Mains protection

[cowasaki]

Thanks. I think that that resettable fuse looks good. Any other alternatives?

nop haven't seen better for this purpose.

I can find them easily right here or even from AliEx.

They are cheap (cheaper than equivalent fuses) and hassle free
with BONUS .. they have the PANEL mounting already

Combine them with a regular NEON you have a cost effective
tripping wired on the mains front end

Save your money for the secondary side.. more complex.
piece of cake
Paul

How do you adjust the trip current on the adjustable version? I can’t find an image.

[PKTKS]

How do you adjust the trip current on the adjustable version? I can’t find an image.

I only use the FIXED version carefully matched to
the isolation transformer primary winding VA.

And that transformer should meet at least the
power of the device I will isolate... otherwise
i will try another method.

And some of them are poorly marked ...
it depends on the fabrication batch

For example I have a batch without good markings..
nevertheless the minimum specs still fits safely margins.

It works pretty well on the primary side
and is cheap as well

Paul

[cowasaki]

How do you adjust the trip current on the adjustable version? I can’t find an image.

I only use the FIXED version carefully matched to
the isolation transformer primary winding VA.

And that transformer should meet at least the
power of the device I will isolate... otherwise
i will try another method.

And some of them are poorly marked ...
it depends on the fabrication batch

For example I have a batch without good markings..
nevertheless the minimum specs still fits safely margins.

It works pretty well on the primary side
and is cheap as well

Paul

There is an adjustable one which looked like a good idea.  I have a resettable fuse board that I made maybe 30 years ago from a kit but I'm not sure what the connections are.  I'll have to reverse engineer it

[AudioFile]

Everyday Practical Electronics (now just Practical Electronics) had a project for a differential probe a short while ago. I built it and it works very well. Quite cheap too (a few tens of pounds I recall).

If anyone interested and can't find it, just shout and I'll dig out the reference.

[Monkeh]

How do you adjust the trip current on the adjustable version? I can’t find an image.

I only use the FIXED version carefully matched to
the isolation transformer primary winding VA.

And that transformer should meet at least the
power of the device I will isolate... otherwise
i will try another method.

And some of them are poorly marked ...
it depends on the fabrication batch

For example I have a batch without good markings..
nevertheless the minimum specs still fits safely margins.

It works pretty well on the primary side
and is cheap as well

Paul

So what you're saying is they're so cheap and nasty they can't even manage to mark the case properly, but you trust them as overcurrent protection?

[BravoV]

Bad advise. Differential probes cost less than an isolation transformer nowadays (MicSig sells a very economic one). Using isolation transformers is just dangerous. There are too many ways to ground stuff. Better keep everything grounded while protected with a low current GFI. That way you can minimise the danger area to the DUT and the probe leads which is much easier to keep an overview on and thus have less chances of a mistake.

NO not here in Brazil
and may be other places and other budgets.

Almost 100% stuff in Brazil is now IMPORTED.
TAXES OF EVERYTHING ... are pretty much 100% - AT LEAST!
sometimes may be more due to extra shipping costs

Prices are for sure the DOUBLE at least may be 3x times
EVERY STUFF YOU SEE on EBAY ALIX and BANGOOD...

3x times of EVERYTHING is not a good bottom line to live with

Paul

Before giving such advice, you have to know which country the OP lives, as standard of living is different, say UK vs Brazil. So does affordability in "general".

What if this discussion about welding, and someone from poor African country, so proud and keep insisting that is just fine using cheap & effective welding transformer, that was salvaged & re-winded as below example. 

C'mon, technically with much safer equipment, "IF" its affordable is much better right ?

[AVGresponding]

Before giving such advice, you have to know which country the OP lives, as standard of living is different, say UK vs Brazil. So does affordability in "general".

What if this discussion about welding, and someone from poor African country, so proud and keep insisting that is just fine using cheap & effective welding transformer, that was salvaged & re-winded as below example. 

C'mon, technically with much safer equipment, "IF" its affordable is much better right ?

