I need an isolation transformer so I don't die

[Speakerguy]

Hey,

I need an isolation transformer for when I work on high power audio amplifiers, etc. Something so that I don't keel over when I accidentally touch the +120V input to the amp, or the B+ rail, or whatever. Is this what I need?

https://www.electro-tech-online.com/custompdfs/2008/08/TRIPP-LITE_POWER-PRODUCTS_5752502PDF.pdf

Or something else? Recommendations welcome.

 

[TRexall]

I think I remember reading you could connect 2 transformers back to back with similar ratings to get isolation from the mains

 

[Mikebits]

I think I remember reading you could connect 2 transformers back to back with similar ratings to get isolation from the mains

Sam Goldwasser agrees with you
**broken link removed**

 

[Speakerguy]

I can do that, but I only have a bunch of 15VAC 3A (45VA) laying around and I need more power. Also does transformer regulation come into play with that method?

The thing that worried me about that Tripp Lite was that it said something about the isolated neutral and ground being tied together??

 

[Larry4911]

It is a good safety practice to use one yes... But, It's not going to stop you getting killed if you touch BOTH conductors. Or if there is a fault and you touch the case and one conductor.

You have to remember you are still dealing with lethal levels of AC. With a system using an isolating transformer you have no earth to protect the case and supply cable. So, if there is a fault and the case becomes live there is no earth path to leak any current and trip any breaker.

Generally it is said that you should remove the supply before doing work. However, this cannot always be done. In these cases BE BLOODY CAREFUL.

Larry

 

[Hero999]

You can do that but the problem is the regulation is very poor and the fully loaded voltage will be much less than the input voltage.

It's also more heavy and bulky.

 

[Willbe]

so that I don't keel over when I accidentally touch the +120V input to the amp, or the B+ rail, or whatever.

If your input voltage from the wall outlet normally varies +/- 5% I'd spec the xformer to drop ~2% at your amplifier full load input wall current.

I'd only use a transformer like this if I had to use grounded scopes or grounded test equipment.

You can also unground/isolate yourself and use only one hand.

 

[Hero999]

That depends on the regulation. I've found that most isolation transformers are actually wound 1:1.05 so the off load secondary voltage is slightly higher than the primary voltage and the fully loaded secondary voltage is the same as the primary voltage.

 

[TRexall]

Another way would be wear rubber gloves and thick soled rubber boots...and make sure your not grounded in any way

 

[Willbe]

Here's two

https://www.bkprecision.com/www/np_searchmodel7.asp?lf=Isolation+Transformers

one for ~$91 per amp of current, the other for $75 per amp, both at 120v.

 

[john1]

I think you would be better off with no isolation,
and a proper RCD unit (Residual Current Device).

 

[Speakerguy]

I do have an $8k non-isolated (grounded) scope, which is another reason I want one. I threw the GFCI on my outlet when I made a mistake with it the other day. Scope survived, thankfully.

Is there anything wrong with the one I originally linked to?

 

[Diver300]

Am I missing something here? I would have thought that the 120 V d.c. would come from a transformer and rectifier.

There needs to be something to isolate the line and neutral from the audio signal, and that would normally be done by the input transformer.

If so, there is very little point of isolating the mains.

 

[Speakerguy]

Touch the Vcc rail of an amp at the same time as the earth ground at an outlet and tell me how it turns out.

I need an iso trafo.

 

[Mikebits]

The first time I tried to hook a scope ground to an unisolated switched mode power supply, I had wished I had not ...

 

[Speakerguy]

I seriously thought I damaged my brand new DPO3000 just a few days ago when I hooked the ground clip onto something in the (working off line voltage) device. BANG! I thought I killed the scope, until I realized the GFCI had tripped. Turned it on, ran it through the signal path correction and everything looks to be OK now.

 

[Pommie]

Touch the Vcc rail of an amp at the same time as the earth ground at an outlet and tell me how it turns out.
I need an iso trafo.

Are you saying that the amp uses the rectified mains as it's power supply. If so, how is the ground line not lethal (assuming full wave rectification)? Wouldn't this mean that the speaker (ground) wires are at a lethal voltage also?

Forgive my ignorance if this is something that is normal with the U.S. 110V system. I've only ever used 240V with grounded neutral.

Mike.

 

[jpanhalt]

You haven't defined the VA that you need. You can get them on eBay for fairly reasonable prices. At least, you will learn some of the brands, if you need to buy a new unit for a commercial setting.
John

 

[Nigel Goodwin]

The first time I tried to hook a scope ground to an unisolated switched mode power supply, I had wished I had not ...

This is why the majority of service engineers don't earth their scopes, it's far too dangerous.

However, I'm not getting involved in that old argument again - basically you shouldn't be messing with any live voltages unless you know EXACTLY what you are doing and why you are doing it.

 

[Hero999]

Lol, we'll have to agree to disagree on that.

I don't know why they don't make double insulated scopes that don't require an earth connection.