I have one common sense suggestion which I've found very useful. I use the same mains lead for my soldering iron and for testing equipment. This means I can't solder equipment which is turned on. I pull the mains lead out of one and put it in the other as I work. So I NEVER solder inside anything that is connected to the mains - certain!
Andy
Andy
One more thing: breadboarding. If you make a breadboard, you'll want to mount the mains transformer securely, and also fix the circuit to the board at strategic points. If you solder a mains cable to it, make sure it is firmly secured somewhere else too so that, in case you accidentally yank it, the cable doesn't come off to touch you. Tie-rips are cheap, so no excuse not to use them to secure the cable to your breadboard or a table leg.
One general remark: make sure that everything sits securely on your table. No wobbly breadboards or amp chassis balancing precariously upside-down on a lousy support. Have everything ready that you need, so that you don't need to search for stuff or put it away while the circuit is powered up.
I once had a scary experience working on an amp. I had done everything right in terms of safety (at least, that's what I thought) until I wanted to move my multimeter. The ground clip was still connected to the amp. The meter dropped off the table, giving a short tug on the black probe. As a result, the amp slid off its supports and came tipping over towards me. Luckily it stopped in time and no damage was done (it was caught by -yes- the mains cord which I had secured to the table), but from then on I take great care that my stuff is well supported before powering it up.
One general remark: make sure that everything sits securely on your table. No wobbly breadboards or amp chassis balancing precariously upside-down on a lousy support. Have everything ready that you need, so that you don't need to search for stuff or put it away while the circuit is powered up.
I once had a scary experience working on an amp. I had done everything right in terms of safety (at least, that's what I thought) until I wanted to move my multimeter. The ground clip was still connected to the amp. The meter dropped off the table, giving a short tug on the black probe. As a result, the amp slid off its supports and came tipping over towards me. Luckily it stopped in time and no damage was done (it was caught by -yes- the mains cord which I had secured to the table), but from then on I take great care that my stuff is well supported before powering it up.
As mentioned in my safety thread, a lot of old equipment (from the 40's through the 60's) had one side of the AC line connected directly to the chassis. Most soldering equipment (and just about everything in the US) had two wire power plug (no ground). In the early 70's Weller came out with a soldering iron that had a 3 wire cord with the soldering tip grounded. I'll never forget the big bang that I got when I stuck that soldering iron into a TV set that was turned off, but plugged in.
The XYtronic iron that I use today seems to have some resistance between the tip and ground to allow for draining static build up, but reducing the "big bang" moments. The fancy Metcal that I use at work will sense any electrical activity at the tip and shut off, even on a 3 volt cell phone.
Ah, the combination of soldering and arc welding! Done that a few times myself
As Mr. Spock would say: "Interesting.." We frail humans..
Certainly one of those major "Oops" moments would be with a power supply circuit that you deliberately and safely discharged or so you thought, and that also had bleeder resistors installed. Some electrolytics have surprising memory, i.e. dielectric absorption.. Minutes later.. (Lots of photoflash caps) zappppppppp! I guess the moral of that story is to check with a meter before soldering in order to avoid arc welding with your solder iron..
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I put a GFIC (Ground Fault Interrupter Circuit) outlet on my workbench, it trips faster than I do when a fault happens...
I have one (GFCI/GFI here in the USA) providing power to my bench as well, and highly recommend it.
That reminds me of my first encounter with mains AC. My introduction to electronics in my mid-teens was with battery-powered AM radios and small amplifier projects from magazines (all with germanium transistors!). I then decided to try a mains-powered project; I don't recall what exactly it was, but it certainly didn't have any kind of chassis! I stood up from the desk and I must have snagged a wire. This was ultimately attached to the mains transformer, and the latter slid off the front of the desk. My natural reactions clicked in, and I grabbed the transformer before it hit the floor. Luckily I managed to let go of it again pretty quickly - after thirty years I can still remember the feeling of that numbing throb in my left hand, and the shock afterwards of realising how much worse it could have been...
I am either very fortunate or have developed healthy habits since that first experience, as I haven't had a mains shock since, despite graduating to valve circuits recently. I did get a tingle a year or so back when I lifted my preamp off its shelf while it was still powered up - I forgot momentarily that the box had no base, and that there was 300VDC under there. Luckily it was only my fingertips that brushed the offending circuit and the rest of that hand was holding the earthed chassis...
Alex
Once you get past electrical safety, the real danger is to your bank account...
Lots of good advice, one more...
Buy a few of those way cheap made in China $5.00 digital meters and connect one of the to every power supply volage and leave them connected until to are finished working on the amp. these will let you know for sure if the power is high.
Also until the amp is working I llike to install a extra "bleeder" resistor that takes the voltage down very quickly. Yes it lowers the overall voltage but I size it to drain the caps very fast when I'm working on the amp then I remove it later
When yo are working on the unpowered amp, to make 100% certain it really is unpowered I connect a clip from B+ to the ground on my bench. This makes sure the caps really are drained all the way to zero. And even after I do this before I touch anything I routinely short it to ground with a screw driver just as an extra triple redundant precaution.
Always if you leave the bench, put the big ground clip on the B+ suply then if some one else does something stupid like plugs the anp into AC a fuse will blow.
Buy a few of those way cheap made in China $5.00 digital meters and connect one of the to every power supply volage and leave them connected until to are finished working on the amp. these will let you know for sure if the power is high.
Also until the amp is working I llike to install a extra "bleeder" resistor that takes the voltage down very quickly. Yes it lowers the overall voltage but I size it to drain the caps very fast when I'm working on the amp then I remove it later
When yo are working on the unpowered amp, to make 100% certain it really is unpowered I connect a clip from B+ to the ground on my bench. This makes sure the caps really are drained all the way to zero. And even after I do this before I touch anything I routinely short it to ground with a screw driver just as an extra triple redundant precaution.
Always if you leave the bench, put the big ground clip on the B+ suply then if some one else does something stupid like plugs the anp into AC a fuse will blow.
While generally all good advice I would recommend not using a directly grounded screw driver to discharge caps, a probe or screw driver grounded through an appropriately sized power resistor (I typically use a >2W 10K resistor for this) is much kinder to the caps, not to mention device used for grounding. (You can actually seriously damage a capacitor by shorting it, low impedance types with significant charge will vaporize internal conductors and foils - and in extreme instances may explode. I've seen it happen before I could "intervene" in the lab..)
Maybe it goes without saying, but if your amp is on the bench and the AC is unplugged there is no safety ground to chassis. If you then clip lead the B+ to safety ground and the caps ARE in fact charged, the entire chassis is now going to be at negative B+ potential... Just occurred to me...
Yes. I agree. Maybe I was not clear. I ONLY use the screwdriver on conductors I THINK are already discharged as a double check that they are in fact dead. If sparks fly then it shows that I made a serious error. I never toch a screw terminal or wire until I first test it be shorting it to ground,
But if you get in the habit of always triple checking then you will survive even a double screw-up.
The idea was that every time before you touch anything, verify if is dead. Not just verify it once but every time, even if you just checked 15 seconds ago. And you likey have a screw driver in hand so it's an easy way to very the conductuctor is at ground potentiall.
That said, I have this one screw driver with a chunk missing. That one, years ago. found a mistake and blew a small bit of metal off the shaft.
Ouch, yes IMHO... I always clip to the internal amplifier grounds - I always unplug prior to doing anything..
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