owenduffy.net – Page 109 – Mostly electronics…

Interpreting temperature rise in ferrite cored RF transformers and inductors

We often see statements by hams where they draw inference from observed temperature rise of a ferrite core at RF. Lets consider the following statement.

The FT-240-43 balun MUST be quite efficient as it barely increased in temperature over a 5 minute over at 100W on SSB.

For the purpose of this explanation, lets assume barely increased in temperature means 5° increase in temperature from cold. Under these conditions, we can reasonably assume that almost all of the heat input to the core is consumed in raising the core temperature. Continue reading Interpreting temperature rise in ferrite cored RF transformers and inductors

Last update: 24th May, 2015, 11:07 AM

A walk through of a practical application of AIMuhf/AIM900A #2

I mentioned at A walk through of a practical application of AIMuhf/AIM900 that I wasn’t all together happy with feed point R at resonance, at 40Ω it was perhaps a touch high for a 2m quarter wave ground plane on a largish vehicle roof.

Repeated measurement of the DC resistance from the coax plug sheild to car body yielded unstable resistance ranging from 1 to 10Ω. If stable low DC resistance is not achieved, this feed line won’t work properly for RF. Continue reading A walk through of a practical application of AIMuhf/AIM900A #2

Last update: 26th December, 2015, 9:01 AM

Report on Hobbyking 40A ESC 4A UBEC 9261000003 / BC3540-14 brushless drive #2

Report on Hobbyking 40A ESC 4A UBEC 9261000003 / BC3530-14 brushless drive reported a set of measurements on a test on elements of a drive system which is the basis for one of my flying quads.

This article reports some interesting characteristics derived from the measurements.

Test configuration:

Gemfan 11×4.7″ SF propeller;
BC3530-14 1000Kv motor;
Hobbyking 40A ESC 4A UBEC 9261000003 (BEMF caps removed);
SimonK commit 02bd8e4ca36a06722efe51bc7cd5130d72a184b8 bs.hex with 2000ns dead time; and
4Ah 4S Lipo battery.

Keep in mind that the application for this drive is a multirotor, and hover speed is around 3800RPM, so efficiency in that region is critical to battery endurance. Continue reading Report on Hobbyking 40A ESC 4A UBEC 9261000003 / BC3540-14 brushless drive #2

Last update: 17th May, 2015, 9:42 AM

A walk through of a practical application of AIMuhf/AIM900

This article describes the use of the Array Solutions AIMuhf/AIM900 to test a mobile antenna installation, a quarter wave whip for 2m with about 4m of RG58 cable which has been previously installed and tuned.

The exercise is motivated by a perception that the antenna is not working as well as it should.

Above is a scan of the VSWR. It indicates problems, there should be a main VSWR dip around the high end of the 2m band (147MHz), but instead the minimum is nearer 160MHz. Clearly there is an antenna connected to the far end of the line in some form (ie the inner conductor is not simply broken), but there could be a high resistance in the inner conductor or shield connection (the latter is common issue with this type of antenna base). Continue reading A walk through of a practical application of AIMuhf/AIM900

Last update: 26th December, 2015, 9:01 AM

A test run of the generic heating / cooling controller with Pt100 sensor

The generic heating / cooling controller (hcctl) is a flexible bang-bang thermostat controller based on an ATTiny25.

The test load is a pot containing 1l of water and a 1200W immersion element controlled by the SSR above (on-off control). The controller board is a ‘fully optioned’ test framework, hcctl is the left hand DIP8 and the other is a TC427 H bridge (not needed for this SSR which can be driven directly from the ATTiny25 output pin) for buffered output and alarm. Continue reading A test run of the generic heating / cooling controller with Pt100 sensor

Last update: 27th May, 2015, 4:31 PM

SPI input for the generic heating / cooling controller

The generic heating / cooling controller (hcctl) is a flexible bang-bang thermostat controller based on an ATTiny25.

The project has been expanded to accept a simple SPI temperature sensor. The test case uses a MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter for K type thermocouples. The MAX31855 is around US$5 at Digikey for singles, but the tests were conducted using Adafruit MAX31855.

