Bench Isolation Transformer

[kg4arn]

 I am looking for a bench isolation transformer.

I thought about modifying a commercial one as described by Toddfun or purchasing?
http://www.mcmelectronics.com/product/72-1097#description
http://www.mcmelectronics.com/product/72-6670

http://www.phcenterprise.com/isolation.html


Any suggestions?

 
 

[nctnico]

A GFI is much safer. Nowadays a GFI is very affordable.

[kg4arn]

My bench power is already GFI protected.
I would like to start working on switch mode power supplies.

What is the safest (in the sense of least dangerous) way to do that?

Isolation transformer for the DUT and a differential probe for the grounded scope?

[Monkeh]

A GFI is much safer. Nowadays a GFI is very affordable.

They really aren't safer.

Let's see, we have two options:
1: An isolation transformer which drastically reduces the chance of a shock
2: An RCD which simply limits the duration but in no way the chance or magnitude of a still potentially fatal shock

I'll take option #1, thanks.

[tom66]

A tip you can also get an isolated variac for both functions in one.

Not all variacs are isolated. In fact most of them aren't. So it's usually a selling point.

[nctnico]

My bench power is already GFI protected.
I would like to start working on switch mode power supplies.

What is the safest (in the sense of least dangerous) way to do that?

Isolation transformer for the DUT and a differential probe for the grounded scope?

An isolation transfer undoes the protection a GFI offers. The nice thing about a GFI is that it shuts down when you touch the hot wire. An isolation transformer just makes two hot wires you can touch by accident.

There is a simple trick when working on switching power supplies if the rectifier and transistors are still OK but the PSU is still not working: apply a low voltage to the primary circuitry to get the switcher chip going. This is the 100% safe way to check whether its working or not. You'll be ably to see waveforms on the secondary side as well.

[Monkeh]

My bench power is already GFI protected.
I would like to start working on switch mode power supplies.

What is the safest (in the sense of least dangerous) way to do that?

Isolation transformer for the DUT and a differential probe for the grounded scope?

An isolation transfer undoes the protection a GFI offers. The nice thing about a GFI is that it shuts down when you touch the hot wire. An isolation transformer just makes two hot wires you can touch by accident.

No, it 'shuts down' after it allows an unspecified current to flow through you to ground for a short time. And if you get between line and neutral, you're stuffed either way.

There is no added safety in an RCD over an isolation transformer. If you knew how one worked, you might realise that.

[nctnico]

My bench power is already GFI protected.
I would like to start working on switch mode power supplies.

What is the safest (in the sense of least dangerous) way to do that?

Isolation transformer for the DUT and a differential probe for the grounded scope?

An isolation transfer undoes the protection a GFI offers. The nice thing about a GFI is that it shuts down when you touch the hot wire. An isolation transformer just makes two hot wires you can touch by accident.

No, it 'shuts down' after it allows an unspecified current to flow through you to ground for a short time. And if you get between line and neutral, you're stuffed either way.

There is no added safety in an RCD over an isolation transformer. If you knew how one worked, you might realise that.

Isolation transformer = two wires with a lethal voltage = bigger chance for an accident. Its better to avoid working on circuits with high voltages present. See above.

[Monkeh]

Isolation transformer = two wires with a lethal voltage = bigger chance for an accident. Its better to avoid working on circuits with high voltages present. See above.

Two wires with a lethal voltage to each other.

No isolation transformer: One wire with a lethal voltage to the other wire, ground, and anything which might be connected to ground, including your feet. An RCD does not prevent lethal current from flowing.

[ivan747]

Isolation transformer = two wires with a lethal voltage when both are touched, however, these wires are floating, so touching ground and one of them poses no risk.

[IanB]

An isolation transformer just makes two hot wires you can touch by accident.

Not really. There are no "hot" wires coming out of an isolation transformer ("hot" meaning to have a dangerous voltage relative to ground).

Isolation transformers are mandated on building sites and industrial work sites because any accident involving cutting a power cord will not result in a dangerous electric shock to the operator. (As long as the two power conductors are only a few mm apart inside the power cord there is almost no chance a human can get in a conduction path between them.)

[Rick]

An isolation transformer just makes two hot wires you can touch by accident.

Not really. There are no "hot" wires coming out of an isolation transformer ("hot" meaning to have a dangerous voltage relative to ground).

Isolation transformers are mandated on building sites and industrial work sites because any accident involving cutting a power cord will not result in a dangerous electric shock to the operator. (As long as the two power conductors are only a few mm apart inside the power cord there is almost no chance a human can get in a conduction path between them.)

