Quick and Dirty SMD SOT Transistor Converters

Introduction: Quick and Dirty SMD SOT Transistor Converters

About: The nickname is because I couldn't spell "Frobscottle". Loving getting back into electronics as a hobby after a break of many years. Now I work as an EPOS engineer, so I spend my days fixing tills in…

Sometimes you need to attach wires to a surface mount transistor. Some reasons you might want to do this are:

  • you have a re-claimed transistor you want to use which happens to be surface mount
  • you want to try something out on a solderless breadboard
  • you need to replace an obsolete transistor and the only replacement is in SMD form
  • you want to attach long wires to it

I tried using cut up pieces of scrap PCB for this in the past, but it is awkward to do. Then it occurred to me that I have some scrap PCI cards sitting in my junk box and edge connectors are almost the right spacing to fit a SOT23 package on.

The edge connectors have the advantage of being gold plated, so are ridiculously easy to solder, and are quite robust.

Supplies

  • SOT transistor you want to put wires on
  • Old PCI card
  • Shears (eg tin snips or aviation snips)
  • Soldering iron
  • Cut off component leads or other thin wire

Step 1: Prepare the Connectors

You need some shears for this which are able to cut the PCB. I have tried tin snips and aviation snips, both work well. Though tin snips give a better cut I don't like the idea of what the board is doing to them! You could of course use a hacksaw.

Cut off the edge connector. Take a little bit of green with it, which you can trim off later if desired. The reason for this is damage limitation, since the snips distort the board, the strip of green provides a bit of protection. It also gives you some margin for error when the cut goes astray.

Select a segment with 3 contacts together and cut it off.

Do any cleaning up you feel is necessary.

If you want to attach a smaller package than SOT23 (I did SC70, pictured) you can try attaching it to just 2 pads, by cutting out a small piece from the centre of one of them.

Step 2: Mount the Transistor

Decide what order you want the transistor legs to appear in. With the single leg at the top, you get pins in the order 1,3,2, if it is at the bottom you get 2,3,1. This might be important to you, or it might not.

Tin the spot where one leg has to go

Position the transistor so one leg is over the spot you tinned

You have to be careful because the transistor wants to slip off the solder, and also the pin has to be positioned at the inner edge of the connector.

Anyway, re-heat the spot you tinned so the one transistor leg is soldered to it.

Check very carefully as the transistor only just fits the connector, the outer 2 legs will be at their edges. You may need to twist it slightly, or re-heat and adjust

Solder the other two legs, then if necessary, apply fresh solder to the first one you did.

Step 3: Attach Legs

This is where some of those component leads you've been saving come in handy, for making component leads!

If you don't save them, what's wrong with you? They are dead useful. Anyway, you can use thin solid core wire instead.

You need 10 to 15mm for each leg. It's easier if you tape the transistor down before soldering. I used "Koptan" tape, which is a lot cheaper than Kapton tape.

Tin the ends of the wires, and tin the contacts with a good blob of solder. Be quick doing this as the transistor joints melt very easily.

Solder each wire on, the centre one being straight, and the two outer ones sticking out at about 20 degrees

Bend the outer two wires so you end up with about 0.1" spacing between them and the centre one.

Trim the legs so they are all the same length.

For a smaller package on a 2 pad board, trim the long pad so the wires from the 2 half pads can pass either end of it.

Step 4: Test and Deploy

Test the transistor using your preferred method. I used a "Hiland" type component tester, which is a brilliant piece of kit.

This particular transistor was dismantled from something. I originally chose it because it's marked PD, which according to the information I first had makes it a BSS84, which is a P type mosfet. Testing proved it to be a PNP however, and further investigation showed it to be a 2SA1171.

I wanted to use this particular transistor on a solderless breadboard, so you can see the demo photo there.

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