Oct 1, 2015

12V LED Backpack Power Supply Circuit

In this article we learn to make a simple 12v LED backpack power supply circuit for powering a 36watt LED lamp, which includes appropriately wired integrated sockets for enabling the attachment of external devices such as a regulated battery charger, ammeter, voltmeter etc. The idea was requested by Mr.  Kevin Bates

The Request

Firstly, I must thank you for tasking the time to look at my project with me. Although I have some understanding of electrics and electronics I am not very knowledgeable . I am an engineer by trade and have a good head for visualization and comprehension; I am also skilled at machine works and soldering.

This project is a hand made wooden backpack with three equal sized compartments ; the bottom two have a single fixed cover held in place with brass wing nuts and the top cover is hinged : -

1. The bottom one; this will house the battery. It is vented via the brass louvered grill.

2. The middle compartment; this will house all the electrical , including those which face out (panel mount) through the outer cover , being the Isolator switch, charging socket, amp meter and volt meter.

3. The top compartment which has the hinge is for storage of daily bits and bobs. The concept: - In our Steampunk world, my wife is a time jumper and I , a demon catcher . I wear the backpack which has a special gun which fires a charged beam of particles (not for real) this interrupt the jumper and demons ability to teleport .

Once captured the demon or jumper is held in stasis. This is where the LED lights come in, representing the particle beam.

The LED controller has an infrared pickup, which controls the motion, color and speed of the lights. The feed to the lights splits into two as it leaves the controller.

One section of lights goes down to my gun and the second section of lights goes down to my belt where it forms a lasso. This section has a hook on the end which can be clipped to a collar my wife wears. This forms the “ Keeper ” in our story, preventing her from Jumping at will.

With the Infrared remote either of us can control the lights movement, color, speed etc. Although the lights are split they remain a total of 36W

The battery will be recharged when the backpack is off , the charger is a self - contained (wall wart) type which came with the battery 12V 6 amps, so the charging socket is only an interface between the outside and inside the battery compartment. No charging circuit is required. 

What I would like to achieve : -

  • A circuit board which will provide me with a stabilized voltage regulation of 12V 3 A via a 5 A switching rectifier? to reduce heat generation, and incorporating some form of circuit protection – Which I will need your expertise to determine .

  • The ability to run the volt and amp meter via removable plugs

  • To use the Isolation switch to isolate the battery from the rest of the system during charging.

  • As an option I would like a couple spare 12v out sockets mounted on the board so that I can expand in the future. I hope this is adequate , if not I will try again, if you ca n bear with me
Kind regards
Kevin Bates

The Design

12v LED backpack power supply circuit

As shown in the above 12V LED backpack circuit diagram, the wiring and the circuit layout appears to be pretty straightforward, and with bare minimum components. The details may be understood as given under:

The input supply is acquired from a ready made 14V/5amp SMPS unit, which can be seen at the extreme left side of the image.

If the user prefers to build the SMPS himself he may do so by referring to the diagram that's presented in the following article

12V, 5amp SMPS circuit

However procuring it ready made would be much easier and hassle free, and is recommended here.

As requested, the output from the SMPS can be seen passing through a changeover switch stage for enabling the option of a direct input to the load or through an ammeter depending upon the position of the SPST switch.

Next, the input supply is allowed to traverse through an LM338 IC stage configured as a current controller in order to manage the correct  amount of amps to the load (LED modules), as well as for the charging of the battery.

The current limit is achieved by appropriately calculating the value of Rx. The entire procedure for calculating the current limiter resistor may be learned from the following article:

Universal current limiter circuit

As indicated in the request, the current controlled 14V supply is terminated into a couple of pluggable sockets, one for enabling voltage measurement externally, while the remaining two facilitate the powering of the LEDs and the charging of the battery via another SPST switch.

The regulated 12V output from this 12v LED backpack power supply circuit can be seen connected with an extra separate socket so that the user is able to access the voltage for powering any other desired 12V gadget through the same.

Feedback from Mr. Kevin

Hello Swag,

Many thanks for your assistance, I took a look at your design but noted a
number of things which I would like to put forward for your
confirmation, please.

- I note a permanent charger SMPT....IN my design this would need to be
the battery - I note a switch on Ampmeter, but I would need to have mine
permanently connected. - You have labelled the Charging Socket as "Output"
but I require it to be an "Input". My charger plugs directly into the wall
and charging will be through this socket.

I have redrawn (in basic form) what you provided me but with my
alterations, would you be kind enough to check the design through and make
sure it is OK.

Many thanks and kind regards


My Response:

Hello Kevin,

Thanks for pointing out the mistakes, however we have another associated problem here, it is the LM338 current controller circuit which needs to be used both ways, that is while charging the battery and also while illuminating the LED, therefore the wiring needs to be done rather in the manner shown in the attachment, please check it and let me know if you any doubts.

Hopefully this will solve all the indicated issues.

Best Regards. Swag

In the corrected diagram shown above, a single TPDT switch is employed and is able to achieve the intended charging of the battery as well as the illumination of the LED through a common LM338 current controller circuit.

So this idea enables both the charging of the battery and the illuminating of the LED to be implemented via a safely calculated current controlled supply, making the unit much compact and cost effective.

Rest of the section of the diagram are self evident.

Feedback from Mr. Kevin

Hi Swag,

Just to say thank you for all your help with this project, I have just ordered all the bits I need to complete.

All the best


Hi Friends, Welcome to my site, a place where you will discover a massive collection of electronic circuit ideas, mostly requested by the dedicated readers and exclusively designed by me for their customized application needs. I have posted more than 1100 circuit designs in this site, if you have a personalized circuit requirement you may feel free to request it through the comment box, if it seems feasible to me then surely you may find it published here with your credentials attached in the post, thanks and please keep reading