So if you touch the output of an Isolation transformer with a voltage tester non of them would appear live... I don't know... I haven't bought any yet:)
If you touch both the live and  neutral wires the RCD device would not open the circuit either, right? It works only if there is fault one one of the wires as far as I understood.
I think the original poster wanted to isolate his DUT so does a GFI offer that advantage ? I think the scope's earth clip would still burn in the case of a short circuit or not?
Lots of question marks 

[Monkeh]

So if you touch the output of an Isolation transformer with a voltage tester non of them would appear live...

What's a 'voltage tester'? A multimeter will indicate voltage. A test lamp will indicate voltage. Any two-pole measurement will indicate voltage. The neon in your little screwdriver probably won't, and one day it'll get you shocked.

If you touch both the live and  neutral wires the RCD device would not open the circuit either, right? It works only if there is fault one one of the wires as far as I understood.

Correct. An RCD detects an imbalance in current between two or more wires. Should it detect a potentially fatal imbalance, it breaks the circuit. But not until after the current has already passed. There is no limiting of current or reduction in the chance of a shock, merely the duration. If you're unlucky or have a heart condition, you can still die. Their main utility, IMO, is that they practically ensure a cut power cable (which includes an earth, at any rate) will become dead before it becomes deadly.

I think the original poster wanted to isolate his DUT so does a GFI offer that advantage ?

No.

[SLJ]

Not really. There are no "hot" wires coming out of an isolation transformer ("hot" meaning to have a dangerous voltage relative to ground).

Let's make this a little clearer for any new users right now.  True there should be no "Hot" wires to ground but you can still get killed by completing the path between the two output wires which are both "Hot" relative to each other.

Also, any isolation transformer you buy needs to be checked first.  Some medical transformers have been known to not be true isolation transformers.

[David_AVD]

Isolation transformers are mandated on building sites and industrial work sites because any accident involving cutting a power cord will not result in a dangerous electric shock to the operator. (As long as the two power conductors are only a few mm apart inside the power cord there is almost no chance a human can get in a conduction path between them.)

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

[nctnico]

Isolation transformer = two wires with a lethal voltage when both are touched, however, these wires are floating, so touching ground and one of them poses no risk.

When testing / measuring on a bench it is easy to create a dangerous situation.

For instance when clipping the scope ground lead somewhere in the primary side of a PSU. All of the sudden there are lethal voltages references to ground on unexpected places and without the GFI protection it is easy to get a lethal shock.

If you try the same without an isolation transformer you'll trip the GFI or blow a fuse when you clip the scope probe into the primary side of a PSU. That will count as a lesson learned and is hopefully an incentive to get a differential probe.

If you weigh all the pros and cons you'll see a GFI is always preferable over an isolation transformer.

[IanB]

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

Maybe that's true. I was thinking that in Europe they would just have an isolated 120 V secondary, but I guess I have to research it some more.

[Monkeh]

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

Maybe that's true. I was thinking that in Europe they would just have an isolated 120 V secondary, but I guess I have to research it some more.

UK site transformers are as described. Although portable units often lack the ground on the centre tap.

[Monkeh]

]If you weigh all the pros and cons you'll see a GFI is always preferable over an isolation transformer.

I disagree. And I love the suggestion that blowing the bollocks out of your scope so you have to buy a new one and an expensive differential probe is better than just practicing working safely and using an isolation transformer (which is often cheaper, easier, and serves multiple purposes).

And I'm still loving the faith in the life-saving properties of the RCD. I'm sure you'll be just fine after 20A passes through you, it was only for a moment!

[ivan747]

What if I connect a GFCI on the isolated side of my transformer? If I am grounding the DUT for measurement I still have some protection with a GFCI.

[alm]

If both the live and neutral coming out of the transformer are floating relative to ground, what alternative path could the current take that would trip the GFCI?

[Monkeh]

If both the live and neutral coming out of the transformer are floating relative to ground, what alternative path could the current take that would trip the GFCI?

Indeed, you would have to ground one leg before the RCD.

[Rick]

The neon in your little screwdriver probably won't, and one day it'll get you shocked.

That's what I meant! My online dictionary gave me "voltage tester" for that screwdriver And it's meant to be a technical dictionary...
Sorry for that.

[Monkeh]

The neon in your little screwdriver probably won't, and one day it'll get you shocked.

That's what I meant! My online dictionary gave me "voltage tester" for that screwdriver And it's meant to be a technical dictionary...
Sorry for that.

Discard the toy.

[Rick]

I thought the work site transformers in the US used an isolated split secondary with the centre tap grounded? (55V AC either side of ground)

I thought I had started having a good understanding of these things now you say the secondary has a grounded central tap ... Is the transformer still isolated in this case? Because its secondary is not galvanically isolated from the earth.