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The D.I.Y safety thread

Discussion in 'd.i.y.' started by Mike P, Mar 12, 2021.

  1. Mike P

    Mike P Trade: Pickwell Audio

    Starting this thread is something I've been meaning to do for a while and safety is something we should all be interested in here on the DIY forum.

    My aim with this thread is to start a discussion resource regarding the subject of safe working practices with electronics, especially live circuits and high voltage circuits but anything relevant is welcome. We're talking isolation transformers, dim bulb testers, RCD's, correct use of handheld multimeters etc.

    I hoping that some of the professional techs and experienced DIYers will contribute with their tips and advice and perhaps @Tony L would consider making this tread sticky if it develops sufficiently?

    I'll get the ball rolling with the subject of:

    240vac to 240vac 1:1 Isolation transformers - What do/don't they do and should I be using one?
    gavreid and Mynamemynaim like this.
  2. booja30

    booja30 pfm Member

    IMO dim bulb should be your first line of defense if you're actually going to be sticking probes around inside and might accidentally short or touch something. But you can't always have a dim bulb connected, e.g. when setting amp bias/offsets.

    It's good to make sure anything carrying mains voltage inside your amp (or whatever) is covered, insulated, etc. IEC inlet blades/tabs, terminal blocks, soft start modules, fuse blocks, etc. should all be covered if you're not sure of your skills. Otherwise it's like playing Operation with a stiffer penalty than a buzzer and a glowing red nose. When I built my Pass F5 and preamps I had lots of areas exposed that would have been bad to touch, but when I was setting bias/offset I used a plastic (Bourns) screwdriver.

    I'm not sure what an isolation transformer can do for you if you use one and accidentally touch something connected to its secondaries. You'll still get a bad shock, right?
    Craig B and Mynamemynaim like this.
  3. Mike P

    Mike P Trade: Pickwell Audio

    Dim bulb testers are great. I have one and a selection of bulbs ranging from 40w up to 200w to use with it.
  4. martin clark

    martin clark pinko bodger

    I'll start with a great example of what isolation transformers are NOT for:

    Isolation transformers - you may think can be used to 'float' an earth-references mains driven oscilloscope (that's all mains-powered o'scopes all of them); allowing differential measurements, and potentially live mains-side measurements on mains -powered equipment

    This is a BAD idea - if you happen to be measuring at high voltage, your whole scope its casework and the metal bits of the probe / any exposed controls inc the BNC sockets - float up to whatever dangerous voltage you've just probed in your Earth-referenced device under test. So bad/fatal shock can still occur.

    A marginally better answer would be to 'float' the DUT - but that does NOT really reduce the potential for shock; and it makes your scope probe the mains earth fault current path from a defective DUT :eek:

    (hint - use a differential probe, or a battery -powered device: and still - be careful)

    More detail on this and sim 'gotchas' in this outstanding primer 'the X-Y-Zs of Oscilloscope use' that Tektronix still maintain, here:
  5. Mike P

    Mike P Trade: Pickwell Audio

    This is a good watch:
    martin clark likes this.
  6. S-Man

    S-Man Kinkless Tetrode Admirer

    Isolation transformers:

    Speaker safety for chronic amp dabblers:
    After any and every change get into the habit of leaving the + speaker wires disconnected at the amplifier end.
    Set some music playing quietly and dab the speaker lead on the output terminal.
    This provides surprisingly effective protection against:
    - no 0V connnected... death squeal
    - dc on the amp output
    - music playing at full volume
    - it may even protect tweeters from an oscillating amp... you may hear a rustling sound

    A speaker guru told me this doesn't work because you can't react fast enough. IME is does work... the death squeal doesn't half make you jump!

    Of course this is not recommended for valve amps, which like to see a load to avoid the possibility of damaging the output transformer insulation.
  7. Mike P

    Mike P Trade: Pickwell Audio

  8. martin clark

    martin clark pinko bodger

    [edited as comments two below] I still think I might look to a known supplier like Airlink, they do isolating transformers for a range of scales and uses. esp since UK Mains tends to b right up there round 240vac often, if you want/need to exploit the full VA

    And that said - if you only need/want small VA - I bought a 240:240 / 43VA EI/dual-bobbin unit from John Wilkinson here May last year

    example - 75VA unit:

    - And the quality is outstanding. Mine is utterly silent, cool-running, <45pF primary:secondary capacitance - and excellent regulation for such a small VA). A source I can recommend from experience.

    tl;dr: you get what you pay for.
    Last edited: Mar 12, 2021
  9. Mike P

    Mike P Trade: Pickwell Audio

    Please could you elaborate/be more specific?

