AC High voltage protection - Page 1
Forum section: electronics / beginners

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 25, 2015, 03:24:46 am
content: I'm currently making a tube amp (my first high voltage project) and I'm soon ready to do some live testing.I seem to remember I've seen something about running mains through a light bulb before going into the amplifier. Does this actually do anything?The video I seem to remember having seen, the guy shorts the live output, and the bulb turns on while he lives.I guess it presents the AC with a resistive load other than me...?There is no need to warn me of the dangers, I'm terrified (almost solder the thing with one hand in my pocket)!Is there a way to protect oneself against high voltage (~270Vpp) AC using a bulb?

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 25, 2015, 03:30:34 am
content: sorry guys, I promised I had googled the thing to death. Of course I stumbled upon the thing just behind the next corner...It seems it protects thegearnot me. Though it seems like a nice quick-n-dirty debugging approach. Kind of likestd::cout << "This is debugging, I promise!" << std::endl;

author: bills
author_rank: Frequent Contributor
timestamp: August 25, 2015, 03:45:50 am
content: Quote from: alexanderbrevig on August 25, 2015, 03:24:46 amNO!!!what you are describing is a dim bulb tester, they are used for testing for shorted components or excessive amp. draw.IE: If the bulb (say a 60 watt) glows bright it means your drawing excessive current if it glows dim it means you maybe able to put full mains power on the dut.to answer your question how to be safe? main thing is not to work on it powered up.Also when testing under power one of the precautions would be a isolation transformer.may I suggest that you wait until you have some more experience before you try working this when it is powered.If you would like I can post some links to a couple of sites where experienced tube (valve) folks can advise you.

author: bills
author_rank: Frequent Contributor
timestamp: August 25, 2015, 03:52:37 am
content: http://antiqueradios.com/forums/index.phpThey have a vintage audio forum with very knowledgeable and experienced people.http://www.audiokarma.org/forums/index.phpyou might find some help here. although it takes time to sign up.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:00:18 am
content: Thanks guysIt's a chicken and egg problem. I can't get more experience by doing nothing, and I can't do what I want without experienceGood point about waiting to test it under load. That sounds like good advice (and probably what I'd do anyways. I have much respect for the VAs).For now:Do not test under powerAlways bleed out filter caps (one hand in pocket)A question for later:Got me thinking. How could one probe certain nodes with oscilloscope?Isolation transformer and 10x probe? Hmm.I'm too scared to just try. I think I need to educate myself more on precautions and procedures with HVAC.Has @Dave done any videos on the subject?

author: fivefish
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:00:48 am
content: The bulb is poor man's substitute for a variac... you just want to see if there is a mains short in the equipment, before applying the full 110/220V to it (say, after a repair job). It doesn't protect you personally.

author: fivefish
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:02:24 am
content: QuoteScope is always ground reference. Don't just go poking it everywhere, not unless you're using a differential probe. Otherwise, you may be creating a high voltage short and that probe lead blowing in your hand.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:06:21 am
content: I'm annoyed at my brain for understanding impedance calculations and things of that nature, but the whole ground loop / earth reference thing I cannot seem to build up any intuition.I get that something can be or not be earth referenced, but why does it matter? Maybe there's something wrong with my assumption that ground is always 0 volts (as in something that's absolute)? Do you know of a good place to quickly see a model of the problem? OR something to read if that's better.EDIT: As always, thank you guys for valuable feedback! RE: Variacs, they're crazy expensive. Do you think I'll need one? I'll probably not do another valve project (I think) as I'm looking to try the digital class D + DSP approach for my next amp.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:17:08 am
content: The bulb debugger is OK. It can be not completely deadly if bulb is very low power, say 7W one from refrigerator. Experimenter will just recieve little bit of shock, if anything goes wrong. To become deadly, the AC should reach about 100mA. So at 60mA it is just shocking to make person unconscious. But I am kidding.Anything with mains voltage is deadly serious. Don't do it, if you are not prepared.

author: bills
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:17:45 am
content: " Always bleed out filter caps (one hand in pocket) "How about a soldering resistor across the filter caps say 10k-30k (just a guess) and just unplug it and wait a few minutes, the caps will bleed them selves. Always check with your volt meter!

author: bills
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:28:08 am
content: Quote from: unitedatoms on August 25, 2015, 04:17:08 amNO not safe!Think about it 7watts at 300volts= 43.0 ma. will still charge the filter caps. unless the amp is using a tube rectifier than it may not light the filament and the only place you would have voltage 43.0 ma.@ 300 v is the output at the power transformer.Still enough to kick you arse.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:37:07 am
content: Quote from: bills on August 25, 2015, 04:28:08 amI was kidding. No way anything with galvanic connection to mains is safe.

