current, but for 8A, you can use just two sets of transistor-resistor. You have to provide very large heat sink since the transistor must dissipate 96Watt of power when a 12V drop voltage is developed across the transistor to supply 8A@12V.