Apr 282018
 

So, I’m learning now to read datasheets a little more carefully.

Finally got around to testing that board, and at first, I had some shorted connections to deal with and one dry joint.  No problem, a bit of scoring with the knife and a tap with the soldering iron, and we’re in business.

Apply 9V to the GPIO pin, and ~3V appears at the output.  Good enough, the AVR will recognise that.

Now for the MOSFET, applying 5V to the input, switches the MOSFET.  Great.  Try it on the actual synth… sure enough I can make noise, but nothing lights up.  What gives?

In this prototype I’m using Fairchild FQI4N80s, which I bought a tube of 50 of them.  Turns out these MOSFETs have 40ns turn-on delay, and a 100ns rise time.

Doesn’t seem like much, but then consider this: our PWM clock is 64MHz.  That means the pulse time can be as low as 15ns.

They also can have a gate threshold as high as 5V… so borderline for this application.  Never mind, I’m sure they’ll be useful in other projects.

I have a couple of options in mind, both by Infineon: the IPS70R900P7SAKMA1 and the IRLU120NPBF .  Out of those two, the latter sounds like the better bet, I seriously don’t need 700V Vds!

It is tempting though to design a board based on the prototype though, as I think if I make room for a TO-220 or TO-251 (IPAK; not to be confused with the larger I²PAK), there are actually a lot of choices out there, and much of the time, the choice is not critical.

If I do that, I’ll be spacing out the KK connectors a bit, as that’s another point: the plugs are just a wee bit wider than the sockets, so I need to make room for that.

Jan 212018
 

So last week, some more parts arrived for this project.  Crucially, some capacitors, and some MOSFETs.

Turns out, the MOSFETs I got last time were not I²PAK … I thought that’s what I ordered last time, but clearly not, because that’s what these are, and they’re bigger than the previous ones.

No matter, lean them back like dominoes and they still fit.  I’ve got a tube of 50 of them.

I was able to put the pull-down resistor in, and I basically fitted the MOSFETs along a track, scoring the track between the legs so they didn’t short out.  For the 12ohm resistor to the drain, I’m doing half-veroboard-PCB, half-point-to-point construction to link the drain pin (annoyingly in the centre) to the outside world.

I’ll need to rustle up a 2-pin KK to act as the power input, and that board is done.  I might add two in parallel on here so a short lead can link from here to the mainboard to supply +12V.  This will go on the right-hand side (lower photo) just past where that jumper connects.

After that, the next step is wiring up the buttons and switches.  The use of 4-pin connectors should greatly simplify the wiring since everything we need is on the one connector.