Power over Ethernet, PoE

June 25th, 2015

It is nice to be able to power a device over the same connection that is used for signalling.  We take this for granted with USB, however with Ethernet devices we typically need a separate power connector.  PoE enables the sending of power down the unused wires in the cable or even on the used wires.

10Mb and 100Mb Ethernet only uses the middle four wires of the eight wires in the connector.  The centre two wires – pins 4 & 5 are used for one signal and the next two wires, ie pins 3 & 6 are used for the other signal.  In fact you can actually use the other two pairs in the cable for a second Ethernet connection in the same cable – pins 1 &2 and 7 & 8 are unused.  So these wires could be used for power.  Typically about 50V is used and the current kept low to reduce the losses / voltage drop in the cable.  The maximum current for 21AWG conductors is 0.36A giving a maximum power that can be drawn of about 36W.

For gigabit Ethernet, all eight wires are used.  however power can still be sent between two pairs of signals with the actual signal sitting on top of the power.

for more information, a good paper on the subject can be found at


Zener diodes

June 25th, 2015

Zener diodes are the oft forgotten solution to simple voltage regulation requirements.
For low current use circuits a simple Zener diode is sufficient as a voltage regulator. What’s more with the addition of a couple of transistors this “regulator” can serve as a very wide input voltage regulator where isolation isn’t required.

The circuit diagram given here fills such a purpose. So let’s examine how it works.

Diode D1 is a reverse polarity protection diode which allows the circuit to be connected to both AC and DC inputs.
R1, R2 and R3 serve to bias the transistors and the Zener diode D2. R1 also limits the current into the circuit so it’s value is quite critical and depends on the load current flowing out of Vout.
The zener diode D2 determines the output voltage. Note that because the diode is connected to the output via a base-emitter junction for 5V output the zener value needs to be 5.6V.

That’s all fine but how do I derive all the component values?

First establish what the output voltage and current are to be. Say in our example 5V and 10mA respectively.
Also determine what the input voltage is to be say 24Vdc ± 20% to 240Vac ±10%.

From the output voltage we know that the zener voltage needs to be Vout+0.6 = 5.6V. That was easy

From the output current and minimum input voltage we can determine what the maximum value for R1 is to be:
R1 = (Vin_min-Vout)/Iout = (24*0.8 – 5)/0.01 = 1420Ω take the nearest lower standard value that you may have say 1k2.

R2 and R3 aren’t critical as such and 47kΩ is a good value for biasing the zener diode as well as the transistors. Basically you want to have a little current flowing through them as possible yet still guarantee that the zener is operating at its nominal voltage.

Q1 and Q2 need to be high voltage bipolar transistors if the circuit is to be connected to mains (300V rating). The same goes for the resistors and D1.

C1 filters out the ripple when the circuit is supplied by AC and depends on the output current and acceptable ripple voltage.

A note of caution though: although the current flowing through R1, R2 and R3 is only small (~10mA) they dissipate power. In our example R1 will dissipate roughly 1200*0.01²=0.12W So to be on the safe side and avoid resistors burning use ½W rated ones.

Finally a note on safety: when connecting this circuit to mains all the parts are referenced with respect of neutral and thus is dangerous. Do not touch any parts of the circuit when energised.

Safety Capacitors

June 25th, 2015

Types of Safety Capacitors:

There are two major types of interference-suppression / AC line filter safety capacitors; namely, type X and type Y. The purpose of these capacitors is to reduce radio frequency interference and to ensure safety from shock and fire.

Null Modem Cable

June 25th, 2015

Every now and again I need a null modem cable and have to go searching for the connections. Here they are:

Null modem with full handshaking

MIDI Interface

June 25th, 2015

MIDI is commonly used to connect musical instruments together and to a computer / sequencer.  A standard 180° 5pin DIN connector is used. with the signal on pins 4 and 5.  Below a circuit to interface this to a microprocessor.

Learning to Solder

June 25th, 2015

Being able to solder is one of the most important skills in electronics. The soldering process involves solder – traditionally a tin – lead mixture that melts below the melting point of either tin or lead. We use this same reduction in melting point when we put salt on ice causing it to melt at a lower temperature.

Soldering involves the solder chemically bonding to both items to be soldered.  To do this , each item must be clean and free from oxides.  Solder wire has a rosin flux in it which chemically cleans the surfaces when it is heated, but it must be delivered directly to the joint.  A common mistake is to put the solder on the iron and then try and solder with this, but by the time you get to the joint the rosin flux has already evaporated.


Watch the video below for a good description of how to solder.

How to solder video

soldering video

Electronics Made Simple

June 25th, 2015

If you grasp a few simple concepts then making your own electronics gadgets is not that difficult. I am assuming that you understand simple concepts like resistors and ohms law, capacitors, etc. There are a few simple building block in electronics

Is my mobile phone killing me?

June 25th, 2015

This is a concern many people have and they get very anxious when they hear there is to be a mobile phone mast to be located near them.

Electronics Made Simple Part 2

June 25th, 2015

Using on the simple circuit we discuss last time, we can make a simple microphone preamplifier by connecting two of the simple amplifiers together.

Near Field / Far Field

June 25th, 2015

In the last blog I said that I would talk about near and far fields. When you have a tiny antenna you radiate in all directions. Some signals are going around the antenna and others are going out from it. Depending on the construction, the electric and magnetic components fall off at different rates in different directions. . The components decrease by different amounts depending on the distance from the antenna. Some at 1/r, some at 1/r2 and another at 1/r3 . The distance from the antenna at which the various components are all equal is defined at the boundary between the near and the far field. This happens to be λ/2π. Where λ is the wavelength of the signal. The near field is often also called the reactive field because when you are in that area you are also affecting the source of the signal i.e. you react with it.