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Old 10th September 2012, 03:22 AM   #1
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Default cheap isolation transformer: will this work?

Hi
Would this work as the basis for an isolation transformer? To go before my Variac. Its for sale on a popular online auction site but i think if i just post the specs then some one might be able to tell me. The bit i am worried about is the "unbalanced DC" but i am guessing that this will be a small number any way

600 - 600 Isolation Transformer - Used to provide isolation between Telstra lines and mains operated equipment..Austel Approved - Impedance: 600 line, 600 Equip. - Turns Ratio:1: 1 - Max Level: 0dBm @ 300Hz - Insertion Loss: 1dB max @ 0dBm300Hz - 5kHz - Freq Response: 300Hz~10kHz 1dB, 0dBm - Return Loss:<15dB, 300Hz-5kHz - Max Unbal. DC:60mA - Proof Test:4Kv RMS - (HWD): 30 x 41 x 33mm - Weight:153 Grams

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Old 10th September 2012, 03:52 AM   #2
DUG is offline DUG  Canada
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Isolation transformer in question is an audio transformer for speech.

"To go before my Variac."

If this is what I think you mean in that you would apply power (120/220VAC) to it as a power isolation transformer then NO, do not hook it up to line level power.

600:600 is in ohms recommended source/load.

0dBm in 600 ohms is about 0.77Vrms

Take care.
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Old 10th September 2012, 03:57 AM   #3
AJT is offline AJT  Philippines
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Quote:
Originally Posted by bullpeters View Post
Hi
Would this work as the basis for an isolation transformer? To go before my Variac. Its for sale on a popular online auction site but i think if i just post the specs then some one might be able to tell me. The bit i am worried about is the "unbalanced DC" but i am guessing that this will be a small number any way

600 - 600 Isolation Transformer - Used to provide isolation between Telstra lines and mains operated equipment..Austel Approved - Impedance: 600 line, 600 Equip. - Turns Ratio:1: 1 - Max Level: 0dBm @ 300Hz - Insertion Loss: 1dB max @ 0dBm300Hz - 5kHz - Freq Response: 300Hz~10kHz 1dB, 0dBm - Return Loss:<15dB, 300Hz-5kHz - Max Unbal. DC:60mA - Proof Test:4Kv RMS - (HWD): 30 x 41 x 33mm - Weight:153 Grams

Thanks

don't even try.....
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Old 10th September 2012, 06:15 AM   #4
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Ok thanks. I am not real smart but not stupid either.
Ok so I want an Isolation transformer. I gather i need a suitably rated transformer with 1:1 turns ratio. Any thoughts were to get one? Preferably for not a lot of money
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Old 10th September 2012, 06:24 AM   #5
AJT is offline AJT  Philippines
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you need an isolation traffo about the same volt-ampere capacity as your variac...or at least as big as your biggest anticipated load....
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Old 10th September 2012, 11:07 AM   #6
DF96 is offline DF96  England
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The weight (153 grams) and size (30x40x33mm) ought to have warned you that this is not a power transformer even if the other technical details went over your head. It is meant for use in modems or telephones, to isolate the telephone line from technical faults and nontechnical users.
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Old 10th September 2012, 11:16 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by bullpeters View Post
600 - 600 Isolation Transformer - Used to provide isolation between Telstra lines and mains operated equipment..Austel Approved - Impedance: 600 line, 600 Equip. - Turns Ratio:1: 1 - Max Level: 0dBm @ 300Hz - Insertion Loss: 1dB max @ 0dBm300Hz - 5kHz - Freq Response: 300Hz~10kHz 1dB, 0dBm - Return Loss:<15dB, 300Hz-5kHz - Max Unbal. DC:60mA - Proof Test:4Kv RMS - (HWD): 30 x 41 x 33mm - Weight:153 Grams
just about every second word tells you this specification is not for MAINS' POWER.
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Old 10th September 2012, 08:34 PM   #8
Enzo is offline Enzo  United States
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1:1 transformers for mains isolation are specifically called isolation transformers. In catalogs they generally have their own page. I don't know what the surplus market is where you are, but in the USA we sometimes find them there cheaper than new.

I have been in the coin-operated amusements industry, which includes arcade video games. And all of them required an isolation transformer for their video monitors. As a result I have a number of iso's in my parts pile from them. You might look at some locel "operators" of arcade equipment and see if they have any old isolation transformers from video arcade games they would give up cheap. Can;t hurt to ask.
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Old 16th September 2012, 08:09 AM   #9
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Two identical AC power transformers wired back-to-back will operate just fine as an isolation transformer in most circumstances. AC mains power into transformer #1 primary, connect transformer #1 secondary to transformer #2 secondary and you'll have isolated power at just slightly less than AC mains power out of transformer #2's primary. There's a slight loss in voltage due to the conversion losses of the two transformers, but since you're also going to be using a variac and many variacs that I've seen will go 10% - 20% or so over the input level you should be able to compensate for the loss if you need to. Find two identical car battary chargers to donate the transformers and you'll get some pretty decent current levels. I like to fuse both the AC input (low amp) and the secondary-to-secondary connection (high amp) just to be super-safe. Here's a link I found using a quick Google search, not my site but it shows a schematic of connecting two transformers together:
Isolation JeeLabs
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