I would give this guy 11/10 for improvisation!

[cowasaki]

Everyday Practical Electronics (now just Practical Electronics) had a project for a differential probe a short while ago. I built it and it works very well. Quite cheap too (a few tens of pounds I recall).

If anyone interested and can't find it, just shout and I'll dig out the reference.

Thanks I would like that.

[Electro Detective]

Before giving such advice, you have to know which country the OP lives, as standard of living is different, say UK vs Brazil. So does affordability in "general".

What if this discussion about welding, and someone from poor African country, so proud and keep insisting that is just fine using cheap & effective welding transformer, that was salvaged & re-winded as below example. 

C'mon, technically with much safer equipment, "IF" its affordable is much better right ?

I would give this guy 11/10 for improvisation!

Meh.. 5 out of 10 from me 

He should have buttoned up the shirt sleeves

Worn gloves or oven mittens, or wrap something around the electrode holder/stinger 

he's too close to the welder and has a cable that would offer an extra meter or two of distance.

isolate both the welder and himself from the ground with wood, cardboard, anything

not sure if he's got a helmet or goggles   

and since I can't see anything else in the picture, the local Fashion Police can add or subtract improvisation points
for whatever style of PPE shorts and flip flops he's sporting.. 

[vk6zgo]

What are people’s views on the best way to protect yourself.

I have a large isolation transformer, large variac and bulb current limiter.

What order?
Which devices?

I work on guitar amps and currently messing with valve amps. My current protection method involves discharging caps and attaching leads etc before turning the device on. Sometimes checking things with the power on is useful. It would also be helpful to protect the device as well.

What are people’s views? At the most the isolation transformer goes into the bulb limited and I have just bought a job lot of variacs.

Valve audio amplifiers  will normally use a built-in power transformer.

The only circuitry on the primary side of such a transformer are the incoming Active, Neutral, & Earth wires, an On/Off switch, a fuse, & perhaps a Mains filter.

There is not much there that needs to be probed with an Oscilloscope, & a DMM will pretty much tell you all you need to know about the primary circuit.

On the secondary side, isolated from the Mains  you will find the High voltage & heater windings.

Classic transformer power supples use a centre-tapped high voltage secondary, & a "Fullwave rectifier"circuit.

https://www.google.com.au/search?source=hp&ei=1aizXZTxHZnZz7sP46uU0Ao&q=full+wave+rectifier&oq=full+wave&gs_l=mobile-gws-wiz-hp.1.0.0l8.2516.5907..8088...0.0..0.285.2271.2-9......0....1.......0..0i131j46j46i275j0i7i30.WtjN4zEIBl8#imgrc=CUBL-vAcbmvdzM:

Here is somewhere you can probe, but two things come to mind.
(1) Your probe needs to be rated for the voltages you are probing.
Tektronix x10 probes are usually adequately rated, but "El Cheapos" may not be.
I find the best probes to use for valve stuff are the big old "clunky" ones that Tek supplied with their old 500series CROs.

A x100 probe would be nice  but it is not really essential.

(2) Don't try to hang your 'scope across the entire high voltage secondary, as you will short out half the winding (note how the centre tap in the drawing I linked goes to common, which is, in this case, the amplifier chassis.)
Your probe and/or 'scope will suffer, the transformer will get"red in the face", & the fuse will probably blow.
You are unlikey to suffer anything other than being dazzled by the spark.

D1 & D2 in the drawing may be solid state, or they may be valve rectifiers.
In the case of the latter, there is "trap for young players"
Some larger or older valve rectifiers have directly heated cathodes, so require a separate better insulated winding ( almost always 5 volts).
This winding will be floating at the DC HT voltage, so don't touch!.
Also, don't try to measure across the winding.

All the rest of the amplifier circuitry will be referenced to Earth , or Common, most likely the ampifier chassis.

After all this  someone will probably pipe up that their Great Uncle found a transformerless audio amplifier in a hayshed in deepest Hampshire, but such things are as rare as hen's teeth!

