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.

Screenshot - 11_05_2015 , 08_36_22

Above the model for 100% flux coupling.

Screenshot - 11_05_2015 , 08_36_05And 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!

Using a Pt100 RTD with my generic heating / cooling controller

Generic heating / cooling controller describes a bang-bang type thermostat based on a AtTiny25.

This article works up an example application using a Pt100 RTD sensor. Pt100 is the designation for a platinum (Pt) resistance temperature detector with nominal resistance of 100Ω at 0°. Pt has a nearly linear resistance / temperature characteristic and high accuracy. Continue reading Using a Pt100 RTD with my generic heating / cooling controller

N2006P PID checkout #1

The N2006P is a inexpensive PID controller, typically for heating and cooling operations. There are lots of similar devices for under A$20 on eBay.

N2006P-01

Above, the controller in a minimal test harness using a Type K Thermocouple for temperature sensing and 40A SSR mounted on a heatsink. (The SSR output should be protected with an MOV for inductive loads.) 480VAC 40A SSRs sell for as little as A$5 on eBay. Continue reading N2006P PID checkout #1

A new impedance calculator for RF inductors on ferrite cores

Some time ago I published a calculator for estimating the impedance of RF inductors on toroidal ferrite cores (Calculate ferrite cored inductor).

Screenshot - 23_02_2015 , 08_09_25

The calculator (input form above) use the core dimensions and complex permeability as the basis for calculation.

There are some popular cores that are not simple toroids and so not suitable for direct use with that calculator. For these cores, a practical method is to measure the inductance constant Al (inductance of 1 turn in nH) at low frequency (where µ=µi), and using the µ’,µ” characteristic from the datasheets, to calculate the impedance at the desired frequency. Note that µ’,µ” are usually frequency dependent for ferrite materials. Continue reading A new impedance calculator for RF inductors on ferrite cores

Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites #2

This is a follow up to Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites.

Steve saw the above article and revisited the FT240-52 measurements which he apparently did, and found them wanting: Continue reading Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites #2

Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites

Steve (G3TXQ) posted a graph comparing Cecil’s (W5DXP) measurements of two turns on FT240-52 and FT240-K.

It is interesting to reconcile the #52 curves with Fairrite’s datasheets. A simple reconciliation is to compare results at the frequency where µ’ and µ” curves cross over. Continue reading Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites

Ferrite K mix

Among forum experts, there are ready recommendations for the ideal ferrite material (or mix) for a balun, often without knowing any detail of the application.

The ‘magic’ mixes include K. Perhaps they are devotees of Sevick.

Over some years I have searched for manufacturer’s data on K mix, and found only two references:

  • Amidon who give a very brief table summarising characteristics, inadequate for RF inductor design; and
  • Ferronics who give characteristic curves, albeit in less common format.

Problem is that Ferronics µi is 125 against Amidon’s 290… so their K materials are different.

One has hoped that an interested competent person might have made measurements of some samples from Amidon to give full characteristic curves, it isn’t that hard. Continue reading Ferrite K mix

LM386 audio power amplifiers

I tested a couple of LM386 audio power amplifier modules.

LM386tThe larger one was a kit using the DIP package, the smaller came assembled and used a SO package. Both cost less than $2 each posted on eBay.

LM386b

They both deliver close to 3Vpk into an 8Ω load at 1kHz when powered from 12.0V. That is close to 0.5W out, but the SO chip cannot withstand the associated dissipation of 0.5W continuous output.

Both handle broadcast program quite happily at 0.5W peak, the chip temperature rise is 15° and 25° respectively.