Steve Gardner is an Electronics and Firmware Engineer in the UK. He works at an engineering design consultancy where he primarily designs electronics subsystems for medical applications. His focus is on analog and power electronics although most design work involves the use of microcontrollers, processors or FPGAs which he is proficient with.
In early 2015, Steve decided to start publishing regular content on his YouTube Channel to share some of his knowledge and personal projects, which he hopes to grow both to engage others in the hobby and also to help complete stalled projects…
Steve started electronics as a hobby as early as he can remember and has always had a fascination with learning how electronic devices work. He has had a full time electronics job for the past 12 years designing a multitude of electronics devices from pregnancy testers to precision surgical cutting tools.
I thought this was an interesting device, a calculator kit that also has a few electronic functions – resistor colour code calculator and LED current limiting resistor calculator. Take a look here: https://www.banggood.com/custlink/mDKDhu0ZKT
You can buy the FY3224S 24MHz Dual-channel Arbitrary Waveform DDS Function Signal Generator for the reduced price of $65.55 during May 2016 by using the following discount code at the link below. The signal generator is featured in a few of my videos and is a good value unit for the functionality it offers.
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.
The electronics adds a few additional features such as an emergency stop button and remote control of the variac, however, it’s worth noting that this is not really a suitable ‘Emergency’ stop button which should really be a fully hardware implemented solution – NOT involving software running on a microcontroller!
The contactors used in this project are a little bit on the physically large side, partly due to the plug-in form factor:
3PCO 10A Plug-In Contactors
Squeezing everything into the box was a little bit of a challenge, so the PCB is tucked in near the variac core:
Variac Control PCB
The electronics also needed their own power supply. If the overall current draw of the electronics and contactor coils was less than about 50mA, I would have been tempted to just use a simple non-isolated capacitive dropper circuit from the mains, however I chose to use a small linear power supply with a toroidal transformer:
Toroidal transformer for logic supply
When assembling the device together, it was important to make sure there was enough clearance around the variac core and the wiper assembly – the whole wiper becomes energised when the variac is powered up!
Here is the completed photo of the internals of the variac:
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