For good reason, too!
Amplifiers, unlike domestic radios, where with a transformerless design, all the " bities"cab be corralled up inside the cabinet, an amplifier is messy!
It has wires leading all over the place, such as audio inputs & speaker leads.

[Electro Detective]

I too got zapped good and proper from an early age    which sort of 'sparked' my interest in electronicals later on,
learning how zaps happen and how to avoid 

Transformer isolation and grounding etiquette aside,

just how reliable are these differential probes ?   

and what's the worst case scenario when they fail and or pushed past their peak voltage/frequency limits,

or just begin to exhibit intermittent or unreliable behaviour?

Surely there are some cons with these and not just the safety pros?

[AVGresponding]

I too got zapped good and proper from an early age    which sort of 'sparked' my interest in electronicals later on,
learning how zaps happen and how to avoid 

Transformer isolation and grounding etiquette aside,

just how reliable are these differential probes ?   

and what's the worst case scenario when they fail and or pushed past their peak voltage/frequency limits,

or just begin to exhibit intermittent or unreliable behaviour?

Surely there are some cons with these and not just the safety pros?

Modern ones use opto-isolators to completely electrically isolate the signal from the scope, and are powered by an internal battery.

As for intermittent or unreliable behaviour, that would be due to either low battery voltage, or low quality. You get what you pay for.

Cons would be a false sense of safety, and a lower usable bandwidth (unless you have cash to burn anyway lol).

[trobbins]

Valve amp testing can be done without a lot of hoopla. AC mains requires continuity and insulation testing first., and then a cheap power meter can give volt and current readings to confirm no adverse levels during amp operation.
Scope or soundcard probing doesn't need to access HV levels in a valve amp, but should use at least a 10:1 probe to avoid loading.  Absolutely no need for diff probe.

[nctnico]

I too got zapped good and proper from an early age    which sort of 'sparked' my interest in electronicals later on,
learning how zaps happen and how to avoid 

Transformer isolation and grounding etiquette aside,

just how reliable are these differential probes ?   

If they are CAT rated then they are constructed to be safe for use on high voltages and mains up to the specified voltage limits.

Modern ones use opto-isolators to completely electrically isolate the signal from the scope, and are powered by an internal battery.

Wrong. Differential probes use a high voltage divider to bring the voltages down to low levels (and thus limit any current to safe levels) and then use a differential amplifier to feed the single ended input of the oscilloscope. The battery is in there to supply the differential amplifier. Constructing a good CAT rated differential probe is a bit if an art and the MicSig one is pretty good.

[PKTKS]

So what you're saying is they're so cheap and nasty they can't even manage to mark the case properly, but you trust them as overcurrent protection?

Nop.. you can pick some Zing-Ear just fine.

the material and finish is very good.
They also are reliable and durable, more cost-effective
than fuses for the purpose of using them on the primary side

The specs are ABOVE 95% of usual bench setups so
the markings are irrelevant. I have some for quite sometime
with so far no problems.

I would like to see some stress tests or comparisons...
either way my transformers are very good quality
and OVER DIMENSIONED for the bench.

No worries so far. They do a very nice fit there
with a simple NEON.

Paul

[AVGresponding]

Wrong. Differential probes use a high voltage divider to bring the voltages down to low levels (and thus limit any current to safe levels) and then use a differential amplifier to feed the single ended input of the oscilloscope. The battery is in there to supply the differential amplifier. Constructing a good CAT rated differential probe is a bit if an art and the MicSig one is pretty good.

Ehhh... my bad, I thought they were isolated.

Looking at the theory of operation, they seem pretty specialised, and pointless for measuring mains waveforms.

May as well just use the 2 passive probe with channel maths method (making sure your probes and scope V/div combine to the correct signal magnitude), and save some cash, or a proper battery operated and double insulated scopemeter.

