Benchtop Power Supply Question

[stj]

Re: Benchtop Power Supply Question

the usual failure point is where the box is then strapped - literally, to the incoming pipework from the street.
in one case i found that the damned gas company had replaced the iron pipe with a poly one and just left the fucking earth strap hanging without telling anybody!

[redwire]

Re: Benchtop Power Supply Question

For ESD, you don't want a negative ion generator because they spew out -ve HV charges which basically charges up everything.
For workbench use there are AC ionizers to neutralize charge but I have never seen one used, they are silly expensive.
I have to have a grounded (dissipative) ESD mat because the winter humidity gets so low I can easily make 10kV walking around, or in getting out of a chair. The LED thingy I made shows stray charges on myself all the time.

My building uses metal piping for earth-ground and it would be very hard to open-circuit that and have power PE up at some potential, but even if it did happen - so what the entire bench mat, soldering iron, scope etc are all at a common potential so I can't see getting zapped.
Many times I took a long run of wire and measured ohms from plumbing pipes to power outlet PE ground to make sure things are OK. DannyX has that groundless building I think.

[stj]

Re: Benchtop Power Supply Question

all good till there is a ground failure at the box and inductive coupling puts several hundred volts at 10's of mA on your earth wire.

been there - felt it, then later tracked it down and fixed it.
several times at different locations.

earth is there for emergencies - connecting yourself to it is unwise.
use a negative ion generator if you want a static-free area.

[Per Hansson]

Re: Benchtop Power Supply Question

if the return is through an earthed tool or an antistatic mat/strap thats some assholes swear by then it could trip.

Assholes?
You mean like 100% of all production facilities and commercial repair facilities in the world?
BTW you are wrong: a wrist strap and mat have a 1MEG resistor in series with ground.
Otherwise it would provide absolutely no ESD protection, quite the opposite!

[stj]

Re: Benchtop Power Supply Question

there are other sources of ground though.
like your soldering station.

[eccerr0r]

Re: Benchtop Power Supply Question

If you put a GFCI after an isolation transformer, earth ground no longer is part of the "hot"/"neutral" of the secondary (after an iso transformer, both poles are "hot" sort of ... or neither because neither pole has any relation to GND due to isolation) so no current will flow from the iso secondary to GND. And since no current flows, it will never trip.

If you have a iso transformer plugged into a GFCI, it likewise will not trip either, same problem.

Note that neither case is a loss of protection, just that it wouldn't trip, or at least I don't see how it could trip as no current would flow to ground as there's no connection and thus you wouldn't get electrocuted either - and the GFCI won't protect you anyway if you put two fingers into both holes of a wall outlet.

[jiroy]

Re: Benchtop Power Supply Question

I think it's time to put some reminders :

1 mA faint tingle
5 mA slight, not painful but disturbing shock
6-25 mA painful shock, loss of muscular control
9-30 mA freezing current, muscular contraction, but can also be pushed away from the circuit
50 - 150 mA extreme pain, severe muscular contraction, respiratory arrest, death is possible
1.000 - 4.300 mA heart fibrillation, muscular contraction and nerve damage, death is likely
10.000 mA severe burns, cardiac arrest, death is probable
Main causes of electrical injury or electrocution

[redwire]

Re: Benchtop Power Supply Question

The DMM fuses one thing I like is they are sand-filled and when you short mains there can be quite an arc due to cable and load inductance. This will basically chrome plate the inside of a vanilla 5x20mm fuse if the arc happens in the fuse.
But these fuses are highway robbery, I've never seen such a cash cow. I want to buy a roll of fuse wire and stick it to the man.
Take a generic 5x20mm 213 fast 5A fuse, at 200% 10A 5 seconds and 50A around 10msec.
When I look at circuit breakers, they are designed to be a little faster than the wiring melting and burning up, that's all.

With an isolation transformer, and your scope grounded to SMPS DC bus (-) so you can probe the primary... a ground fault or leakage will NOT trip the (upstream) GFIC because it's current flowing to GND, Line and Neutral remain symmetrical as far as current I think.
Placing the GFIC on an isolation transformer secondary should work?

[momaka]

Re: Benchtop Power Supply Question

Putting a GFCI after an isolation transformer is kind of pointless, isn't it? It would never detect a case that it should shut off.

I was just thinking about that and forgot to mention it.

Actually, it may not be pointless to have it after an isolation transformer.

After all, most GFCI work by comparing current draws between Live and Neutral... or just two AC lines, in case neither is referenced to ground, which would be the case with an isolation transformer. So technically speaking, a GFCI should actually work after an isolation transformer.

If you don't ground-reference any part of the device, then you can touch any one energized part without getting a shock (but not TWO energized parts with high enough potential and current capability... though that extends to any electronic circuit with or without GFCI or ISO transformer.) And if you do ground-reference any part of the device (again, o-scope or other test equipment), the GFCI would not trip until you conduct current to ground through your body. In that case, an imbalance between the two AC lines will be seen by the GFCI, and so it should trip.

So in short, having a GFCI *after* an isolation transformer may not be a bad idea after all. (Or am I missing something here?)

