Category: Electronics

Transformerless power supply for PAROT

This article documents design of a capacitive transformerless power supply for operating low voltage, low power logic from power mains. The intended application is PAROT (Duffy 2013), though it has potentially wider application.

(Microchip 2004) gives a method for design of a capacitive transformerless power supply for operating low voltage, low power logic from power mains. The equations seem simplistic for a circuit whose apparent simplicity belies the complexity of an optimal design that properly tolerates supply voltage and load variations. For that reason, a SPICE simulation was used to refine a design.

The immediate application is for the PAROT chip driving a 40A SSR.

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Above is measured characteristic of a Fotek 40A SSR, it seems typical of several similar types on hand. It appears that much smaller SSRs in the 2A range require fairly similar current. Continue reading Transformerless power supply for PAROT

Last update: 11th June, 2016, 5:50 PM

Fox flasher MkII – high power 2 LED solar powered beacon

Fox flasher MkII – owenduffy.net described an animal deterrent based on an STC 8051 microcontroller and running from a single LiPo cell.

This article describes a further development using a solar cell, shunt regulator, 1S LiPo cell with protection board, and two high power red LEDs.

FF100Above, the unit constructed in a medium size Jiffy box, and a 6V 0.6W PV panel fixed to the top with silicone adhesive. The LDR is fixed to one end with silicone adhesive.

Two SM 1W red LEDs are fitted to opposite sides. They are 120° LEDs, the holes are countersunk to provide for light dispersion and the LEDs clamped to the inside with small brass brackets and heat sink rubber, a little silicone adhesive seals the holes. Continue reading Fox flasher MkII – high power 2 LED solar powered beacon

Last update: 22nd June, 2016, 3:28 PM

Field strength survey of an M40-1 short helical vertical on 40m

This article documents a field strength survey of an M40-1 short helical vertical on 40m.

This test is more a feasibility study of the experimental method and apparatus than an absolute measure of the antenna.

The antenna under test is described at AUT – MobileOne M40-1 40m helical.

Field strength was measured using a small square untuned loop and VK3AQZ RF power meter (RFPM1), and data was captured using A prototype data logger for RFPM1.

Power meter

VK3AQZ RF power meter (RFPM1) described my build and calibration  of the RFPM1.

RFPM00Above is the RFPM1, shown with two probes, but only one probe is required for this procedure, the other is disconnected. The RFPM1 directly reads input power in dBm.

Loop antenna

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The loop antenna used was described at (Duffy 2007). It is a small square loop (600mm sides) fed in one corner with a 1:1 voltage balun. Continue reading Field strength survey of an M40-1 short helical vertical on 40m

Last update: 15th March, 2020, 4:25 PM

USB-A to DC 5.5/2.1mm power cable – current carrying capacity

The growing popularity of 5V plug packs and Li-ion power banks with USB A connectors provides a convenient source of power for some projects, and a USB-A to 5.5/2.1mm DC cable is a possible connection option.

Scouring eBay turned up some sources, but one can never assess the quality of the things because usually there are no meaningful specification offered, and lets face it, they are Chinese.

USB-21

Above are two sample 1m cables that I purchased, the left one for about A$1, and the right for about A$3.50 (posted).

Loop resistance of the cables was measured with Kelvin probes to assess their current carrying capacity from a voltage drop perspective.

DC loop resistance of the one on the right was 0.16Ω, so the maximum current for a 5% voltage drop is 5*0.05/0.16=1.6A… not quite a 2A rating.

DC loop resistance of the one on the left was 3.3Ω, so the maximum current for a 5% voltage drop is 5*0.05/3.3=0.075A… not even a 100mA rating.

This is not surprising, experience with USB-A to USB-micro cables has revealed similar variation, and an explanation why so many of these cables are hopeless in battery charging applications.

Last update: 20th April, 2016, 7:52 AM

Making sense of LED output figures

Browsing eBay for some high power LEDs for a current project created frustration in trying to wade through the stated performance figures (to they extent that they can be relied upon).

LEDs are often headlined as having some luminous intensity in candelas, but while that might seem to be a good measure of the ‘brightness’ of the LED viewed on-axis, it gives no information about the spatial distribution off-axis and the total luminous flux output or flux density.

I wrote a little online calculator that can be of assistance in finding the total luminous flux and flux density give luminous intensity and apex angle, Calculate luminous flux (lm) from luminous intensity (cd) and apex angle (°). (Note that specified luminous intensity is usually on axis and should be discounted by perhaps 20% to provide an average luminous intensity over the cone angle.)

Example 1

An example, an eBay seller advertises:

Specifications:
Source Material: InGaN !
Emitting Colour: 0.5W 10MM HI POWER White 0.5W LED
LENS Type: Water clear
Luminous Intensity-MCD: Typ: 290,000 mcd
Reverse Voltage: 5.0 V
DC Forward Voltage: 3.2 ~ 3.4V
DC Forward Current: 100mA
Viewing Angle: 40 degree
Lead Soldering Temp: 260¡ãC for 5 seconds
Power Dissipation: 500mW

Does it appear rational? Lets calculate average luminous intensity at 80% of 290cd, 232cd. Lets assume the viewing angle is the half power beamwidth.

Screenshot - 15_04_16 , 09_50_50

Above is a calculation from the specifications. Of concern is the calculated luminous efficiency of 266lm/W, it is perhaps three times or more the expected value, so it questions the accuracy of the claims. Even at 0.5W input, the luminous efficiency is unrealistically high. Continue reading Making sense of LED output figures

Last update: 15th April, 2016, 2:20 PM

Precision GPS experiment #1

This article reports an experiment to evaluate the usefulness of precision GPS for the purpose of location data for automated antenna field strength surveys.

The experiment was conducted with the rover located in a fixed location 13km North of the reference station at Symonston and with very wide view of the sky, about 7:00 am 07/04/2016.

Only GPS satellites were used for the rover.

The software was RTKLIB v2.4.3b8.

IMG_0505

The GPS was a UBLOX LEA-6T with a small patch antenna (as sold for small UAVs). The LEA-6T provides binary data as used by RTK for carrier phase measurements. Above is the GPS and a USB-RS232 adapter. Continue reading Precision GPS experiment #1

Last update: 8th April, 2016, 1:16 PM

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

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.

XML-T6This article describes a work around  that makes the thing usable (IMHO). Continue reading Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’

Last update: 4th April, 2016, 12:15 PM

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.

Ultrafire18650Above, the cells are clearly marked 3000mAh, way short of the advertised 4200mAh… but what is their actual capacity.

Screenshot - 04_03_16 , 19_45_49

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

Last update: 4th March, 2016, 8:06 PM