BPAmp: July 2009

This is an excellent post from the diyaudio forums by member pacificblue ;

A hobby should be a source of fun. Following certain rules of safety should help with that. I invite everybody with an adequate background, formation or experience to contribute to this thread.

First things first. What qualifies me to start this thread? I am an electrical engineer from the field of industrial automation, i. e. I design control systems for the industry. Safety is vital there, so it forms part of my day-to-day activities. There is of course a disadvantage in that: I may take certain things for granted, so please ask for further explanations, if something is not clear.

I will distribute my contributions across several posts and try to maintain things easier to read and digest that way.

This thread cannot replace good books on all the topics. And before you start, find out which local regulations and standards apply and meet them.

The five safety rules.

We use them in Germany, and they will exist in a similar form in other countries. Where they do not exist, copy them.

Before you open up your electric and electronic equipment:

- Disconnect from mains.
- Secure against re-connection.
- Check that no voltage is present. (Measure!)
- Ground and short.
- Cover or fence off neighboring live parts.

What ‘disconnect from mains’ means, should be pretty obvious.

‘Secure against re-connection’ means that you must make sure that nobody else connects the device again to mains, while you or anybody else is servicing it. Imagine, what happens, when you are just touching the Phase wire, and somebody plugs the cable into the wall outlet. Arrange the mains cable so that nobody can take it and plug it in without your permission and without drawing your attention.

‘Check that no voltage is present’ requires of course that you measure it. If you have several devices, you might have pulled the wrong plug. If your power supply has big capacitors, they may still be charged. If your only means of disconnection is a switch, that may be stuck, etc. So make sure that no voltage is present.

‘Ground and short’ is a measure that people like to skip, although it is relatively simple. All you need are a pair of jumpers with alligator clips. Connect the Phase, Neutral and PE wires with those jumpers and you are safe. If somebody takes the mains cable and plugs it in, the fuses will blow, but no voltage will be present to put you in danger. Make sure that the jumpers and alligator clips withstand the short circuit current. Use wires with at least 1,5 mm² or AWG16 and robust clips. Don’t forget to remove them, before you re-connect the device to mains!

‘Cover or fence off neighboring live’ parts. When you are at work, you will touch neighboring parts unintentionally sooner or later. The cover will save your health and life. Use a rubber mat, sheets of plastic, pieces of dry wood, anything that prevents you from touching live parts by chance. But keep an eye on the temperature. The cover should not restrict the air flow and cause overheating.

One more rule that is not included in the five, because it is a general rule, whenever you work. Never work alone! Procure that somebody is nearby to come to your rescue, if something bad happens. That person should of course know what to do. E. g. not try to pull you away from a live wire, when you got stuck to it, but to switch it off instead.

How can I check my device, when it is switched off?

Sooner or later you will come across something that does not work. Or you want to do some measurements or tests. Organize your work so that those situations are as rare as possible and as short as possible. Work with the utmost care. And use at least one of the following measures.

- Connect the device under test to an outlet with an earth leakage circuit breaker (ELCB). That will switch the circuit off, as soon as too much current flows to ground. Use the lowest leakage current rating you can get your hands on, it should be below 50 mA. Use the ELCB's test button before you start to work on your device to make sure that it works.
If you don’t have a ELCB-protected wall-outlet, there are multiple outlet strips with integrated ELCBs available. Buy one for your workplace.
- Use rubber mats below you on the floor and below the items on your work-desk to increase your resistance, thus reducing the current that can flow through your body. Those mats must be big enough, so you don’t leave them while at work. They must be robust, so you don’t wear them through, while at work. They should be heavy and sticky, so you don’t move them away, while at work. The thicker the better.
If you use certified work-shoes with rubber or plastic soles that protect you from voltages up to 1000 V, be aware that the rest of your body can still make contact to ground through different paths (table, chair, etc.), so don’t rely on those shoes only.
- Use 'handcuffs', i. e. use electrically conductive bands on your wrists that are connected to PE with a wire. That way, if you touch a live part, the current will only flow through your hands to your wrist and from there to ground. Your heart will not be in the current path and hopefully not be affected.

As a DIYer don't try to make Class II equipment. It is likely that you cannot design it right and even if you can, you cannot test it according to the relevant standards. So don't.

- Go for Class I.
- Make a drawing of, what you intend to do and refer to it. Any changes to the circuit should be reflected in the drawing for later reference.
- Provide adequate fuses, as big as necessary and as small as possible.
- Connect all conductive parts that can be touched without the use of a tool to PE.
- Connect as many conductive parts as possible that can be touched in any other way to PE.
- Make a safe connection to PE that cannot come loose on its own.
- Make the connection so that you cannot disconnect it unintentionally.
- Make sure that the PE connection is the first to make contact and the last to loose contact. If you use connectors, only use connectors, where that is guaranteed. Make the PE wire as short as possible, but leave it so long that is always has 10 or 20 mm more clearance than the Phase and Neutral wires. That way, if the cable is torn out, the PE will be the last to come off.

From human safety to device safety.

After securing your own life and health and the life and health of people and pets around you, you will want to secure the device as well. It has cost you time and money, so of course you want to.

It starts in the design phase.

- Make sure that all wires and traces can cope with the currents that can be expected on worst case conditions.
- Make sure that all components can take the voltages that will be present on worst case conditions.
- Make sure that all components can take the temperatures that will be present on worst case conditions.
- Provide adequate heatsinking and air circulation.
- Install as many automatic protection systems as you can against any fault you can think of. If protection depends on you, what will happen, when you are not present or when you are distracted?

In the building phase, human safety is usual the bigger issue, so be careful with any tools that can hurt you. Don’t build everything at once, but bit by bit. E. g. if you build an amp, finish the power supply before you start the audio section, then concentrate on the protection systems, later on the case. Make sure that the protection systems are the first in place and that they work.

Testing.

You have finished, what you were building and want to find out, whether it meets your expectations…
Now slow down. It is normal that you experience kind of a high at this moment, because you have completed the biggest task and you will suffer from impatience to see that device in action. Take a breath and let the high and the impatience wear down, before you start the next step.

- Don’t just connect everything and switch it on.
- Use a dim bulb tester, also referred to as light bulb tester in series with the equipment, if you don’t have other means of current limiting, e. g. a laboratory grade power supply.
- Don’t connect anything else to the device as yet.
- Step away from it, before you switch it on. Use a shield between you and the device to protect you from injuries, should something blow up.
- When nothing has blown, measure, measure, measure. Check that all voltages are correct. Check that all currents are correct. Check that all temperatures are correct. Complete all settings. Make sure that everything remains stable for an extended amount of time.
- Only, when you are sure that everything works within its specifications, remove the bulb tester.
- Again measure, measure, measure. Check that all voltages are still correct. Check that all currents are still correct. Check that all temperatures are still correct. Check the settings again. Make sure that everything remains stable for an extended amount of time.
- When everything is still okay, connect other devices. Use cheap or old devices for testing that you won’t miss too much, if something goes wrong. Don’t use your 15000 € speakers on that new DIY amp of yours, before you know for certain that they will survive.

And then, enjoy.

Link

BPAmp

Tuesday, 28 July 2009

Safety First

Friday, 24 July 2009

DIY PCB

Wednesday, 22 July 2009

Protection bulb tester