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Isolation transformer connection
Isolation transformer connection
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Old 3rd April 2011, 06:46 AM   #1
pirastro is offline pirastro  Singapore
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Default Isolation transformer connection

I have a pair of Stancor Isolation Transformers connected in series as below:

Isolation #1 Isolation #2
------) (------ ------) (------
) ( ) (
) ( ) (
------) (------ ------) (------
230 120 120 230


1. If I connect the ground only between #1 and #2 would I risk electrocution?
2. If I tie the ground across TAPs, would I negate the benefits of isolation?
3. If I connect it in parallel, would this be equivalent to balanced transformer as discussed elsewhere in the forum?
4. Currently the transformer is causing trips quite frequently, how can I reduce this problem?

Thank you all for your informed advise.
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Old 3rd April 2011, 01:36 PM   #2
pirastro is offline pirastro  Singapore
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reposting diagram

Isolation #1 Isolation #2
------) (------ ------) (------
) ( ) (
) ( ) (
------) (------ ------) (------
230 120 120 230
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Old 6th April 2011, 04:08 PM   #3
pirastro is offline pirastro  Singapore
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Reattaching as image file thanks.

Click the image to open in full size.
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Old 6th April 2011, 04:37 PM   #4
sofaspud is offline sofaspud  United States
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You have a 2-wire input and a 2-wire output. The 2 are hot and neutral.
I don't know what you mean by "the ground".
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Old 6th April 2011, 06:09 PM   #5
zigzagflux is offline zigzagflux  United States
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I would ground one of the 120V leads between the transformers. Effectively acts as a shield between first primary and last secondary.

I am not familiar with code requirements in Singapore, but they will govern what you are supposed to do with the 230V secondary with regards to grounding. I would strongly support either grounding one lead (now becomes the neutral) or grounding the center tap (which you do not show as being available).

Floating systems in general are not friendly, and should be avoided unless you plan on performing open heart surgery..
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Old 7th April 2011, 01:43 AM   #6
metalsculptor is offline metalsculptor  Australia
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There is no need to ground any of the outputs from the isolation transformer but the amplifier case and any exposed metal must be connected to the power outlet earth and ultimately to the building earth, I am assuming that the code in Singapore is similar to Australia most commonwealth countries derived their electrical code from the UK

AFAIR only extra low voltage equipment is exempt from the case earth requirement. I doubt that an isolation transformer would make the equipment fall under the double insulated earth exemption.
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Old 7th April 2011, 02:30 AM   #7
Ruwe is offline Ruwe  Europe
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I second zigzagflux and would ground one of the 120V lines.

I have an isolation transformer in my audio system and it works better if the secondary is referred to ground ... much more quiet at startup.
In floating mode you'll notice that the voltages between the two lines, with reference to ground, are quite different. In my case I had 40VAC and 80VAC, respectively. I am not sure if that is of any significance to the connected end devices down the line, but at least it's not pleasing to the eye.
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Old 7th April 2011, 10:27 AM   #8
AndrewT is offline AndrewT  Scotland
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a 230:120Vac feeding a 120:230Vac will not give 230Vac.

You need to look at turns ratio for the two transformers.

You can measure turns ratio fairly closely by measuring the open circuit secondary voltage.

Then take account of the voltage drops through the four windings.

If either of the transformers has a dual primary then use the dual primary as the isolated output. The centre tap of the dual primary goes to PE. This gives you an isolated ~110Vac from each of the two output windings. ~220Vac from the out of phase windings.
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Last edited by AndrewT; 7th April 2011 at 10:32 AM.
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Old 9th April 2011, 07:08 AM   #9
pirastro is offline pirastro  Singapore
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Thank you all for your advise, the transformer actually has 3 wires, I managed to obtain the the actual schematic from Stancor:

Click the image to open in full size.

I traced the lead in and lead out ground are completely isolated. If I short the ground between the transformer pair it would still be floating. I am concern on the proper connection and safety aspect of the connection.

Since I have two pairs, what are the modes of connection for the ground? Should I short all the 4 grounds then?

I noticed there is a SH sign on the schematic, does it signify the shield is connected to the primary ground? Should I connect the secondary to earth separately

There seem to be two school of thought here
1. to ground at the transformer
2. common ground at amp

What are the pro and cons of each? Thanks
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Old 9th April 2011, 05:24 PM   #10
zigzagflux is offline zigzagflux  United States
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Assuming you will be connecting 3-1 and 4-2 in the middle of the drawing, and your statement that you are concerned about safety, then I recommend:

Tying all grounds together. It looks like the shield is already bonded to ground, but if not it should be.

Grounding 4-2 in the middle of your drawing.

Bonding 4 (far right of the drawing) to ground. Don't be surprised to hear many suggestions to leave the final secondary isolated. Besides the fact that code does not permit this (at least in the US), there is no demonstrable benefit to leaving the winding floating. On the contrary, there are documented IEEE standards that detail the problems with ungrounded systems.

You will have no ground loops with this method, and will have fantastic common mode filtering of RF and noise in your power supply. If you wanted to reduce differential noise, you could add a X capacitor across the middle 120VAC lines.
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