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need some help here guys!
i'm building an AC power supply with switchable "earthed/floating" output for working with an oscilloscope.i've sourced some isolation transformers but am unsure of the current rating of the windings i will need for working on guitar amps.what current amperage would you think will be safe? i don't want to buy "overkill" but also don't want to be under spec.
thanks

#1 - the scope needs to be grounded at all times.

#2 - watch this video: https://www.youtube.com/watch?v=xaELqAo4kkQ

#3 - if your intention is to use the ground probe on anything besides ground, get a differential probe instead.



I hope you're not doing this because of Uncle Doug. What he does in his video re: isolation is beyond dangerous / stupid.

That's a death machine. The only safe way to do this is with an isolated input adapter or a battery powered scope.

thanks Josh, i've seen that and several other tutorials.i'm confident with the procedures and know the scope needs to be grounded,the "floating" is for the device under test.i've seen a nicely priced transformer which is 240V to 240V "one to one" ratio and the current rating is 1.7amp and 250VA,this is what i'm unsure of.

geoff 1965 wrote: ↑

Tue 01/12/21 7:09 pm

thanks Josh, i've seen that and several other tutorials.i'm confident with the procedures and know the scope needs to be grounded,the "floating" is for the device under test.i've seen a nicely priced transformer which is 240V to 240V "one to one" ratio and the current rating is 1.7amp and 250VA,this is what i'm unsure of.

Why not just get a differential probe? I paid under $150 for mine.

i'll look into that Josh, the floating is just an option and i would still use my current limiter and be very careful,been electrocuted twice in my life and that's enough!
can you go into more detail with the "isolated input adapter" Jack?

geoff 1965 wrote: ↑

Tue 01/12/21 7:34 pm

i'll look into that Josh, the floating is just an option and i would still use my current limiter and be very careful,been electrocuted twice in my life and that's enough!

A differential probe is a better option than an iso transformer. I have one of those too, but I don't really use it. If I need to do that (which I really almost never do), the differential probe is what I would grab. The one I have is a Micsig DP10013. If I was gonna spend more than that (which I'm not), I would probably look at ProbeMaster. I have some passive probes coming from them I'm looking forward to trying out.

As usual you get controversy when watching tutorials on YouTube!
most people recommend isolating the devise under test but i’ve just watched Terry at D-Lab signal trace from the input jack to grids & plates of preamp to primary and secondary output on a fender champ that had a grounded input chord and he ground isolated the scope! it’s hard sometimes determining the correct method when you are new to using a device.

geoff 1965 wrote: ↑

Tue 01/12/21 8:50 pm

As usual you get controversy when watching tutorials on YouTube!
most people recommend isolating the devise under test but i’ve just watched Terry at D-Lab signal trace from the input jack to grids & plates of preamp to primary and secondary output on a fender champ that had a grounded input chord and he ground isolated the scope! it’s hard sometimes determining the correct method when you are new to using a device.

Yikes! He's usually good. BUT to be fair, he uses a lot of cheap old scopes, so if he destroys one, he's probably only out like $20. lol

Mine was only £50 but I still don’t want to fry it!

geoff 1965 wrote: ↑

Tue 01/12/21 7:34 pm

can you go into more detail with the "isolated input adapter" Jack?

It's the same as the differential probe Josh mentioned.

I just want to add that the problem here isn't the risk of equipment damage. All AC-operated equipment that isn't earth grounded carries a charge on its chassis. The amount of current that can be drawn from the chassis to ground depends on several things. The big factor usually is capacitance between the windings of the power transformer. That's why the patient monitoring equipment in hospitals uses special low-capacitance transformers. The equipment is usually not earth grounded due to the risk of electrocution if a patient touches two things at once.

That's the same concept you're facing when you "unground" your scope. Capacitance within the isolation transformer continues to connect the chassis to the AC line. It's not a direct connection, of course, but it will usually carry sufficient current to stop your heart. And if you happen to touch the scope ground (such as a probe clip lead) and the chassis of a grounded appliance at the same time, you become part of the current path. In fact, touching a supposedly isolated item can also create a shock hazard, depending on the polarity of the signal leaking to the chassis in that particular piece of equipment.

This is why manufacturers abolished the use of so-called death caps across the AC line. It was like installing an especially leaky power transformer in every amplifier!

My apologies if you already know all this, just thought it was worth mentioning.

Jack

geoff 1965 wrote: ↑

Tue 01/12/21 9:22 pm

Mine was only £50 but I still don’t want to fry it!

Well, yeah, that's about $68. More expensive than half of his scopes.

TriodeLuvr wrote: ↑

Wed 01/13/21 12:52 am

I just want to add that the problem here isn't the risk of equipment damage.

Well, sure, killing yourself testing something is worse than exploding a scope. But that £50 scope can be a big deal too.

My plan was to build a unit with isolation transformer/variac/bulb limiter.but going to look more deeply into this,just seen one with a variable current trip which seems a good idea.

Just picked this zenith variac up for the project,massive 8amp/2KVA so plenty current draw.not sure about isolation transformer yet but i’m thinking of 2 outputs from the variac,one with a series bulb limiter and one direct with a fuse/trip protection.

In one of the Dlab videos Terry is drinking wine in the beginning and halfway through the video he’s probing the live socket pins with the probe in his right hand and his left fingers resting on the metal chassis.

The isolation transformer is out of the equation, this 8 amp variac takes up 3/4 of the steel cabinet. fitting a NTC thermistor in series to input to protect from inrush current these big variacs can have,output is simple limiter/direct with voltage/current meter.

Why use a 2A fuse on an 8A variac? If you're going to limit it, I would use a second fuse to limit it on the output side, and have an easily accessible fuse holder to swap it out for different amps or DUTs that might need more.

Finished housing the variac,ac output is switchable between bulb limited or direct fused “good suggestion Josh” tested with a 500mA fuse,I can get a range of fuses to suit testing now,better than a current trip which is limited to one level.
bridge rectified/filtered the ac post out socket so i’ve around 430 VDC as well.

But more importantly, how fast does it go with that racing steering wheel?