Choosing a toroidal magnetic core – ID and OD

One sees lots of articles describing inductors and transformers wound on rectangular cross section ferrite cores, and in explanations, the OD seems to be an important parameter but little consideration is given to ID.

Inductance of an inductor on a rectangular toroidal core depends on many factors, and among them ID and OD. Inductance is proportional to ln(OD/ID).

Above is a plot of the factor ln(OD/ID) against ID/OD as a percentage. It can be seen that for ID/OD approaching unity (ie a radially ‘thin’ toroidal core) that the characteristic is almost linear, and inductance is proportional to the radial thickness of the core. Continue reading Choosing a toroidal magnetic core – ID and OD

VU3SQM directional wattmeter build – #4

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

At long last, some PTFE rod arrived to permit assembly of the transformers.

For reasons discussed in an earlier article, the transformers use a larger core than the original VU3SQM. They need to stand above the board, and whilst that compromises the mechanical strength of the assembly, it should have better performance. Continue reading VU3SQM directional wattmeter build – #4

VU3SQM directional wattmeter build – #3

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

The parts have arrived and construction commenced.

Above, the PCB populated with the SM parts and soldered. It was soldered in an IR reflow oven. Continue reading VU3SQM directional wattmeter build – #3

VU3SQM directional wattmeter build – #2

VU3SQM directional wattmeter build – #1 laid out the first steps in design review and build of a directional wattmeter.

This article canvasses the issues of the display.

Intention is a digital based display (though not to exclude an analogue meter or bar graph type displays).

So, the output of the AD8307 needs to be digitised.

Let’s first consider the nature of the AD8307.

It is a log detector, so it provides a ‘DC’ voltage proportional to the log of the input signal, but the ‘DC’ voltage can vary very quickly.

The chart above from the AD8307 datasheet shows that the unfiltered response to a burst of RF has a rise time of well under 1µs. Continue reading VU3SQM directional wattmeter build – #2

VU3SQM directional wattmeter build – #1

VU3SQM offers an interesting directional coupler based on a Sontheimer coupler, and using AD8307 power sensing for a nominally HF coupler. I must say that I am not a fan of Sontheimer couplers… but that is what the board uses.

This article lays out a preliminary design review to assist in selection of appropriate toroids, and ordering of the needed parts.


Above, the top side of a PCB. Continue reading VU3SQM directional wattmeter build – #1

STC-1000 firmware change

I have an inexpensive (~$10 inc post) digital temperature controller that was observed to have very high RF emissions and was unusable because it was incompatible with radio operations here.

There has been enhanced firmware for a STC-1000 controller based on a PIC chip, but in my experience, most seem to be made with an STM8S003F3 chip. More recently, what appears to be a port of the PIC software to STM8 has been published at

The enhanced firmware is directed at home brewing for control of long running fermentation processes etc, incorporating storage for a number of multi-step programs (profiles). It is not really suited to more general applications like a fridge controller as for example it is more difficult for the common operation to set the set point.

I thought it might be entertaining to try it out.

Installation of the firmware was straight forward, I had the necessary programming adapter and software.

Interestingly, after updating the firmware, the device would not start on its original power supply. My suspicion is that the new firmware runs the LED display at higher duty cycle and the power supply could not withstand the load.

Since the power supply had RF emission problems, it was removed and the device converted to 12V DC.

Above, the large components of the switched mode power supply were removed, a Polyswitch protective device installed and input routed to the output terminals of the old 12V output switched mode power supply (white wire is -ve, brown wire is +ve). Continue reading STC-1000 firmware change

On Thevenin’s theorem – #2

On Thevenin’s theorem looked at a simple source network to demonstrate some key characteristics and limitations of Thevenin’s equivalent circuit.

The example network used was linear in V,I for all V,I combinations possible. Let’s now look at a network that is not linear for all V,I, but is sufficiently linear over a sub range to be usefully modelled using Thevenin’s equivalent circuit.

Black Box for discussion

For the purpose of discussion, we have a Black Box with just two terminals and is a source of DC voltage and current, and the internal implementation is hidden from us.

A series of measurements is made with different load resistors attached and the voltage and current at the terminals is recorded and plotted uniformly stepped currents.

The V,I characteristic is clearly non-linear, but on closer examination there are two fairly linear regions, from 0.008 to 0.060A and 0.08A to 0.1A. It is a device that is usually used in the region below the knee, and for our application, let us concentrate on 0.008 to 0.030A. Continue reading On Thevenin’s theorem – #2

ISP adapter for Arduino Pro Mini / Pro Micro

I have started using Arduino Pro Micros recently, and sourced inexpensive clones from China.

Experience is that all manner of inexpensive small microcontroller modules from China are likely to have issues with the bootloader: it isn’t there, it is back level, not suited to the actual clock speed.

I have come to routinely install a current / known / working bootloader to avoid wasting time down the track.

The Pro Micro does not have an ISP header, and the QFN package does not suit a chip adapter, so the next option is an adapter that can connect to the board with no pins, male or female headers, top or bottom.

Above is an adapter built on a small piece of Veroboard. If you are ging to copy it, make it one row of holes higher. I did initially, and in a miscount of rows, I incorrectly removed the top row. The black mark identifies the pin 1 of the Pro Micro, and the adapter connects to the side with the /RST pin.


The headers on the adapter engage JP6, preserving the pin ordering, pin 1 to the black mark on the veroboard.

Continue reading ISP adapter for Arduino Pro Mini / Pro Micro

IoT – exploration of LoRa – part 3

This series documents a set of experiments to explore LoRa for a telemetry application. Note this is simple multipoint to point LoRa, it does not use LoRaWAN.

IoT – exploration of LoRa – part 1 outlined a simple direct RESTful submission to Thingspeak from the LoRa – Wifi gateway.

IoT – exploration of LoRa – part 2 outlined a simple RESTful submission to Node-Red from the LoRa – Wifi gateway.

This article describes a direct MQTT submission from the gateway. In this case the gateway converts the binary LoRa payload into more friendly MQTT key,value pairs.

The block diagram above shows the information flow between the main elements. Continue reading IoT – exploration of LoRa – part 3