# Archives 2013

#### Kirchoff Laws

Kirchoff law consists of two laws relating to electronic circuits.

The two laws are named after Gustav Kirchoff, who described them in 1845.

Law one deals with Current (I) within a circuit, and law two deals with Potential Difference which is also commonly known as Voltage (V).

LAW 1 – THE CURRENT LAW

The current entering any junction in a circuit (i.e a component connection point on a PCB) is equal to the current leaving that junction point.

To state this law algebraically we can state:

Before you run away screaming, I will explain how to understand the algebraic equation above.

The thing that looks like a bit like a drunken ‘E’  is called an epsilon. It means the total addition of the currents entering and existing the point in a circuit.

‘I’ is the symbol for Current

‘n’ is the total number of connections (or branches) flowing towards, or away from the particular point in the circuit (known as the node point).

LAW 2 – THE VOLTAGE LAW

The Sum of all the voltages around the loop is equal to zero.

V1+V2+V3 – V4 = 0

Where V4 is the Voltage supply.

#### Vehicle Ammeter

Vehicle Ammeter add on accessory suitable for a wide range of vehicles including 4×4.

The Ammeter is used to measure current (I) drawn through a circuit, and indicates up to 30 Amps.

Please ensure that the equipment that you intend to connect it to, does not draw more than 30 Amps current.

An ammeter is important for monitoring how much current is being drawn by your circuits. An ammeter is connected ‘in series’ .

The best way to imagine a series connection is to think of two people holding hands. These two people are a wire between the battery / voltage supply , and the piece of equipment.

To fit an ammeter, you connect it between the hand of each person. If you are not competent to do this, then consult an auto electrician.

Once you have your ammeter connected, it will display the current(in amps)that the piece of equipment connected after the ammeter (the end persons other ‘free’ hand.

Reasons that you would want to know the current drawn is if you have added accessories to your vehicle, such as a 4×4 winch. This may mean that the current drawn exceeds the current supplied by the charging system when lots of electrical systems are on simultaneously.

Example of electrical systems include headlights, air conditioning, rear heated windscreens.

We are a Uk supplier, and aim to dispatch within three days.  For more help and information, contact Craig via  my business website www.yesway.co.uk

£15.99

#### Star-Delta Starter

Star-Delta (or Wye-Delta)  starter is used in electrical engineering to start larger three phase motors, typically over 4 KW (5 HP).

The Star-Delta starter is chosen, rather than the cheaper D.O.L (Direct On Line) for larger motors, because when a motor is started a surge current is generated for a short initial period. This surge current is typically 5 – 7  times greater than  the normal operating current of the motor, and can cause problems due to the fact that suddenly a large load is appearing on the electrical circuit of your ship or building.

If you started a large induction motor, rated at 50 Amps, then the surge current being between 5 – 7 times that figure, would mean a current appearing on your electrical system of between 250 – 350 Amps. This surge current could potentially cause a ‘blackout’ on board a ship, or at least put strain on the generator.

Even on land, large surge currents are undesirable, and many countries require Star (Wye) – Delta starters to be used to start motors rated above 5 HP or 4 KW.

Operation

The Star-Delta Starter basically consists of three contactors, a timer, and an OCR (Over Current Relay).

When the Star- Delta is first switched on, the electromagnetic coils in the ‘Line’ & ‘Star’ contactors are energised. The magnetic field produced by the coils (in conjunction with the metal cores fitted) closes the contacts of both contactors. This allows voltage to flow through the line contactor into one side of each of the three (three phase) windings.

The Star contactor is also initially closed, and this creates a ‘star’ winding configuration, by shorting the other side of each of the three windings together.

Having the motor windings connected in Star means that surge current is reduced when the motor initially starts up.

At motor startup, a timer is also started.  To be continued………………..

#### Comparison of True Versus Reactive Power

Comparison of True Versus Reactive Power

When a  resistive load is connected, no phase shift is occurs between the voltage and current.

The Power that is the  result of the resistive load  performs work in the circuit. This is      known as “true” power, and is measured in watts.

By  comparison, an inductive load causes a 90-degree phase shift between voltage and current.

To calculate the power that results from an inductive load, multiply voltage by current. When either voltage or current is zero, the power is zero. This type of power is known as  “reactive” power. It counteracts the true power in an electrical circuit. It is measured in reactive volt-amperes.

Inductive loads are loads from electrical equipment.  Electricity circulates through coiled or wound  wire. An inductor is a component which produces a magnetic field, when a current is passed through it.

Examples  include relays, motors, solenoids, & transformers.

As a general rule, if an electrical load involves movement, it is classed as being an inductive load..

Inductive loads can cause ‘blowback’ and must be protected by diodes. Diodes allow the current to flow in one direction only.

Blowback is caused by a surge in voltage. This is caused when the magnetic field in the inductor collapses.

Resistive loads are most often are found  in circuits where electricity  produces heat, light, or both forms of energy, Resistive loads do not produce movement.

Examples of resistive loads include Kettles, Incandescent Bulbs, and Heaters.

(c) 2013 Craig Miles

#### Safeguarding Danger

Safeguarding Danger of Schools Radio Communications Activity, by Craig Miles, P.G.C.E

Radio communications enable school staff to communicate using devices such as walkie-talkies, however wrong equipment choice can result in breaches of data protection & safeguarding.

Due to the introduction in the last few years of the licence free radio system, known as ‘PMR 446’ , many schools have chosen these short range handheld radios, as they offer a cost effective solution to their communication needs.

Professional PMR 446 radios are often advertised as having ‘private channels’, which is reassuring to Head Teachers, who believe that conversations about school business are secure, and free from other users.

The ‘private channel’ is achieved by one of two techniques, known as CTCSS & DCS. These techniques use special signals to only allow you to hear conversations from radios programmed with the same signal setting.

It is a misconception that the channel is private, as the speech being transmitted is often unencrypted, and can be heard by anyone monitoring the channel, for instance if they turned their own ‘private channel’ settings off.

Schools need to check whether their handheld ‘walkie-talkie’ radios are secure, and that their staff are trained in correct usage of radio communications, not only to comply with data protection requirements, but also potential safeguarding danger issues.

For help with ensuring your schools communications are secure and compliant, complete the enquiry form below.

Ensuring safeguarding when using radio communications in schools

Written by Craig Miles of  www.Yesway.co.uk  (01522) 740818  © 2013-2023