## Chinese 18650 Li-ion cells – Ultrafire capacity test

I purchased a torch (flashlight) on eBay recently. It was described as using CREE T6 LED array, and supplied with two 4200mAh 18650 Li-ion rechargeable batteries with charger for A\$25 inc post.

Above, the cells are clearly marked 3000mAh, way short of the advertised 4200mAh… but what is their actual capacity.

Above are the results of discharge tests, the first digit is the cell number and the second is the test. The first test is charged with the supplied charger, the second test is with my charger. Continue reading Chinese 18650 Li-ion cells – Ultrafire capacity test

## Measuring common mode choke Zcm using a two port VNA

There are some who insist that it is not possible to make practical measurements of a common mode choke using a one port analyser, and recommend the ‘S21 method’

## S21 method

The ‘S21 method’ means different things to different people.

Above, (Agilent 2009) describe the common methods of impedance measurement using a VNA. The first method is often supported with direct display of R,X, and possibly a Smith chart presentation of Γ. Continue reading Measuring common mode choke Zcm using a two port VNA

## Updated: Calculate initial load line of valve RF amplifier

Calculate initial load line of valve RF amplifier was written as a companion to my RF power amplifier tube performance computer tool to provide a starting point for building a model, but as it turns out, the initial load line (and related values) is a very good estimate and further modelling may not be needed.

Although written for an application to valves, it is quite applicable to any active device, keeping in mind that it assumes a linear transfer characteristic.

The update provides for both single ended and push-pull configurations.

For example, the requirement is for a single ended Class C bipolar amplifier to deliver 25W from a 13.8V DC supply. What is the ratio for a broadband output transformer to 50Ω.

Above is the solution. The required Rl is 3.3Ω, and the required turns ratio is (50/3.3)^0.5=3.9. a 1:4 (turns) transformer would be selected for a prototype. Bear in mind that output power would fall to around 20W at 12V DC supply.

Another example is the common 100W 13.8V Class B push-pull design.

With a requirement for around 3Ω collector to collector (or drain to drain), a transformer with 1:4 turns ratio would be selected.

## Identifying ferrite materials #2

A correspondent wrote about Identifying ferrite materials asking how to use the method on an analyser that does not display X, but displays R and |Z|.

The method described calls for making a winding of the least number of turns for which measurement can reasonably be made, and finding the lowest frequency where R=X.

If your analyser does not display X (or |X|), you can exploit the relationship that |Z|=(R^2+X^2)^0.5. When R=X, |Z|=1.414R… so you would look for the lowest frequency where |Z|=1.414R.

Having found that, compare it with the table given or datasheet graphs to find candidate mixes.

## Reconciling my #52 choke design tool with G3TXQ’s measurements

A correspondent wrote with concern of the apparent difference between graphs produced by my #52 choke design tool with a graph published by G3TXQ of his measurement of 11t on a pair of stacked FT240-52 cores.

I published a note earlier about my concerns with a similar graph by G3TXQ compared to the Fairrite datasheet, and he reviewed the data, found the error and published a corrected graph.

The corrected graph above might at first glance appear different to my model’s graphs, and the first obvious difference is that G3TXQ uses a log Y scale (which is less common). The effect of the log scale is to compress the variation and give the illusion perhaps that in comparison with other plots, this balun has a broader response.

To compare the two, I have roughly digitised G3TXQ’s graph above and plotted the data over that from my own model (with linear Y scale). Continue reading Reconciling my #52 choke design tool with G3TXQ’s measurements

## Identifying ferrite materials

This Jan 2012 article has been copied from my VK1OD.net web site which is no longer online. It is for reference from other related articles. The article may contain links to articles on that site and which are no longer available.

One often wants to identify the type of material used in a ferrite core for use at radio frequencies. This article captures advice that the author has offered in online fora stretching back more than a decade, yet it seems uncommon knowledge.

The most common method is to make some measurements to determine the initial permeability µi, usually at audio frequencies, and to compare that to a table of µi for common core materials. This method might well indicate several mixes that have similar µi, but each may be quite different at higher frequencies.

The suitability for use at RF usually depends much more on complex permeability at radio frequencies than it does on µi at say 10kHz.

Above is a plot from the Fair-rite data book showing the complex permeability characteristic of #43 ferrite material. Continue reading Identifying ferrite materials

## Lithium battery – 1S protection boards

Some of my projects use a single Lithium cell for power, and the ready availability of low-cost battery protection boards offers opportunity for better projects.

Above, a 1S board rated at 4A and which sells for about A\$1 posted in lots of 5.

Specifications:

Description:
New and good quality
Use BM112 protection chip + AO8810 MOS tube
The protection board is used to protect the battery overcharge, over discharge, so can’t use as a charger,when you want to charge the battery you need to use the dedicated charger,because the protective board has a time to response to the short circuit, can’t to connect too large instant impact current, such as drills and so on

The main performance parameters:
1. PCB Size: 39 * 4 * 2mm
2. Overcharge protection voltage: 4.2750 ± 25MV
3. Over-discharge protection voltage: 2.88 ± 75MV
4. The overcurrent protection: 4-8A
5. Continue working current: > 4A

Note: Only for the equipment which instant start-up current less than 4A,those starting current instant is great, such as high-current motors, drills, etc., are not suitable for use.

Above are protected battery assemblies based on the board and a 1200mAh LiPo (sells for about A\$4) and Panasonic 18650 Li-ion 3400mAh (sells for about A\$12). The connectors used are 3A rated JST RCY connectors as used in RC battery applications and readily available with tails for way less than A\$1 each set.

The 18650 cell has tags spot welded to the battery contact points, the LiPo has contact tags as supplied.

To use these, the power source needs to supply about 4.5V so as to ensure charging when necessary. The power source needs to be current limited unless you choose to depend on the protection boar’s limit.

## A low cost home made USB CI-V interface with open collector and solid Windows drivers

There are four common options for USB-serial adapters:

• Prolific;
• FTDI;
• WCH; and
• Silabs.