Dual Voltage Regulated Power Supply





Introduction: Dual Voltage Regulated Power Supply

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

Component List:
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

I bought some generic plastic enclosure for the project. Drilled some holes on it to have proper heat dissipation. During the test, the MJ2955 generated heat but not very much, I can still touch it. Connecting it to the CD-Rom was staright forward and it played right away.

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.



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    Can any one please help to identify the MJ 2955 pin out put? What is the voltage and the amp of the transfomer output? too hight voltage of the LM 7805 will make the IC hot, any advice? I'm beginer for this project, i use it for my CD ROM also.

    Help plz. im confused about "MJ2955 NPN Transistor " when i look up mj2955 it says that it is pnp and the 2n3055 is the npn version so which one do i use??

    I went to Radioshack the other day to buy the parts I needed for this project (I'm only using the 7812 side, though), and I was able to get the 7812, the MJ2955 (known as the 2N3055 at Radioshack), and 5 10 Ohm 1/2 Watt resistors. As you can see, I'm leaving out the 4700 uF 16 Vdc capacitor and the 1 Ohm 1/2 Watt resistor, but I was hoping that I could replace the capacitor with the 4700 uF 35 Vdc capacitor and the resistor with one of the 10 Ohm 1/2 Watt resistors that I found at Radioshack. By the way, for my project, I am taking a 15V battery supply (from 10 normal AA batteries though I would love to use the 10 rechargeable AA batteries that I have that go out a little over 12V which is enough to not work for my situation) and running that hopefully through this circuit and into a DC-DC power supply to power a Mini-ITX computer that runs about 18W (batteries are 2.5Aph providing about 30W which will definitely work).

    You could have connected a diode from the 7805's GND pin to GND as well; this will boost the output relative to GND with 0.7, which is then dropped over D1 resulting in a 5V output.

    Hi can any one help. I'm looking for a circuit that i can connect to a 12v van battery & step it down to 5v 2amp. its for an LCD picture frame. I saw this circuit & was wondering if i could just built the 5v part & if that would work??

    morph1664 you can use a 78T05 type 3 pin positive fixed voltage regulator in a TO-220 package which will handle up to 3A with heat sinking. The input voltage needs to be kept 2.2V-3V above the output voltage but they will handle an input of up to 35V. Remember though the higher the input voltage the more heat the regulator has to dissipate. If it the junction gets too hot (about 125 degrees), regulation will begin to fail. This series (78T0x) of devices can be used with a series–pass transistor and suitable heat sinking to supply up to 15 A at the nominal output voltage. I think you'll find that most specs are quoted at 25 degrees Celsius with a maximum power dissipation of 25W.

    A circuit to do what you want (a simple voltage regulator) is usually contained in any datasheet for a given device. You only need at most two capacitors to complete the circuit. Do a quick search for 78T05 data sheet.

    A better more stable switching supply would be made by using an LM2576 switching regulator which will supply 3A but at higher efficiency and reduced heatsink size (or even no heatsink in many cases). They also have better current limiting and thermal shutdown features than linear regulators. The component count is a just a few more with a catch-diode, choke and two capacitors being needed to complete the circuit.

    Hi Everyone... Can someone provide me a functional circuit regulator for a 70amps24v and 70amps12v PMA. It's for my friends experiment. I'm trying to help him actually. It will be great it if has a built in ovecharge protecton. Thanks guys... more power to all

    can someone help me?i have this transistor that says PL on the top, 7805 in the middle,and 2 MC at the bottom...but i dont know if its a voltage regulator can someone please reply me back if it is?