I was slightly bored, and I was thinking about how I used to be scared of trying surface mount parts, so I created this. I hope at least one person finds this helpful (or at least amusing).
Tutorial: Stripboard-based Adapter Boards for Surface Mount Transistors
Or
How I Learned to Stop Worrying and Love the SOT-23
MOTIVATION:
Through-hole JFETs are getting more expensive as manufacturers phase them out. To say that the supply is being rapidly depleted would be a bit overdramatic, probably, but the available supply is definitely decreasing and will likely continue to do so. However, surface mount JFETs continue to be available at reasonable prices, and they are suitable for guitar pedal building if you can adapt to using the tiny form factor. Adapter boards are commercially available to convert the surface mount form to a conventional through-hole terminal spacing, but these boards are sometimes more expensive than the transistors themselves.
GOAL:
To cheaply and easily modify the SOT-23 JFET to work as a drop in substitute for the through-hole equivalent transistor.
In this tutorial, I will be using a MMBF5457 and modifying it to substitute for a 2N5457. The same steps will work for other transistors, such as the MMBFJ201 as a substitute for a through-hole J201. These are the pinouts of the 2N5457 and MMBF5457:
Instead of using a custom etched PCB, I'm using a tiny piece of stripboard (Veroboard).
The final product will look like this:
MATERIALS:
One piece of stripboard, 3 rows × 4 columns, and
one MMBF5457 transistor.
You will also need a little bit of wire (and a soldering iron with solder, of course).
STEPS:
Cut:
The board requires only a single trace cut. Be careful not to make it too wide though. The source and drain terminals of the transistor will need to straddle this gap.
Prep surface:
Lightly apply solder to the spot where the transistor will be mounted. This will make subsequent soldering easier.
Place:
This is the correct location and orientation for the transistor. Tweezers may be helpful for placement.
Tack:
To successfully solder the transistor, it is helpful to tack it down with a temporary solder joint at one of the transistor terminals. Depending on how much solder is already on the board you may be able to simply press down on one of the terminals with a hot soldering iron tip. Here, I grabbed a small blob of molten solder on the iron and unceremoniously plopped it onto the gate terminal while using my fingernail to hold the transistor still.
Solder:
Once the transistor is temporarily fixed in place, solder the unattached terminals before returning to clean up the initial tack with a proper solder joint.
Jumper (and optionally clean flux):
The last key component to our adapter is a wire jumper connecting the top holes of the left and center traces. I have also taken the opportunity to clean up residual flux on the board using a toothbrush and isopropyl alcohol. You could wait to do this clean up later, or you can probably get away with not doing it ever.
At this point, the adapter board is effectively done. The bottom three holes are electrically connected to the drain, source, and gate terminals, respectively, of the transistor. You can solder header pins or wires to these holes and connect them to the appropriate places in your circuit and enjoy. If, however, you want a through-hole device with a minimally-bulky, reasonably-robust set of solid leads, I suggest the following additional steps.
Leads:
Prepare the leads by bending three pieces of wire similar to the jumper used previously, but with one side substantially longer.
Insert the wires into the copper side of the board like this.
Solder the wires into place at both holes and along the length of copper in between. Then, bend the leads straight, clip them to the desired length, and trim the ends poking out of the other side of the board.
CONCLUSION:
Congratulations! You now have an inexpensive through-hole transistor equivalent made by adapting a surface mount transistor.
