AD587 Battery Powered 10V Reference Tutorial

Following on from a previous post discussing the Analog Devices AD587 precision 10.000V voltage reference, I built a portable device to utilise the chip.

Some requirements of the project were:

  • 10.000V ±5mV output
  • 10mA output
  • Battery operated device
  • Visual, low battery indication
  • Small, aluminium housing
  • Clear front panel
  • Low cost (under $50), readily available components

A low battery indication was a desired feature to prevent the device being used in an important test and the battery level drops low and compromises the AD587’s performance. A simple green LED will suffice. Output performance of 10.000V (± 5mV) couldn’t be compromised so there is no protection to prevent high current draw from the chip, I’ll just have to be sensible.

Battery powered AD587 10V reference.
Battery powered AD587 10V reference.

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Energy Meter Hacking – Reading EOUT Pulses

Following on from a recent teardown of a low cost appliance energy meter, I’ve done a bit more hacking of the device. As you may recall, I identified that one of the pins on the meter’s chip (EOUT) output a train of pulses proportional to the energy consumed. Some tests with a multimeter seemed to confirm this because the average voltage of EOUT changed with the appliance wattage.

I was hesitant to connect my logic analyser to the meter to measure the EOUT pin because of potential differences in voltage levels. To resolve, this I quickly whipped up a small board with a 4N25 opto-isolator to provide some voltage isolation between the internals of the energy meter and my logic analyser.

Again, I will repeat the obligatory warnings prevail. Do not:

  • Attempt or copy any of this if you do not fully understand or appreciate the hazards of mains power
  • Open the meter whilst connected to mains power
  • Perform any measurements whilst the meter’s case is open
  • Connect another mains powered device to measure the chip. The energy meter’s power supply is not isolated from active, neutral or earth.

The circuitry that allows for isolation of voltage levels is based around a 4N25 opto-isolator. An opto-isolator  allows for a signal to be transferred using light, this provides an airgap which provides electrical isolation.

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Ridiculously Simple Flex Sensor

New Scientist recently detailed the construction materials for the simplest flex sensor I’ve ever heard of – a graphite pencil and a piece of paper. It is obvious when you think about it, but as New Scientist details in in their article Pencil lines as sensors, 21st March 2015 (No 3013) edition,

“MacGyver would be proud. Drawing a rectangle on a piece of paper with an ordinary pencil can create a sensor.”

Draw me a flex sensor?

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TO220 Single and Dual Gauge Packages

A friend of mine recently asked me if I knew that there are two package types for humble TO220 devices. I didn’t understand what he meant. He then asked if I had heard of single gauge and dual gauge TO220 packages. Nope still no idea. Finally, he asked if I had seen TO220 voltage regulators such as a 78L05 device with a thin heatsink?

Yes, I had seen these before, in fact I have a couple. I hadn’t really paid a lot of attention to the first time I had seen one of these components with a thin heatsink. I thought that it was perhaps a counterfeit device. That is not the case (excuse the pun). A thinner heatsink is a type of TO220 package.

Below are two TO220 devices, a 7812 12V regulator manufactured by On Semiconductor and a TIP31 NPN transistor from ST Microelectronics. A quick glance and their shape is instantly recognisable.

TO220 devices
TO220 packages for a 7812 12V regulator and a TIP31 NPN transistor.

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MJE3055 Power Transistor Teardown

Whats inside a power transistor? Well some semiconductor to be a transistor of course. How large would you expect the transistor’s die to be?

My guess is, that for a device that can conduct up to 10 A, the die would occupy the bulk of the transistor’s package. What’s your guess?

MJE3055 NPN transistor
An MJE3055 NPN transistor ready to be dismembered.

Lets find out with a teardown of sorts with the following MJE3055 NPN power transistor that comes in a TO-220 package.

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Skillet Reflow 0603 SMD Capacitors = Tombstones

A recent project involved using an electric skillet to reflow solder some APA102C RGB LEDs to a PCB. The aim was to make a device similar to the Adafruit Neopixel Strips, but arranged in a 3×3 square and using the APA102C LEDs.

Honestly, each board that was made was a fail. Electrically, they all worked correctly. The failure was during the manufacturing stage – reflow soldering very small SMD components.

Just as an aside, Adafruit Neopixel devices use WS2812B LEDs which have some very strict timing requirements – this can be problematic for some devices that use software interrupts. Whereas, the APA102C LED is a chip that does not suffer from strict timing.

Each small PCB, contains 9x APA102C SMD LEDs and 8x 0.1uF 0603 ceramic capacitors for power supply decoupling. Each APA102C LED is the large white square with white circle. Between the rows of the APA102C LEDs are the SMD pads for the 0603 size capacitors.

PCB layout for APA102C LEDs
Top surface of the PCB layout.

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Sparkle Eliminating Diode

Whilst flicking through an old user manual for a Toshiba Tosdic 215D single loop controller I found this gem.

sparkle eliminating diode
Don’t forget to use a sparkle eliminating diode

The user manual suggests that when interfacing inductive loads to the Tosdic 215D’s digital outputs, the user must implement sparkle eliminating diodes.

My Japanese language skills are relatively poor so I’m not sure if sparkle eliminating diode is a literal translation or a mistake from the original Japanese user manual.

Personally I like sparkle eliminating diode as it is visually descriptive. A much better explanation than the traditional freewheeling or flyback diode.