A desk review of the MiniPa100 kit – #1: characterise the output transformer

This article is one in a series of a desk review, a pre-purchase study if you like, of the MiniPa100 kit widely sold on eBay and elsewhere online.

One of the first questions to mind is whether it is likely to deliver the rated power, so let’s review the MOSFET output circuit design from that perspective.

Sellers mostly seem to need to obscure the MOSFET type in their pics, so essentially you buy this with no assurance as to what is supplied, no comeback if the supplied MOSFET is not up to the task. Online experts suggest the MOSFET is probably a MRF9120 (or 2x IRF640 in a 70W build). The amplifier claims 100W from 12-16V DC supply.

Note that this module does not include the necessary output filter which will lose 5-10% of the power from this module.

In this case Carlos, VK1EA, connected a sample output transformer (T2) core from a recently purchased MiniPa100 kit to a EU1KY antenna analyser. The fixture is critically important, it is at my specification. Continue reading A desk review of the MiniPa100 kit – #1: characterise the output transformer

Last update: 11th June, 2021, 7:41 AM

SNTP synchronised clock v1 – display options

The SNTP synchronised clock (ssc) is an ESP8266 based time of day clock with an LED display.

Code development progresses and a working prototype exists with three display options.

Directly supported 4 digit 7 segment LED displays

Several common driver boards are supported directly by the source code, most of these and variants can be purchased online for small money.

HT16K33

Above is a large 4 digit 7 segment LED display (1.2″ or 30mm) using the HT16K33 driver chip. Continue reading SNTP synchronised clock v1 – display options

Last update: 21st May, 2021, 7:02 PM

SNTP synchronised clock v1 – a preview

The SNTP synchronised clock (ssc) is an ESP8266 based time of day clock with an LED display.

Design criteria

The design criteria are:

  • small, portable, powered from a 5V USB power supply;
  • synchronised to a SNPT (simple network time protocol) server;
  • flexibility for a range of displays (74HC595 static, TM1637, HT16K33);
  • configurable time zone offset and daylight saving offset;
  • configurable from a web page;
  • switchable HH:MM and MM:SS display formats; and
  • switchable daylight saving mode.

The concept is that it is synchronised to net time, and the only adjustment needed through the year is to flick the daylight saving mode.

The hardware comprises:

  • ESP8266 dev board of some kind;
  • 5V to 3.3V power supply; and
  • 4 digit 7 segment LED display, preferably with a colon in the middle.

Above is a development prototype using a Wemos D1 mini ESP8266 board and 4 digit display using 74HC595 shift registers. Continue reading SNTP synchronised clock v1 – a preview

Last update: 20th May, 2021, 8:39 PM

Fix for a certain TM1637 LED display

There are a miriad of low cost displays for hobbyists in online shops, particularly targeting Arduinos where libraries exist to drive the most common chips.

This article looks at a 4 digit LED module particularly suited to a digital clock display. The driver chip is a TM1637, and it requires Vcc and Gnd, a data IO wire and data clock wire.

Above is an example that just didn’t work. Continue reading Fix for a certain TM1637 LED display

Last update: 14th May, 2021, 6:37 PM

Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’ #2

On review of the Ultrafire XML-T6 torch, I found the mode switching / mode memory so dysfunctional that it rendered the torch useless in my evaluation.

At Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’ I gave a fix for that revision of the electronics, and updated it with description of a later fixed production model.

Years later, I bought two more of these due to switch failures on the originals… and guess what, the flash on power on returns.

Let’s pull them apart.

They have a new revision / version of the LED driver PCB, and it has provision for a resistor in parallel with the capacitor, but the resistor pads are not populated.

Above, the LED driver board with a 100k resistor added, it is the far component. This was an 0805 part that was on hand, but ideally should be a 0603. Continue reading Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’ #2

Last update: 12th May, 2021, 2:20 PM

ESP8266, ESP32 reset – the (ugly) detail

Like many microcontrollers, the ESP8266 and ESP32 series contain an in-Silicon bootloader which can be initiated at chip reset by holding a pin low at the moment of reset.

Documentation is not helped by less than common terms (like EXTRSTB for a pin that is really a /RST, and it is labelled EN on the ESP32). The other relevant pin is known as GPIO00, but can be thought of as a /BOOT bin.

Automatic bootloader initiation

The firmware is uploaded using ordinary RS232/TTL, and it is possible to use modem control signals to control the /RST (EXTRSTB) and /BOOT (GPIO00 or IO0) pins, indeed the common convention is to use RS232 signals RTS for resetting the MCU, and DTR for boot selection.

So, it is possible to connect RS232/TTL /RTS to /RST and /DTR to /BOOT, and ESPTOOL will automatically initiate the boot loader when accessing the chip.

Above is a simulation of that type of direct connection. The RTS/DTR scenario is that from ESPTOOL, but it can be seen that even if the RTS transition was delayed somewhat the /BOOT pin is low and when /RST rises the chip will initiate the bootloader. The critical timing is that /BOOT is low when /RST transitions from low to high. Continue reading ESP8266, ESP32 reset – the (ugly) detail

Last update: 17th June, 2021, 6:31 PM

(How) does this balun work? A variation on the theme …

My article (How) does this balun work analysed a balun configuration sent to me by a correspondent, apparently published on Youtube channel TrxBench.

Essentially, my analysis was that it comprises two 12t winds of two wire transmission line in parallel on the ferrite ring. The potential benefit was that the characteristic impedance Zo of each transmission line is probably close to 100Ω, and the parallel combination is probably close to 50Ω.

Online experts following fashion are opining that a low Insertion VSWR balun is better made with two wire line(s) than winding a single 50Ω coax line. They make these claims without evidence, I am not convinced.

In that vein, here is a variation on the TrxBench balun above.

The designer describes it: Continue reading (How) does this balun work? A variation on the theme …

Last update: 8th May, 2021, 9:14 AM

Some pretty woolly thinking on measuring Thevenin equivalent source impedance of a ham transmitter

A ham seeking to optimise his station based on some measurements with a VNA and some modelling of a matching network posted the results of a test in the process.

The radio is an Icom IC-7300. I bypassed the built in tuner, transmitted a tone into my external tuner, adjusted it for SWR=1. I then disconnected the tuner from the radio, and measured the impedance looking into the tuner with a VNA. Surprisingly, (to me anyway) the result was a pretty good 53-j3 Ohms at 14 MHz.

What should we / have expected? It is an interesting case to study. Continue reading Some pretty woolly thinking on measuring Thevenin equivalent source impedance of a ham transmitter

Last update: 3rd May, 2021, 6:37 PM