    Manufacturers website here
  10. martin clark

    martin clark pinko bodger

    Apologies, @Mike P - I glossed the maker name & had that confused with some, erm, chinese-made rubbish. My mistake; my earlier comment edited.

    But I might wonder how much of that low retail price is actual transformer - given the casework etc. That said - it could well be all you need, at a good price :)
  11. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    Variac, takes all the fear of that first switch on, I started out repairing TVs and one would have been handy I used to do the quick on off and check for smoke.

  12. mega lord

    mega lord Centre tapped

    Always discharge psu capacitors before starting work on a piece of equipment.
    OldSkool likes this.
  13. grahamdocman

    grahamdocman pfm Member

    Some very simple basics - advice previously given to me:
    Possibly useful to those of us who do or are planning basic and more simple "kit" / amp board etc. constructions and don't have access to variacs, isolation transformers, scopes etc.

    1) a decent multimeter with long well insulated probes (two is even better as if you get unexpected readings you can check them)
    2) several pairs of colour coded flying leads (e.g. 60cm long) with alligator or other insulated clips at each end. This can sometimes make it easier to work at a distance from the gear when testing
    3) make sure everything is powered down and discharged prior to attaching any leads, attach to the various "test points" , connect other end of leads to multimeter probes, then switch on and you are some distance from the most 'dangerous' bits - not just to avoid a chock, but if something goes BANG - e.g. Capacitor
    Its tedious but SAFE​
    4) an insulated mat / board to put your gear on
    5) several simple methods of discharging PSU's - I use a 1K 5W wire wound resistor for the my PSU's and I don't disconnect until the residual charge is less than 30mv
    Again tedious but SAFE and also reduces the risk of you destroying a PCB component
    All obvious I know, but I always remember one of my lecturers saying:
    "you pass exams not by being clever and thinking of answers no one else thought of, but by stating and doing the bleeding obvious boys and girls ....... that's where the easy marks are".
    martin clark likes this.
  14. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yep this is a top tip. Until a a new amp has been in use for several months I disconnect the red lead from the speakers when I switch off and then do not reconnect it until after the amp has been switched back on and seems OK ie no magic smoke escape, burning smell etc, and then gingerly touch the red lead to the output to be sure of no DC or oscillation before then connecting it up properly. Quite a faff each time you switch the hi fi on but worth it.
    wow&flutter, glenn jarrett and S-Man like this.
  15. S-Man

    S-Man Kinkless Tetrode Admirer

    For commissioning bits of kit:
    1) Check all 0Vs are connected together using a mutimeter - e.g input socket to output socket to power star to signal star to transformer centre tap etc.
    2) Check all PS wires are the right polarity in the right place. Recheck that the bridge rectifier terminal match where the smoothing caps connect e.g. follow the + lead and make sure it goes to the + terminal on the cap etc
    3) Check (lack of continuity) between power rails and OV. A DVM will usually count up or down in this situation.
    4) Power up via a variac and wind up very slowly from zero. If the variac starts to hum - power down immediately. At very low volts check that supply rails are the right polarity.

    Follow the usual power up procedure from then.
    337alant and martin clark like this.
  16. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Possibly the best and most useful item after a multimeter and 'scope has proved to be a dual rail current limited PSU.
    If the superb PSU I have had not been a swap with a mate for a 'scope it would have paid for itself twice over in saved output devices, drivers, fuses, emitter resistors etc over the years!
    a.palfreyman likes this.
  17. a.palfreyman

    a.palfreyman pfm Member

    Yes, a current limited bench supply to power you circuits for checking prior to installation. Really useful for checking out amp boards etc as it will stop excessive currents. You can then dim bulb or variac your psu with dummy load attached. As S-man said, double check your connections, then walk away and have a cuppa and then come back with fresh eyes and treble check. Surprising what you can miss when you're 'in a hurry to finish'
    glenn jarrett and gavreid like this.
  18. a.palfreyman

    a.palfreyman pfm Member

    I remember the instrumentation engineer at work saying if you're working on valve gear with psus of say 3 to 400vdc, if you can, connect a wire to the 0v plane and attach your clip to that so it won't inadvertently pull off with potential to touch the HT. Then hold the probe in your preferred hand and put the other behind your back so you can't make a potential leak circuit across your hands (and therefore your heart). And wear stout boots as these will help just in case.
  19. davidsrsb

    davidsrsb pfm Member


    The one you have missed with HV DC is never work alone and train the other person to kill the power.
    You cannot let go of DC.
    martin clark likes this.
  20. grahamdocman

    grahamdocman pfm Member

    NEVER work with live voltages when you're tired,

    don't be tempted by: ".......just one more connection to solder and I'm finished"

    or: "......... I'm almost finished so I'll carry on"
    davidjt, cctaylor and martin clark like this.

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