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Old 01-28-2015, 02:18 PM   #1
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Default DIY isolation transformer

From another thread http://www.electriciantalk.com/f2/trippy-84946/

It seems as though I can build my own isolation transformer from warehouse scrap?

If I take two 120v to 12V core and could transformers and connect the two 12V "output" wires from each transformer together, that should give me 120V in and 120V out....correct?

Transformers are out of my scope and I barely understand the concept, much less the details. What I don't want to do is destroy my new PA equipment.

I'm thinking, in a simple transformer I will have two wires in (120V) and two wires out (12V).

So....if I attach an appliance cord to feed one transformer, connect the 12V leads (connecting the transformers) and put receptacles on the 120V leads of the second transformer and I have an isolation transformer device ??

It seems simple enough. I just need to find a couple of matching transformers (landscape lights?) with enough wattage to supply my gear (5-10 amps or 600/1200watts)

Am I on the right track?
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Old 01-28-2015, 02:56 PM   #2
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This isn't my specialty either, but I'm pretty sure an isolation transformer doesn't have any physical connections between the two windings. In your case, I might be wrong, but it seems that would just add impedence to the circuit.
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Old 01-28-2015, 03:05 PM   #3
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The second xfrm wouldn't have any connection. Just the ground feeding through.
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Old 01-28-2015, 03:11 PM   #4
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anything that is not an auto-transformer is pretty much an isolation xfm, your scheme will work but will have losses. 8 amps is about 1kva, or about the size and weight of a small bowling ball.
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Old 01-28-2015, 03:13 PM   #5
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Quote:
Originally Posted by bobelectric View Post
The second xfrm wouldn't have any connection. Just the ground feeding through.
He is taking the secondary of the first xfm (12v) and feeding into the secondary of the second xfm (12v) 120 volts in 12 volt out/ 12 volt in 120 volt out -
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Old 01-28-2015, 03:41 PM   #6
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Quote:
Originally Posted by xpertpc View Post
He is taking the secondary of the first xfm (12v) and feeding into the secondary of the second xfm (12v) 120 volts in 12 volt out/ 12 volt in 120 volt out -
You don't have to explain it to me, I can picture the concept in my mind. Thank You.
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Old 01-28-2015, 03:54 PM   #7
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Quote:
Originally Posted by bobelectric View Post
You don't have to explain it to me, I can picture the concept in my mind. Thank You.
Sorry Bob I misinterpreted your post. I was confused at first and thought others may also be stymied.
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Old 01-28-2015, 04:04 PM   #8
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It's cool, expertpc, but I kinda don't like these young Tom Edisons trying to reinvent proven uses of xfmrs to save money over safety.
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Old 01-28-2015, 04:05 PM   #9
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I found a scrap "600 watt" landscape light transformer/timer. It is actually two 300 watt transformers providing two 12V circuits.

The only thing that confused me was that the low voltage side had three wires. I didn't disassemble to see what was going on.

I found some other stuff including some 300W transformers that we used to use for undercabinet puck lights (before switching to LED) and one 750 watt.

From reading the replies, I still have no idea if this will work Maybe I'm lesdixic ?


Quote:
It's cool, expertpc, but I kinda don't like these young Tom Edisons trying to reinvent proven uses of xfmrs to save money over safety.
The point isn't to save money. I'm trying to educate myself. If there is a safety issue, let me know.

And I'm not young.


And, the fact is that safety is the thing that started this whole venture. The equipment tripped a GFCI circuit (no ground faults existing) and I'm trying to figure out how to make it work properly with GFCI protection.

Last edited by 220/221; 01-28-2015 at 04:10 PM.
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Old 01-28-2015, 04:12 PM   #10
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Research on the internet or go to the real site, www.MikeHolt.com
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Old 01-28-2015, 04:18 PM   #11
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Research on the internet
WTF do you think I'm doing?



And, **** Mike Holt's site.
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Old 01-28-2015, 04:22 PM   #12
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Ok, before this gets out of hand...

220/221, you are going to waste your time and money working on "making" an isolation transformer.

First off, you don't really need need one, you need to make sure your PA gear is set up properly and grounded properly, and yes, by that I mean AVOID using GFCI protected circuits.

I have over 15 years experience in doing live sound in all kinds of situations up to and including full concert hall rigs....and GFCI's are never used in such rigs. BUT proper grounding is followed to the letter. AND in the hundreds of shows I have done, no one has ever gotten zapped.

Second, any homemade isolation transformer will have two big issues: first, as was mentioned already, losses in efficiency. Second, safety. Choosing the right combination of transformers to handle the necessary current will be tricky (Landscape transformers won't cut it, the primaries would only be good for less than an amp or two.) because it is the PRIMARY windings that MUST be wound with heavy enough wire to handle the current. Such transformers are hard to come by (and I strongly discourage the use of the microwave oven transformers mentioned in the other thread..they operate in the kilovolts on the "normal" secondary side.) Do you really want to risk someone getting fried if your homemade transformer fails? Or catches on fire because it couldn't handle the current?

