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 Post subject: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 4:48 pm 
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Hello everyone,

I am looking to buy an Isolation Transformer from Mouser. The used ones I see on ebay look questionable at best, so I have found two that may fit the bill. However, I have a some of questions, and if someone could maybe give me some advice, that would be great.

1. Both data sheets show the output grounded. I thought the output on a true isolation transformer was not grounded. Yes?
2. Could either one of these be used as is, as my Hallicrafters SX-140 has the old two blade non polarized plug?
3. Could I just remove the ground wire from the output socket?
4. Out of curiosity, which transformer looks better suited to the task at hand, which would be old radio restoration.

Thank you,

73
Paul Johnson
KN4FIF


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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 7:11 pm 
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https://www.mouser.com/datasheet/2/177/171-1390017.pdf
https://www.mouser.com/datasheet/2/410/ ... 067353.pdf
Here’s the links


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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 7:45 pm 
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Both are true isolation transformers. The key is that the hot and neutral are not grounded.

The way I like to think of things is that I want to be able to ground the chassis of the DUT. If it has no transformer ( eg an AA5) then the iso transformer allows me to do that.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 9:43 pm 
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The cord to the primary is a 3-prong cord, grounding the input to the iso trans? Clip it off if you don't want it or use an adapter.
But the 2-prong plug on the radio doesn't care if there is a grounded outlet. Again you could use an adapter to cut off the ground at the secondary of the iso trans if you are working on something with a 3-prong plug.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 9:56 pm 
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I think the way to go is don't use the output ground (3 prong plug to DUT)

but keeping the input ground to wall Socket hooked up as that ground is actually used for grounding the shield in or around the transformer for noise and in case the transformer develops a internal to case short.
Think about using a fuse to save ISO-Transformer.
I would also use a G.F.I. to help safety wise.
So since most older radios only have a two prong cord there would be no issues.

IMHO.

You can always check if both input and output grounds are connected together by using an OHM Meter if you don't get a resistance reading between the two then it's probably good.
After editing fifties beat me to the punch first and his is a good Idea!

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Last edited by Radiosmoker on May Thu 16, 2019 10:06 pm, edited 2 times in total.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 10:02 pm 
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My ISO also had a ground wire from the input to the output. I simply opened it up, and D/C the lead.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 11:00 pm 
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Once again, the ground does not negate the benefit of an isolation transformer. It's **OK** to disconnect it, but it doesn't provide any benefit....Your hot-chassis AA-5 is going to get grounded sooner or later to some piece of test equipment.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Thu 16, 2019 11:09 pm 
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The only thing that matters is the two voltage sourcing leads from the secondary.
As long as neither is connected to "ground", you're good to go.

Modern "isolation transformers" just isolate noise from the power line. They have
one side of the secondary connected to ground, and will NOT work for our use.

You can usually remove that ground connection to make them work for us.

- Leigh

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 2:57 am 
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The third (ground) connection is there by convention so if anybody plugs something in with a three wire cord expecting ground to be there, it will be. There is no point in disconnecting it as the two prong plugs that belong on vintage radios won’t make contact with it anyway. You’d just be depreciating what could be a useful feature if you ever wanted to plug something in for EMI immunity like a grounded tip soldering iron.

Either transformer will work for antique radio repair. I knew Fred Hammond and can vouch for the quality of the transformers his company makes, but Triad is a very good company too. I don’t think you’d go wrong either way.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 6:59 am 
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I bought a 400VAC Toriod brand medical isolation transformer for $50 off of ebay. I am going to wire it up to my dim bulb and use it, made in USA, cheap to buy and will do everything I need it to do.


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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 8:59 am 
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A lot of great information here. I want to thank you all.

It’s going to be a couple of weeks before I can order anyway, so I’ll have time to mull this over.
It’s spring and the wife has projects too.

73,
Paul


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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 3:12 pm 
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impalacustom wrote:
I bought a 400VAC Toriod brand medical isolation transformer...
Modern medical isos do NOT isolate the AC source. They only isolate the AC line from noise generated by the equipment being used.

The ones I've seen have a jumper between one side of the output AC winding and safety ground. This jumper can usually be removed, resulting in the isolation we need.

- Leigh

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 3:43 pm 
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If it is like this one, it appears to be labeled as a floating secondary.


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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 6:53 pm 
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The least expensive method is to simply build your own ISO, out of two identical power transformers, wired secondary to secondary, and enclosed in a case. One primary connected to an AC line cord, the other to an AC receptacle.

Simple, cheap, and you know there's no ground connection in it.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 6:57 pm 
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It seems that for once, fifties and I agree. ^ ^ ^ :lol: :roll:

- Leigh

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 6:58 pm 
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^ Wonders will never cease... :wink:

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 7:38 pm 
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One point to realize if building an iso as described (two transformers back-to-back)...

You would expect the output voltage to equal the input voltage, but it won't. That's because of internal losses, and the way they're compensated in transformer design. The turns ratio is adjusted to add secondary voltage equal to the internal losses. When you connect a transformer backwards, that winding imbalance remains.

- Leigh

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 7:42 pm 
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That's actually a positive aspect, however, given that the sets we work on were designed for 110 VAC, and line voltages today are around 121. So losing a little in the transformer losses would make it about right.

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 8:10 pm 
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fifties wrote:
That's actually a positive aspect, however, given that the sets we work on were designed for 110 VAC, and line voltages today are around 121. So losing a little in the transformer losses would make it about right.
I agree.

But if a newbie just built one of these and is testing it, he might not realize why the voltages differ from what he expects.

- Leigh

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 Post subject: Re: Isolation Transformer Advice
PostPosted: May Fri 17, 2019 9:06 pm 
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Leigh wrote:
fifties wrote:
That's actually a positive aspect, however, given that the sets we work on were designed for 110 VAC, and line voltages today are around 121. So losing a little in the transformer losses would make it about right.
I agree.

But if a newbie just built one of these and is testing it, he might not realize why the voltages differ from what he expects.

- Leigh

That's called a learning experience... :wink:

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