MAX31855

Thermocouples bring two challenges for hcctl:

low noise amplification of very low sensor voltage;
compensation of the ‘cold junction’ temperature; and
high resolution ADC.

The MAX31855 provides a solution to all of these challenges in a single inexpensive chip. Continue reading SPI input for the generic heating / cooling controller

Last update: 15th May, 2015, 5:45 PM

Adafruit MAX31855 checkout

Intending to enhance my generic heating / cooling controller to read SPI temperature sensors, I purchased an Adafruit MAX31855 module on eBay from a local supplier for about A$26 posted.

The module you might have guessed uses a MAX31855, a Cold-Junction Compensated Thermocouple-to-Digital Converter for K type thermocouple with an SPI interface. The Adafruit module includes a regulator and level translators to use it in a 5V system.

This article describes a simple checkout using a BusPirate V4. Conveniently, the MAX31855 module can be powered from the BusPirate. The thermocouple input is provided by a thermocouple calibrator.

After a short wait to allow both devices to stabilise at ambient temperature, a test was run. The BusPirate session is as follows. Continue reading Adafruit MAX31855 checkout

Last update: 15th May, 2015, 5:45 PM

Coupled coils – a challenge for hams!

One frequently sees discussions of coupled coils in ham fora, and the advice of the forum experts is commonly sadly lacking.

An example is the thread Impedance matching transformer where the OP is encouraged to make a transformer for 2:1 impedance transformation ratio based simply on turns ratio and a Rule Of Thumb for minimum number of turns.

Lets review a design where two windings of say 10µH and 20µH are wound on a toroidal core. With no flux leakage, the turns ratio would be 1:1.414. The model is a simple one of coupled coils and ignores self capacitance.

100% flux coupling

If there was no flux leakage, the mutual inductance is (10*20)^0.5=14.14µH, and we can build a three component model of the coupled coils along with the intended 100+j0Ω load.

Above the model for 100% flux coupling.

And above, the response of the network. At 7MHz, the input impedance is 48.7+j8.7Ω, not perfect, but close (VSWR=1.2). Continue reading Coupled coils – a challenge for hams!

Last update: 12th October, 2015, 8:47 AM

BLDC ESC – effect of insufficient dead time

This article reports measurements on a brushless drive used for an RC quadcopter.

Dead time is the time allowed between turning one FET in a half bridge off and the other FET on to ensure that they do not both conduct simultaneously, even for an instant, as very large currents may flow. The dead time specified in ESC configuation is an ‘additional’ delay added over and above circuit delays.

Measuring a stable properly synchronised drive system at minimum speed can expose the effects of insufficient dead time on total drive input current. At minimum speed, very little power is delivered to the propeller and most drive input power is lost in various system losses, so drive current becomes very sensitive to system losses. Continue reading BLDC ESC – effect of insufficient dead time

Last update: 11th May, 2015, 9:24 AM

Report on Hobbyking 40A ESC 4A UBEC 9261000003 / BC3540-14 brushless drive

This article documents some a test on elements of a drive system which is the basis for one of my flying quads.

Test configuration:

Gemfan 11×4.7″ SF propeller;
BC3530-14 1000Kv motor;
Hobbyking 40A ESC 4A UBEC 9261000003 (BEMF caps removed);
SimonK commit 02bd8e4ca36a06722efe51bc7cd5130d72a184b8 bs.hex with 2000ns dead time; and
4Ah 4S Lipo battery.

The dead time was chosen for compatibility with F-40A which I also use. Dead time of 1200ns is sufficient for the 9261000003.

This is quite a low cost drive, the 300W+ motor cost about A$14 and the 40/60A 6S ESC just under A$20 at the time of purchase.

The tests were conducted at 4S as a stress test (up to 170% of rated current), the quadcopter uses 3S. Continue reading Report on Hobbyking 40A ESC 4A UBEC 9261000003 / BC3540-14 brushless drive

Last update: 6th May, 2015, 5:36 PM