BTW, back when I was in university and rented a small duplex in a 50's house as my "dorm", none of the outlets had ground. They were all 2-prong. Looking at electrical codes, the only way I could have a legal 3-prong ("grounded") outlet was by using a GFCI outlet with ground left floating. So that's what I did during my stay for the one outlet where I had to plug my computer equipment.

[stj]

Re: Benchtop Power Supply Question

interesting theory.
i'm not sure if it would trip after a transformer.
it really depends on the return-path.
if the return is through an earthed tool or an antistatic mat/strap thats some assholes swear by then it could trip.

btw, in europe they are usually 30mA trip,
the 7mA ones are too damned sensitive.

[eccerr0r]

Re: Benchtop Power Supply Question

Exactly hence the comment I'd never know if I was touching an energized node whether it was for probing (touching the node and touching the grounded oscilloscope) or not. However if just that GFCI was there, it'd trip and the circuit would go dead.

Thus indeed they are for different purposes and the GFCI really doesn't help you do debug, only certain classes of mishaps... and all the cases the GFCI can protect, the isolation transformer also covers without cutting off power...

Putting a GFCI after an isolation transformer is kind of pointless, isn't it? It would never detect a case that it should shut off. The GFCI needs to be between the iso transformer and mains power -- and even then it shouldn't detect anything nor would it be deadly as no current would flow anyway. At least you can plug in other equipment to the GFCI and have those things protect you, instead of loading down the iso transformer for no good reason.

[momaka]

Re: Benchtop Power Supply Question

it doesnt,
an iso transformer can kill you as easy as mains
they are to isolate the device from ground for your scope.

Yup.

It actually kind of goes both ways, though.

For example, if you touch something energized in a device connected to an isolation transformer and there is nothing providing a ground reference to this device, then you won't get electrocuted if you "ground" that energized part of the device with your body. So in that case, the isolation transformer can provide a little extra safety (after all, there is no "Live" and "Neutral" at the output of an isolation transformer - you just have two AC outputs not referenced to ground.) However, if you touch two energized parts in the device that have enough potential difference between them (and ability to provide sustained high enough current), then you definitely can get electrocuted.

Now imagine your bench is equipped with GFCI.
Without an isolation transformer, if you touch any "Live" energized part in a device, then the GFCI can detect unequal current between L and N (i.e. some current "sinking" into ground) and thus should trip. But if you connect an isolation transformer behind the GFCI and you provide a ground reference to your device... then any current "sinking" into ground won't be detected by the GFCI, because as far as the GFCI is concerned, there is equal current going between Live and Neutral into the isolation transformer. So in a case like this where you do have GFCI and an isolation transformer, the GFCI will become ineffective, because it will not be able to "see" any "stray" current going to ground through your body or anything else.

Thus, I think an isolation transformer can go both ways in terms of safety, depending on how it's used. But like you say, the main point of an isolation transformer is not to protect the user, but rather to provide a line output that is not referenced to ground so you can connect and o-scope or other grounded test bench gear to the primary of a power supply. Once the o-scope or other test equipment provides a ground reference point on the circuit, though, then any other parts in the circuit can become dangerous to touch if they have high enough potential and current capability above the ground-referenced part of the circuit.

[stj]

Re: Benchtop Power Supply Question

i'm using 32x6.3mm 600v FFF rated

[eccerr0r]

Re: Benchtop Power Supply Question

still think those 10mm "midget" super fast HRC fuses are way too expensive and they're rated only 600V.

[stj]

Re: Benchtop Power Supply Question

they are only $ARM$LEG if you want 1KV version,
i buy the 600v ones for about 2-3 $

think about it, are you *realy* gonna be reading current above 600v?

[eccerr0r]

Re: Benchtop Power Supply Question

unless you get those $ARM$LEG ultrafast fuses like they use in DMM's
I think I'd prefer the nuisance trip over having to pay another $ARM$LEG for another fuse, alas right now if I expect a short, it's a DBT that takes the place of a fuse...

[stj]

Re: Benchtop Power Supply Question

fuses are slow - read some datasheets

[redwire]

Re: Benchtop Power Supply Question

I once worked for a company that bought a bankrupt company that had made products with electronics in the pipeline monitoring industry.

While we were boxing things up, workbenches an absolute pigsty, I got blasted by mains line cord someone had left plugged in with bare wires sticking up, in the pile. Guessing a disgruntled ex-employee did it. So I got my hand basically copper plated from the vapors.

So I kind of like a workbench with less than full mains current for whatever I am working on or repairing.
A 5-10A fuse is looking pretty good right now because they can pop fast and it's an uncommon happening for me.
These B circuit breakers I have not worked with, but they seem a bit slow.
An electronic fuse could work but fixing SMPS etc there is inrush current so I would expect it to nuisance trip.

[eccerr0r]

Re: Benchtop Power Supply Question

time to go with electronic fuses (a triac with current sense that cuts off drive when current is exceeded by the tiniest of margins, even for a moment?)

[redwire]

Re: Benchtop Power Supply Question

Did you ever trip one on your workbench, like a B? That's 3-5x rated before they trip so a 3A breaker trips at 9-15A according to the charts.

I looked at mini circuit breakers MCB DIN-rail and they all seem to be about protecting wiring. So they are looking a bit slow and extreme in the magnetic trip part of the curve.