author: bills
author_rank: Frequent Contributor
timestamp: August 25, 2015, 04:50:19 am
content: I knew that

author: unitedatoms
author_rank: Frequent Contributor
timestamp: August 25, 2015, 05:10:16 am
content: Quote from: bills on August 25, 2015, 04:50:19 amGreat. Phew. I actually recall now, that 10mA is the max current meant to be deadly. It was too long since I learned EE basics back in 80s in academy.

author: deadlylover
author_rank: Frequent Contributor
timestamp: August 25, 2015, 05:11:06 am
content: Quote from: alexanderbrevig on August 25, 2015, 04:00:18 amYup, check outI never set up probing, or hand probe high voltage circuits while they are live. I wait and make sure the caps (which should have bleeder resistors) are discharged before I change the probe configuration, always using hooks and grabbers.It's a bit of a pain in the arse waiting a few minutes each time, but I just do other tasks while waiting.A differential probe is almost essential for safe probing of high voltage stuff, quite expensive for a hobbyist but well worth it IMO if you'll be doing lots of HV stuff.You aren't directly rectifying mains AC are you? Best to use an isolation transformer instead.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 25, 2015, 01:01:10 pm
content: Nobody has asked the OP whether the amplifier has a Power transformer as part of the power supply .If it doesn't have one,it either uses a switchmode power supply,or is that most deadly of things-A Transformerless,or "AC/DC " supply..If it is the latter,my advice is to cease & desist now!!!In such power supplies,the chassis is connected to one side of the incoming Mains---the Neutral side,if the plug or power socket are wired correctly.The "Active" or "live" side is then rectified directly.All OK,if a bit horrible,if it is the Neutral on the chassis---but with Active on the chassis,touch it & a real Earth,& in the words of an old Japanese Safety note:-"It is utterly sufficient to send you directly to the heavens"Such supplies were widely used in some countries,with heavy insulation to protect the user,but "homebrew" versions usually don't have such protection.There is no justification for such supplies in an amplifier built in 2015 --------If you don't like linear transformer type supplies,use an SMPS,they are reasonably safe,plus are probably a factory made unit,in any case.This brings us to an "old style" transformer type supply.The series lamp idea is to protect against short circuits,or things which are not exactly shorts,but draw excessive Mains current.They are mainly used when restoring a very old piece of equipment which may have problems.The Primary side of a transformer tyoe supply is quite simple,& may be checked out without power applied,(AND with the cord unplugged from the Mains socket)using the DMM's Ohms range.With this,you can check whether your ON/OFF switch actually does what it is supposed to do,& doesn't short Live & Neutral together-------don't laugh,I did that as a beginner!!Once you get past that hurdle,you should,with the switch "ON" be able to see the resistance of the transformer primary across Neutral & Live.Check for shorts to chassis,& hence to Mains Earth,on both sides of the power cord.With tube rectifiers,the next step was to remove the tube,then fire the amp up ( having taken the DMM probes off the circuit.off,& switching that instrument back to its highest AC range.)This will test all the primary & secondary high voltage ac wiring,plus the filament wiring.This step is a bit messy with solid state rectifiers,& I would suggest be dispensed with.Instead,check the rectified HT circuitry for shorts,Incorrectly polarised electrolytics,etc.Visually check all the wiring twice,& if all looks ok,unplug the tubes & fire it up.No smoke?-switch the DMM to dc & look for correct HT voltages out of the power supply.Go to ac on the DMM & check for the presence of filament voltages on the appropriate pins.Finally,turn it off,restore all the tubes & turn it back on-------if it is an Audio amplier which feeds a Loudspeaker,make sure one is attached,of the right impedance & appropriate power rating.Safety---easy----this thing uses high voltage DC anode supplies,so you can't just poke around inside like you might with something having a 5v or 12v supply.( I was going to say "a solid state circuit" but many such circuits use voltages approaching those used with vacuum tubes,so it is no longer a valid comparison).There are also places you can get bitten by 200-300v ac from the transformer secondary,& the Daddy of them all,is of course,the Mains!.There is no need to be terrified,though,you do hundreds of things every day,which are more hazardous.As a former Australian Prime Minister said in a different connection:-"We should be alert,but not alarmed".