Isolation transformer wiring

[sharpi31]

I've purchased a 230V 1000VA isolation transformer. This has a single primary and a single secondary.

See the attached pic.... Should I connect the secondary neutral to ground?

[dave_gerecke]

Hello sharpi, Is the chassis of the component(s) you are plugging into the iso tranny connected to the earth conductor? If so, then connecting the secondary of the iso tranny neutral would be unnecessary. If the component(s) do not have the chassis connected to earth, it would be wise to do so, for your own and others safety. One of the purposes of an isolation transformer is to create a balanced power signal to the component you are powering. This tends to help reduce hum problems. The iso tranny also helps filter out high frequency signals on the AC lines, due to the intrinsic low pass behaviour of a transformer. If you connect one of the legs of the secondary of the transformer to earth, you may actually induce noise into the AC power going into your component(s). Hope this helps.

Peace,

Dave

[TheMG]

Typically in isolation transformers the GND connects to GND. The secondary of the transformer is left floating.

This is safe, because as the name implies, the isolation transformer isolates the power from the mains, in other words the secondary winding is at a zero voltage potential to ground.

Connecting the GND to the secondary actually defeats most of the purpose of an isolation transformer.

[sharpi31]

Thanks for the replies. The attached components will all have earthed cases, so I'll not attach a secondary to ground.

[sharpi31]

I installed the isolation transformer with neither secondary connected to ground.

The transformers in both my preamp and power amp have started to buzz - not dramatically, but louder than when connected directly to the mains.

The isolation transformer is a 1000VA (and remains cool during use) so I don't think the problem is a lack of current...

Any ideas?

[sharpi31]

Problem solved!

The transformer was not truly 1:1, so my kit was getting 254VAC when the mains was sitting at 242VAC. The high voltage was causing the buzzing.

I swapped primary and secondary windings to make a step down (rather than step up) transformer. Now my kit is getting 230VAC and transformer hum is lower than when the kit is connected directly to the mains.

Sound wise, the addition of the isolation transformer seems to have added some warmth and fine detail. I was worried that running my amp from the transformer would reduce bass power but this hasn't happened - if anything, bass focus and depth has increased.

I guess my 25wpc chip amp doesn't suck too much current so the 1000VA isolation transformer isn't being worked too hard.

[Pano]

Originally posted by sharpi31
[B] Now my kit is getting 230VAC and transformer hum is lower than when the kit is connected directly to the mains.

Hey that's cool! The UK has some high mains voltages. I'll bet a lot of gear is happier at 220-230.

I guess my 25wpc chip amp doesn't suck too much current so the 1000VA isolation transformer isn't being worked too hard.

Yeah! No doubt !


The balanced power thing became popular in recording studios about 10 years ago. Those guys seemed to love it. Mostly for the lower noise floor.

[AndrewT]

Originally posted by panomaniac
The UK has some high mains voltages. I'll bet a lot of gear is happier at 220-230.

no.
the UK had and still has 240Vac mains power.
It used to be +-6%.
We have been forced to respecify it to conform to the EC requirement of 220/240Vac.
We complied with this by increasing the -ve tolerance to -10%, but without changing our supply voltage.
We are now 216V to 254V, instead of the former 226 to 254Vac. But we are still a nominal 240Vac mains supply.
As far as I know we never deliberately use the 216 to 226Vac range for load shedding. Instead we use frequency to adjust to the load as demand varies.

[Pano]

Thanks Andrew, that's all news to me.

Kinda surprising, the change, as you are an island nation. The EU is not going to force you to give up your big, bulky mains plugs are they?


Back in the old days I saw 245 or even 250 on British mains. Luckily not every day. That had to be pushing some of the gear that was really made for 220V but sold in 240V counties.

[AndrewT]

Originally posted by panomaniac
Back in the old days I saw 245 or even 250 on British mains. Luckily not every day. That had to be pushing some of the gear that was really made for 220V but sold in 240V counties.

I regularly see 238 to 248Vac on my supply.
The 220/240Vac unified spec is specifically there to ensure manufacturers build equipment that is safely capable of operating properly over the full range of available voltage.