Hitachi 890-2430 Deuterium UV Lamp Test and Theory

This is a short walk-through on deuterium UV lamp theory and a test of the lamp in operation from its rather special power supply. PSU failed. Lamp failed. Complete fail. The lamp comes from the teardown of a Hitachi L7450 …

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Teardown of a Kappa CF15 CCD Camera From a Microscope

Teardown and on-the-fly analysis of the circuits found inside a Kappa CF15 CCD camera. I got the camera with my Nikon SMZ-2T stereo microscope, that I use for electronics repairs and component identification. The camera was defective and taking it …

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Cisco 9900 Series 50-1000MHz RF Signal Splitter Teardown

Cable TV Networks This Cisco headend/hub RF combining and splitting networks is used in Hybrid Fiber-Coaxial networks. HFC is a telecommunications industry term for a broadband network that combines optical fiber and coaxial cable. Advanced HFC networks include numerous 2-way …

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DIY 30W LED Work Lamp From a Discarded Street Light

I found a discarded street light head at a scrap yard and wanted to give it a new life instead of being shredded. Re-use, re-purpose or re-invent. Do not just throw out stuff, see if you can use it in …

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Tandberg TT1200 DVB MPEG-2 Decoder Teardown

Teardown of some old and discarded DVB MPEG-2 Decoders that have been used in a large buildings distributed TV network. Despite that units look the same on the outside, they turned out to be very different hardware revisions on the …

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Teardown of Servomex CO2 Analyzer 1400 Series

Old discarded equipment from a agriculture laboratory. Unfortunately damaged from the rough handling. This CO2 analyzer measures the CO2 levels in emissions using the principle of infrared absorption. It is used after a gas preparation system and set-points can be …

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German Weather Balloon Teardown, Vaisala RS41-SGP Probe

During my vacation at the island of Langeland, Denmark, I found a washed up radioprobe from a German weather balloon on the beach. It is a Vaisala RS41-SGP single use instrument in a styrofoam enclosure that can measure air temperature, …

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Teardown – Eaton Powerware 30 kVA UPS

I had the chance to take this Eaton Powerware 30 kVA / 27 kW (9355-30-N-7-2x9Ah) uninterrupted power supply apart to look for parts worth salvaging for future reuse in other projects. Looking up the type on the Eaton website we …

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Resistor Color Codes Chart

Resistor color code bands explained

Resistors color codes should be read from left to right, when the grouping of the rings is to the left. If it is a 5% or 10% resistor, the gold and silver ring must be to the right. The silver and gold rings are for tolerance, and they can never be the first ring.

Resistors with 3 bands are all 20% tolerance resistors and should be decoded as a 4 band resistor.

Resistors with 6 bands are special resistors with a temperature coefficient band. These are used in equipment where the resistors ability to withstand a certain temperature, is critical.

On SMD and other small resistors, the value can be given in a numerical way, like “103F”. These numbers should be decoded in the table as a 4 band resistors.

In order to write Ohm values easily in schematics, R is used for sub 1000 Ohm values, as 0R1 = 0.1 Ohm. K is kilo and is used for thousands as in 4K7 = 4700 Ohm. M is mega and is used for millions as in 10M = 10000000 Ohm.

Resistor values table

Color1st band2nd band3rd bandMultiplierTolerance
Black0001 Ω
Brown11110 Ω1% (F)
Red222100 Ω2% (G)
Orange3331 KΩ
Yellow44410 KΩ
Green555100 KΩ0.5% (D)
Blue6661 MΩ0.25% (C)
Violet77710 MΩ0.1% (B)
Gold0.1 Ω5% (J)
Silver0.01 Ω10% (K)
First band, second band, third band, forth band, fifth band and sixth band

It can also be a good idea to use a resistor color code calculator, to get the correct values for a band resistor. If you are designing a electronic circuit that uses transistors, you might be interested in using the transistor base resistor calculator.

Published September 24, 2013. Updated November 1, 2021.

Homopolar motor

What is a homopolar motor?

The name means that the electrical polarity of the motor never changes. The axis of the rotating part of the motor is parallel to the magnetic field from the permanent magnet. Lorentz forces is what makes the motor turn, a conductor conducting current through a magnetic field will respond to a external magnetic field and will gain velocity in what direction the fields happen to be aligned. The hand rule applies here.

This motor is restricted to a single turn coil which limits it to small voltages around a couple of Volts and it has a low torque.

Homopolar generator

The homopolar motor will also work as a generator if we remove the battery and apply the motion to the rotating disc. It is only capable of supplying a couple of Volts but at very high currents, generators can be put in series for higher voltages. Very large homopolar generators are used for high current short circuit testing.


Warning: this motor operates at high speeds up to 6000 RPM, the screw can at any time loose contact and fly through the air.



Michael Faraday demonstrated this motor concept in 1821 at the Royal Institution in London. The homopolar motor is the first electrical motor to be built. His construction was a large and crude concept that he himself describes as “This apparatus may be much reduced in size, and made very much more delicate and sensible.”

The original article in Quarterly Journal of Science:


How can you perform this experiment yourself?

You will need the following four items to make this experiment.

  • 1x 1.5 Volt battery. I use a AA battery
  • 1x iron wood screw.
  • 1x piece of copper wire.
  • 1x round neodymium disc magnet.

Follow these four steps to make it work.

  1. Place the head of the screw in the centre of the magnet.
  2. The screw must be touching the positive terminal of the battery. Sides of the battery are insulated.
  3. Hold the wire with your finger against the negative pole terminal of the battery.
  4. Gently let the other end of the wire touch the side of magnet, completing the circuit.
  5. Watch as the magnet and screw starts to spin faster and faster.



A quick experiment that can be made from common household items. It is a simple practical demonstration of the basic electro magnetic theory, that would help many understand the force around a wire easier.

Published August 4, 2011. Updated October 31, 2021.