[nctnico]

Wrong. Differential probes use a high voltage divider to bring the voltages down to low levels (and thus limit any current to safe levels) and then use a differential amplifier to feed the single ended input of the oscilloscope. The battery is in there to supply the differential amplifier. Constructing a good CAT rated differential probe is a bit if an art and the MicSig one is pretty good.

Ehhh... my bad, I thought they were isolated.

Looking at the theory of operation, they seem pretty specialised, and pointless for measuring mains waveforms.

Quite the opposite. CAT rated differential probes are designed to measure mains like voltages.

[madires]

The real fun starts when you're monitoring multiple signals with your scope. As you know, all channels of a standard scope have a common ground connected to earth/PE. So all the ground clips of the probes are connected to each other. If you like to monitor signals referenced to ground and one signal not referenced to the same ground you need differential probes. Otherwise the ground clip of that channel's probe would short something in your DUT. Another classic example is monitoring waveforms on the primary and secondary side of an isolated PSU.

[001]

Do You protect mains from You? Why You place fuse in primary windings only? 
Is it best way to add two fuses    after variac for 100% and 50% range?

[AudioFile]

Found it.  It was in the January 2016 edition.

It uses opto-isolation and the spec includes:
Input-output isolation >10G Ohm
Max working isolation voltage: 1.4kV peak, 1kV RMS
Input resistance: 2M Ohm / 10pF
Linearity: +/-0.05%

It cost me 50AUD for the 'hard to get' parts including the pcb.

I guess the copyright police will have me if I post a photocopy here, but back numbers are available I believe.

BTW, you can access lots of back copies of EPE on Issuu for free: https://issuu.com/search?q=everyday%20practical%20electronics, though sadly not the most recent ones.

[Messtechniker]

Thanks Audiofile for the hint. Just ordered the PCB and most of the components.
From GB of all places.

The only component I need to hunt down is the power switch.
The usual suspects Mouser, RS, Farnell are out for cost reasons.

Maybe I will fit two separate power switches to the side of the box.

[Electro Detective]

Do You protect mains from You? Why You place fuse in primary windings only? 
Is it best way to add two fuses    after variac for 100% and 50% range?

I have fuses everywhere and PPE    when playing with suspect electronicals 

It's a lot cheaper and less stress blowing a fuse and learning a lesson
rather than toasting equipment and learning the same lesson..     


Popping a question here: shouldn't variacs be fused on their output as well as the input to protect them?

It's recently dawned on me the manufacturers rate them for the output current, yet never provide fuse protection either way,

or put an inline fuse on the input, which may not pop soon enough in a low voltage high current 'burn baby burn' windings satay style scenario 

or they put a totally wrong value fuse = NO BLOW rated   


For the money they charge, they could include some form of switch on surge tamer circuit too 
cold or bitchy mood variacs can pull in some serious breaker trip power on turn on 

Is there some sort of 240 volt surge thingie (that actually works) that can be installed in a variac or via external box ?
I think I saw something posted at EEVblog ages ago somewhere 

[PKTKS]

Same with the hand and fingers.. touching some parts is necessary
and BEING GROUNDED (pulse ground) may be deadly.

Rubber gloves

YES Definitely by far the best advice on top of others.

Everything passing 120 VAC I am always protecting myself
with some sort of touching aids.

With my bench ISOLATED,  ground is out of equation and
I can touch things just fine.

This will not save you from making the classic mistakes
but with your equip ISOLATED and you wearing gloves
that death sarcasm  is ruled out.

best comment so far.
Paul

[madires]

Popping a question here: shouldn't variacs be fused on their output as well as the input to protect them?

I think it's not absolutely necessary but would provide additional protection. I've seen some commercial isolation transformer and variac combos with a circuit breaker at the output (and fuse at the input).

[Electro Detective]

Rubber Gloves may not be that safe with sweaty hands and lots of salt in your food,
but a way better bet than exposed hands

Best to have a few pairs, and flip the moist ones inside out to dry and reuse  in the event you like being cheap like me 

Rubber gloves with thin cotton gloves underneath are the way I roll in any  -ZAP!- scenario