Saturday, 18 July 2015

Prototyping with Small Outline Transistors

A recent circuit idea I've been fooling with required a complementary pair of MOSFETs. I'm tripping over the familiar n-channel devices, but p-channel MOSFETs don't seem to breed in the m0xpd junk box. Accordingly, I had to spend some of my hard-earned money...

The easiest (close-to) complementary pair I could source (from traders on the familiar auction site - cheaper than going to a major component supplier, once the minimum order quantity and shipping is factored in) was the 2N7002 and the BSS83. They cost pennies and arrived in the blink of an eye. The only problem was that they are in SOT-23 packages, making prototyping a rather different proposition to something in a leaded package like a TO92.

As regular readers will know, I like to use solderless breadboards for breadboarding (what else). I needed a method of working with the SOT-23 transistor in a prototyping context.

Here's my solution. I'm sure it isn't original - but it has worked well for me, so it might be worth sharing...

The SOT-23 package has three pins on a 0.89 mm pitch. This is close enough to a 1mm pitch to ba able to fit on three adjacent "lands" of a SOIC ("Small Outline Integrated Circuit")  PCB footprint. Such a footprint will be found on adapters, which can be used to "convert" a SOIC to a dual-in-line format and which can be had very cheaply from on-line retailers.

Here is a SOT-23 transistor, placed on a 16 pin SOIC to DIL adapter...


In the photo above left, you can see the the lands are too short to allow convenient access to the pins on both sides of the SOT package. In the photo above right, you can see that the SOT-23 device doesn't quite bridge the gap between the two rows of lands for the IC (although one of my guys at work has succeeded in soldering the device in place in this position, creating solder bridges between the transistor pins and the nearest lands).

I found an 8-pin SOIC - DIL adapter, in which the lands are rather longer...


The transistor's pins are accessible on one row of the smaller PCB's long pads/lands. You can also clearly see the mismatch between the SOIC pitch and the SOT-23 pitch - not enough to cause a problem over only three pins.


So, I made up a single 8-pin "device", carrying a pair of complementary MOSFETs...



The transistors - too small for me to hold - are now easy to use in the solderless breadboard.

Here's the finished "device" in use in the prototyping exercise...


You can see how small it is in contrast to the neighbouring TO92 (a voltage regulator).

Now the prototyping is done it is time to use the circuit - but that's another story.

...-.- de m0xpd

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