To protect ourselves and the owners of any projects you wish to embark on please note that we will not be held responsible for errors and omissions in circuit diagrams, the non working thereof and/or the event of you being electrocuted by accident or intentionally.
First things first though. Our mains voltage in South Africa to our house or flat or dwelling is anywhere between 220 to 240V single phase. If you do not work responsibly this voltage can kill you. When working on circuits and until such time you have experience to work on these circuits live you will always need to switch the mains off and remove the plug. Never just the one.
Test your earth leakage. This is a life saver. Don’t ask me how I know.
Your mains supply must have the live component (the brown wire in single phase) switched. Never the neutral. Switches can fail and stick. When working with live circuitry use the industry standard – insulated shoes, one hand behind the back and always someone else close by. Never barefoot and never alone.
Always make sure that your distribution board is in good working order and has not been tampered with. The electrical outlet must be sound and in working order – meaning that live is switched and the switch is working properly. No jumpers, bridge and/or direct connections. Your circuit should always have a main on/off switch.
Rectified and filtered – Direct Current
Radio valve or tube gear often runs upwards of 300V DC. The transformer secondary (output) voltage once rectified and smoothed by a filter capacitor is 1.414 times the output voltage. The output is DC which can be just as deadly.
Where possible always ensure that your circuit is earthed when using a metal chassis. An isolation transformer (primary:secondary = mains:mains) is often used when troubleshooting. A variac is not an isolated transformer (in most cases).
Fusing and Circuit Protection
All electronic circuits must contain fusing and/or circuit protection of some sorts. Understand Ohm’s Law and power surging to grasp the sizing of these devices. Circuit protectors can be fuses or fusible links. Variacs are often used to power up circuits, especially higher powered designs whether tube or solid state, amplifiers or control circuits. Split rail supplies must be fused on both the positive and negative rails. Do not be disappointed to find semiconductors burn before the fuses blow. This is normal. Fuses prevent fire. Light bulbs of various wattages can be used in the input circuit (live mains).
Go ahead and have fun!