A friend of mine who runs an electronic shop wants to install an old cd-rom to be used as a stand alone cd-player in his truck. His problem was to find a suitable power supply for this purpose. A cd-rom uses 2 power supplies, 5 Volts which is used in its logic circuit and 12 Volts for its servos. With these parameters I needed to make a dual regulator that produces 5 Volts and 12 Volts output in one input voltage. Trucks usually uses 2 lead-acid batteries so that will be around 24 Volts.
The cd-rom uses around 1.5 Amperes of current for the 5 Volts and 2 Amperes for the 12 Volts. Considering all the needed parameters, using two 7805 regulators for the 5 Volt supply will suffice however we need higher current for the 12 Volts. Using an "old-school" technique, we will be utilizing one 7812 regulator but putting an additional transistor to beef up its current capacity up to 5 Amps. I know its an overkill but better safe than sorry.
Step 1: PCB and Assembly
IC1 and IC2 - 7805 series regulator
IC3 - 7812 series regulator
Q1 - MJ2955 NPN Transistor
R1 - 1 ohm / .5 watt resistor
R2 - 10 ohms / .5 watt resistor
C1 and C2 - 4700 uF / 16 Volts electrolytic capacitor
Using the diagram, make the PCB, you can see it in my previous blog. Put sufficient heatsink for all the IC's to maintain a normal heat.My PCB design for the diagram, this conforms with my plan to put all the regulators and transistor in one heatsink.I had bolted all heat generating components using only one heatsink, as you can see, the center is the MJ2955 transistor, two 7805's on the right side, and the 7812 on the left.Soldered all the components including the MOLEX connector for the CD-ROM and this project is ready for testing.
Step 2: Enclosure and Testing
Theoretically, this is also applicable to supply Hard Drives but I still have to test this myself. I'll post updates soon.
Color Configuration for the MOLEX connector:
Yellow - 12 Volts
Black - Ground/Negative
Red - 5 Volts
- Double check this before connecting to your CD-ROM, this will FRY your device if inverted.
Step 3: Alternate Diagram
To address the comment of koocotte regarding the different voltages from two 7805 regulators, I have added two diodes in forward bias on each output in the circuit.
Step 4: Voltage Spike and Voltage Drop Issues
-To control the positive and negative voltage spikes from mechanical relays when turning a vehicle off (toma's comment), a simple zener bridge will do the trick. This will "pre-regulate" the input voltage going to the regulators.
1N5359B zener diodes - 2 pcs.
-To compensate for the voltage drop caused by the diode that we connected in the outputs (+/-.7 volts), just replace the two 7805's with 7806's. This will give us roughly 5.3 volts in the output.