If you REALLY have a hard-on to try using an isolation transformer, buy one made by a reputable manufacturer. Google is your friend to find one of the right capacity. 5kva would be the safe minimum for the rig you have. Anything less in capacity and you are inviting trouble.

Last edited by guest; 01-29-2015 at 01:24 PM.
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Old 01-28-2015, 04:40 PM   #13
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bobelectric out.
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Old 01-28-2015, 04:49 PM   #14
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Quote:
Originally Posted by mxslick View Post
Ok, before this gets out of hand...

220/221, you are going to waste your time and money working on "making" an isolation transformer.

First off, you don't really need need one, you need to make sure your PA gear is set up properly and grounded properly, and yes, by that I mean AVOID using GFCI protected circuits.

I have over 15 years experience in doing live sound in all kinds of situations up to and including full concert hall rigs....and GFCI's are never used in such rigs. BUT proper grounding is followed to the letter. AND in the hundreds of shows I have done, no one has ever gotten zapped.

Second, any homemade isolation transformer will have two big issues: first, as was mentioned already, losses in efficiency. Second, safety. Choosing the right combination of transformers to handle the necessary current will be tricky (Landscape transformers won't cut it, the primaries would only be good for less than an amp or two.) because it is the PRIMARY windings that MUST be wound with heavy enough wire to handle the current. Such transformers are hard to come by (and I strongly discourage the use of the microwave oven transformers mentioned in the other thread..they operate in the kilovolts on the "normal" secondary side.) Do you really want to risk someone getting fried if your homemade transformer fails? Or catches on fire because it couldn't handle the current?

If you REALLY have a hard-on to try using an isolation transformer, buy one made by a reputable manufacturer. Google is your friend to find one of the right capacity. 5kva would be the safe minimum for the rig you have. Anything less in capacity and you are inviting trouble.
Back in my early 20s, I was playing out in some really sketchy places. I remember one place we played was a hole-in-the-wall dive. We kept tripping the breaker, so we ran an extension cord to another circuit. I had gotten into the habit of touching the tuners of my guitar to the microphone to see if it sparked, and make sure I wouldn't get shocked. I did that after changing the PA to another circuit, and BOOM...a rather large fireball, and the breaker tripped.

We packed up and left. Never went back to that place.
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Old 01-28-2015, 04:49 PM   #15
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I think the 3 wires are to allow for 11, 12 and 13.5 volts or something similar to compensate for voltage drop or increase lamp life.
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Old 01-28-2015, 05:37 PM   #16
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Quote:
If you REALLY have a hard-on to try using an isolation transformer, buy one made by a reputable manufacturer.
Electrical work never gave me a boner. I am just trying to solve two problems.

Equipment trips GFCI and equipment hums/buzzes.

Isolation transformer is suggested a lot but I'm not going to buy one unless I know it will work.

In researching, I thought I may be able to build the same thing as they sell. That way I have not wasted $300, just some time.
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Old 01-28-2015, 05:44 PM   #17
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Quote:
Originally Posted by mikey383 View Post
Back in my early 20s, I was playing out in some really sketchy places. I remember one place we played was a hole-in-the-wall dive. We kept tripping the breaker, so we ran an extension cord to another circuit. I had gotten into the habit of touching the tuners of my guitar to the microphone to see if it sparked, and make sure I wouldn't get shocked. I did that after changing the PA to another circuit, and BOOM...a rather large fireball, and the breaker tripped.

We packed up and left. Never went back to that place.



Back in my early 20s ?

1920s ?

How old are You ?




Oh ! You said your 20s ! SORRY !
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Old 01-28-2015, 05:53 PM   #18
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How do you plan on keeping gfci protection on the secondary side of the isolation transformer if you did use one?

Since a true isolation xfmr is exactly that......the primary and secondary coils are physically and electrically isolated from each other so then a GFCI receptacle on the primary side would only protect against faults on the primary side. If the fault occurred on the secondary side then you the primary side gfci likely wouldn't work.......unless I'm missing something here.

As far as I can see.....you would have to install the GFCI receptacle on the secondary side of the xfmr and then you're right back to the same issue again.
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Old 01-28-2015, 06:12 PM   #19
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In the original post, the suggestion to use an isolation transformer
was not that it be used as an isolating transformer, but be used as
a filter choke.

So you don't have to lift any earths or isolate any earths
you simply need some inductance to suppress any spurious
stuff that may be upsetting the GFCI.

So using two transformers back to back will work,
provided the trannies have the right power capacity,
there will be a small loss but it's not significant.

The earths can be carried thru
remember we want a filter choke not an isolating tranny
for this application.

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Old 01-28-2015, 06:20 PM   #20
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It works in concept, but I don't know about real life.

A 480/240 transformer isn't exactly 1:1, so if you flip it around, you don't get exactly 1:1. It is compensated to fill the gap between theory and reality.
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