Milli ohm meter getting warmer ...

So the aforementioned PCB arrived back, have assembled. 

I had to play with resistor values and so on,  but now I have a circuit that will pump 500mA through the device under test. There is a x100 amplifier which boosts up the voltage developed across the DUT to a suitable voltage for an Arduino 10 bit ADC to convert. 

So, a 100milli ohm resistor will develop a full scale reading. I'm going to have to add a range switch to lower the current if I want to measure bigger resistors than that. But then we have multimeters that can go down to a few hundred milli ohms in general. 

In general, quite pleased so far. I am going to assemble this into a bench power supply which happens to have one of my old boards that can read switches, write data to a flourescent display (which is only 30 years old). I am also going to float the power supply ground so we don't liberate magic smoke from things when I am using it. 

Yet Another Transistor Matcher - my version

 I found an article or a blog post on how to match transistors with a bit of slick interface. Matching transistors is pretty essential for exponential converters and ladder networks for filters on synths. Now you can do it manually with precision measurements, or you can use this circuit to show a quite groovy method. 

YATMA

I adapted it slightly - I have a whole drawer of old LF13331 analogue switches which I have used with an Arduino to sequence through, and a x1000 amp on the back side. It takes up to 8 NPN or PNP transistors, and plots their Vbe on a scope. Now since I am an old git and I like to be able to see the components I am soldering,  I am using pretty much all PTH components. How quaint, I hear you say. 

So, I have a small KiCad PCB made by Seeed Studios: 

The small Arduino Pico on the top just runs a small loop that switches two of the 16 switches on, connecting a current source (a chosen resistor) to one of the eight transistors in the ZIF socket on the other side. I had to put the ZIF socket on the back, so I can mount the board backside to the front panel with the socket sticking through. 

The jumpers can be moved to allow either NPN, or PNP (or simpler devices) to be selected. Then, the big BNC socket on the board is used to connect to a scope - I found the small old world scope managed to hold a sync much better than the digital one. 

The net effect is in this bit of a scope picture. I have AC coupling on so the tops of each step slope slightly in this picture - but you can easily see how you can select pairs of transistors with similar characteristics. 2 and 4 in this case are really close, 6 and 8 quite close and so on. It's surprising how quickly the levels change with a finger on the tranny - so it takes a bit of time to settle once the transistors are put in the sockets. 

And it works!  Quite pleased to be honest. 

More Kicad stuff ...

 Playing more with KiCAD these days. I designed a milli-ohm meter (it's not original - it's another of Rod Elliots designs, but I have done a schematic and laid it out. Quite pleased with the results. KiCAD is slowly getting easier to find my way around - clearly a lot of hot key work needed to make it slick. 

Class B DIY

I thought I would finally get around to building the amps from the bits that I bought a long time ago.

I came across the pages at Elliott Sound Productions a long time ago, and since I am no good at designing analogue electronics beyond an emitter follower or a long-tailed pair, I bought the components. I filed them safely in a box, and forgot about them.

More recently, I thought it would be fun to exercise some layout skills and engage my friends at Itead Studios in creating a small PCB.

So I am sure that Mr Elliott won't mind me showing the results of this small bit of labour. It's his design, just I didn't buy one of his PCBs - I thought it would be fun to try my own. I am going to build five of them, after all - two for the Studio, where the reference monitor is currently running on an ageing Cambridge P60; one as a test amp for the laboratory, and two in bridge mode for a subwoofer project, to drive a Peerless 850140 10 inch driver I bought 17 years ago from Wilmslow Audio. I've been buying little bits and pieces from them for more than 40 years.

Now if only I could lay my hands on the heatsink I put aside for this project when I was young ..

  

The first prototype board. I couldn't find any BC546A in the lab, but I did find some MPSA42. They're high voltage NPN transistors so they will be fine. The pinout is opposite to BC546A so they are in backwards compared to the silk screen. 

PS: I have a handful of the blank boards left over; if anyone would like one or two, please drop me a line via Google, and we can arrange for them to be sent.