I recently purchased a used 10A Carroll & Meynell Variac from eBay for use in the lab, however the variac often caused the 32A B-curve MCB in the consumer unit to trip due to the high inrush current of the variac core. To prevent this from happening and to add a few additional features I created the soft start circuit outlined on this page.
The idea of the soft start circuit is to limit the inrush current whilst the variac core is first magnetising. There are many ways to achieve this and most involve adding some form of resistive element in series with the transformer to prevent the transformer appearing as a very low impedance to the mains AC supply.
The method used in this project was to use a high power resistor in series with the transformer to limit the current. Once the current has settled, the resistor is shorted out by a relay to then allow the full load current of the variac to be drawn from the mains.
I “rebuild” my Geiger counter, the SBM-20 tube was initially inside the box, i have put this one inside a 32mm diam plastic tube, for more convenience, wired through a XLR3 cable. This counter is from “Electronique-Pratique” n°368, a french electronic magazine. Shem, pcb, and PIC hex & C source code available.
I had previously used the Adafruit TFT display using my library (ported from the Adafruit Arduino library). I decided to optimize the library to improve drawing speed. The same display I use comes with a 4-wire resistive touch-screen as well. I decided to write a simple library for the touch-screen and give Protothreads a try. To incorporate all this, I thought it would be cool if I used these to make a simple game. Tic-tac-toe came to mind as a fun little demo.
Many PIC16 family devices include an internal temperature indicator. These devices include the PIC16F72X device family, PIC16F1XXX device family, and the PIC12F1XXX device family. The temperature indicator is internally connected to the input multiplexer of the ADC (Figure 1). Refer to the specific device data sheet for more details.
These devices incorporate an internal circuit which produces a variable output voltage with temperature using internal transistor junction threshold voltages. The indicator can be used to measure the device temperature between -40°C and +85°C. The circuit must be calibrated by the user to provide accurate results, as the equation coefficients will vary between devices.
One problem we found was trying to prototype code using this microcontroller as unlike Arduino and any ARM microcontrollers there isn’t a small easy to use prototyping board available for the PIC24 chip. Microchip make an Explorer 16 Development Board which is designed to work with the PIC24 microcontrollers but it is large and fairly expensive and is designed to work best with other Microchip addon cards.
With this problem in mind we decided to design and build a small prototyping board that would work with the PIC24FJ128GC006 as well as one of Microchips DSPIC33EP256MU806 dsPIC series microcontrollers.
The prototyping board was designed with removable daughter boards for the microcontroller.