Introduction: 9V to 5V Regulated Power Supply - SMD

About: I have been working in IT since the mid 1980's. Most of that has been database and application development. I've been working on Internet application development since the late 1980's. And, we're back in Tass…

I have wanted to make an SMD version of my 9V to 5V Regulated Power Supply for a while now, but I've been waiting for the arrival of some 0.1uF SMD Electrolytic capacitors to arrive from China (bought from eBay). The first lot that I ordered didn't turn up, so I had to get the vendor (tiaochongyi) to resend.

The vendor was very good and resent the capacitors as soon as I alerted them to the failure to arrive. The main hassle was really having to wait again ... I'm a bit impatient, but then, when you order components from China, sometimes the parts don't arrive. It's just a fact of life.

This project was brought about by my desire to make my modules as small as I can. I can still save more space on this board while still satisfying the interface constraints. In this case, I am using a barrel socket (2.1mm x 5.5mm female socket) for input power and 4 male pin headers for connection to a solderless breadboard.

It turns out that I didn't measure very well and this module doesn't actually fit on my solderless breadboard (out by a bit).

Also, my original circuit design was wrong too ... I had the 9V power connecting on the wrong side of the SOT-89 L78M05 ... my excuse was that the component in Fritzing had ALL of the legs of the SOT-89 marked as PIN3 OUTPUT ... I was able to correct this with a sharp knife and a jumper wire, as you will see in the circuit. Until I did that, the module was simply pushing out the full 9V which is, as I'm sure you'll agree, pretty pointless.

Step 1: Design the Board

The SOT-89 LM78L05 voltage regulator has the pins arranged Output, Ground, Input, this is the opposite from the through hole LM7805 regs that I've used up to now, so I managed to get the pins mixed up in my original design.

The above Fritzing design has this problem corrected. So, if you want to build this, please ignore the populated boards that are missing the jumper (that's how I fixed the circuit).

I wanted the board to be pretty much the same as my through hole version. It needs to take 9 volts in and deliver a regulated 5 volt supply ... this will be used connected to a solderless breadboard as well as a design module for my Arduino projects.

I wanted to do this using surface mount device (SMD) ... also known as surface mount technology (SMT).

To that end, I am using:

  • 1 x LM78L05 5 volt regulator (SOT-89 SMD);
  • 1 x 220 ohm resistor (0805 SMD);
  • 1 x super-bright white LED (0805 SMD);
  • 2 x 0.1uF Electrolytic capacitors (0405 SMD);
  • 4 x male pin header (through hole);
  • 2 Pin 5.5*2.1mm DC socket 2.1 (through hole)

The DC socket and the pin headers are mounted on the (bottom) non-copper side of the board. I'm using 4 pin headers rather than 2 so that I have better mechanical connection to the solderless breadboard.

Step 2: Populate the Board. Hand Soldering SMD Components.

Soldering SMD/SMT components can be challenging.

The approach that I use is to tin one of the connection points for the SMD component with a small blob of solder, hold the component onto the board using some spring tweezers so that the leg to be soldered is sitting on or very close to the solder blob, and then heat the blob of solder so that it melts and welds to the leg. Of course, you can't hold the iron onto the solder too long or you risk burning your component or ruining the solder.

Once I have one leg welded to the board, I then go over the rest of the legs of the component making sure that I get a decent solder fillet.

If I end up with too much solder (happens a bit) or a solder bridge, then I use some solder wick (braided copper strand) to suck up some excess solder.

In the first image, I have the VCC of the power socket connecting into the OUTPUT side of the LM78L05, which is kinda stupid. After soldering the board up I gave it some power and noticed that I registered 9 volts everywhere (well on the positive rails, anyway). So I figured that I had made a mistake.

As a carrier for a white LED, the circuit was certainly working ... as a 5 volt regulated supply ... it was rubbish :)

Step 3: Fix the Circuit

To fix the circuit, I wicked off the solder on the power socket VCC leg and then I cut the copper trace with a scalpel. To cut the trace, I cut in two places, at the connection to the Output 5 volt rail and at the union of the VCC leg with the board. I then used a sharp metal probe to remove the copper trace between the two cuts so that I ended up with a 2mm gap in the trace (no electricity is going past THAT cut!).

I then cut a short length of jumper wire and soldered it to the VCC on the left-hand side and to the Input leg of the LM78L05 regulator (pin 3).

With the power re-routed, I gave the circuit some power and measured the voltage at several points around the circuit. Thankfully, the power at the output was now an acceptable 4.97 volts. (Whew!)

The four blobs of solder that you can see near the middle of the circuit are the positive output (left hand pair) and the negative output (right hand pair). This is a poor design because there isn't enough space between the power socket and the pin headers to fit on the solderless breadboard. The design that I've placed on this instructable has been corrected so that there IS enough space, but I would still print the design out and check the spacing if I were you.

This version of the SMD 5V Regulator will do service via some jumper wires inside another project, so I'm not concerned about using this module.

Step 4: Assets

You can download the Fritzing circuit and the etchable PDF from here.

Good luck and happy soldering!