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Author Topic: Isolation Transformer Recommendation  (Read 1406 times)
K8AXW
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« on: July 18, 2019, 11:56:39 AM »

Greetings All:

I'm embarrassed to have to admit that after 63 years in electronics I have never owned an isolation transformer.

After reading numeros horror stories about the necessity of owning such a device and my fairly new purchase of a scope I am now scrambling to determine the SIZE!  I can guess "the bigger the better"  or "what do you intend to power."

So to get this started....let's assume the  biggest thing I would use it for would be my transceiver. It seems to me that a place to start would be the current draw of my transceiver, both in RX and TX modes. 

So my question can be boiled down to "what do you guys and gals use or recommend?
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AA4PB
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« Reply #1 on: July 18, 2019, 12:20:19 PM »

Doesn't your transceiver run from 12VDC? If so, your power supply should provide isolation from the hot side of the AC power wiring. Isolation transformers are needed for old AC/DC radios where it is possible for the metal chassis to be "hot" (connected to the hot side of the AC power line, depending on which way the plug is inserted). Variacs (variable isolation transformers) are used if you need to slowly bring up the AC voltage to an old radio that may have defective capacitors in it.

The thing to remember with scopes is that the probe negative lead is connected to the scope case which is connected to the grounding pin on the AC power plug. You don't want to connect the probe ground clip to anything that has any voltage on it in reference to the AC power ground. On the transceiver (or other equipment with a power transformer in it) you can safely connect the probe grounding clip to the chassis of the device.
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Bob  AA4PB
Garrisonville, VA
AC2EU
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« Reply #2 on: July 18, 2019, 02:29:07 PM »

I have an extra 1.5KVA commercial isolation transformer by ACME.
It will power anything you need - trust me - I never had anything too power hungry for it.
The shipping would be a deal killer though. It weighs about 45 lbs..

The only thing you really need it for is hot chassis testing. Not too much of that in the modern stuff unless you want to hook up a PFC circuit to a scope for diagnostics. The SMPS supplies isolae the chassis like the old 60Hz transformers did only they are smaller and run in  the kilohertz range.
If you mess with antique radios, you DEFINITELY need one.
For small radios a 100 watt unit should be OK.
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WA7ARK
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« Reply #3 on: July 18, 2019, 05:48:30 PM »

You definately need an isolation transformer if you intend to debug ac-line powered Switch-Mode power supplies. You also need to keep one hand in your pocket...
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Mike, WA7ARK
WA3SKN
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« Reply #4 on: July 19, 2019, 03:55:32 PM »

What are you "isolating" for?
Power?
Audio?
RF?

-Mike.
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W9IQ
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« Reply #5 on: July 19, 2019, 04:08:07 PM »

A 200 to 300 watt isolation transformer will serve most of your needs.

It would be rare to need to isolate a transmitter for troubleshooting but a 300 watt transformer would handle intermittent transmissions of a 100 watt hot chassis transmitter.

A much more common practice is to put the scope and other test equipment on the isolation transformer to ensure the test equipment grounds are isolated. Unless you are using an old tube based, mainframe scope, 200 watts would handle that.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K6BRN
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« Reply #6 on: July 19, 2019, 10:03:19 PM »

Something to be aware of....

Many new "isolation" transformers bridge the safety ground from input to output due to regulations and liability issues.  You have to cut this link yourself to provide true isolation of the ground, as well as the hot and neutral lines.

Brian - K6BRN
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W9IQ
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« Reply #7 on: July 20, 2019, 03:10:44 AM »

Well, that is annoying - but good to know.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KX4QP
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« Reply #8 on: July 20, 2019, 05:40:33 AM »

FWIW, if you don't mind having to use your own meter to really see what your output is (the built-in meter is, um, cheap, and reads only in AC Volts), Amazon has 20A variacs for under $50, free shipping from a US fulfillment center with Prime.  Two grounded outlets on the front, and you can dial the voltage continuously from 10V to 130V.  I thought I couldn't afford a Variac until I found these.  The 5A version is cheaper, but not enough so to bother with (I think I'd have saved about $5).  I've currently got my HP-23A plugged into mine, once I can run both concurrently (short on antennas at present) I'll be putting my Hallicrafters S120 in the other variac outlet (finally get the isolation people keep telling me that radio needs).
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WB4SPT
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« Reply #9 on: July 22, 2019, 06:28:08 AM »

FWIW, if you don't mind having to use your own meter to really see what your output is (the built-in meter is, um, cheap, and reads only in AC Volts), Amazon has 20A variacs for under $50, free shipping from a US fulfillment center with Prime.  Two grounded outlets on the front, and you can dial the voltage continuously from 10V to 130V.  I thought I couldn't afford a Variac until I found these.  The 5A version is cheaper, but not enough so to bother with (I think I'd have saved about $5).  I've currently got my HP-23A plugged into mine, once I can run both concurrently (short on antennas at present) I'll be putting my Hallicrafters S120 in the other variac outlet (finally get the isolation people keep telling me that radio needs).

zero isolation with a variac...
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KX4QP
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« Reply #10 on: July 22, 2019, 03:19:38 PM »

zero isolation with a variac...

I wondered about that -- seems to me I recall them being an auto-transformer with variable tap, which can't very well be isolated.  Then again, I'm still a little fuzzy on how, say, transforming 120 V to 12 V and then back to 120 V improves safety in any way.  You still have 120 V 60 Hz, (assuming you're in the USA), and 30 mA across your chest will put you into v-fib (and kill you if there's no one around to administer CPR and/or defibrillate).  You have the same current potential (assuming the 12 V segment has heavy enough wire and the transformers can handle the high current in the low voltage wiring).  I just don't get it.
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W9IQ
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« Reply #11 on: July 22, 2019, 03:49:52 PM »

An isolation transformer doesn't typically go down to 12 VAC and then back up to 120 VAC. Instead it has only two windings, each with the same number of turns. Placing 120 VAC on one winding induces 120 VAC on the other winding. Because there are no connections shared between the windings, isolation is achieved.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
VK6HP
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« Reply #12 on: July 22, 2019, 05:44:55 PM »

It's true that if you get yourself across the secondary conductors of the isolation transformer you're likely to be in bad shape, give or take activation of circuit breakers etc. on that circuit.  But the idea is that you are protected from the most common fault situation in which you, yourself, are between an active conductor and ground.  Since the transformer secondary circuit is isolated from ground, your body does not complete the circuit as it would if you touched an active conductor on the primary (mains) side. There are various other reasons for using an isolation transformer but that's the basis of the safety motivation.

With that in mind, you can see the importance of what Brian and Glenn are saying about grounding on the secondary side: such grounding defeats the safety role.  In an RF environment particularly, you have to watch that the ground circuit is not inadvertently completed by links such as, for example, coax shields. Avoiding such connections, and checking the secondary circuit for isolation, is made easier if the good industry practice of connecting only one piece of equipment to a given isolation transformer is followed.

73, Peter.

« Last Edit: July 22, 2019, 05:47:08 PM by VK6HP » Logged
VK2TIL
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« Reply #13 on: July 22, 2019, 08:47:10 PM »

This is an illustration of different transformers, showing how the earth/ground operates;



I'm fortunate in having one of the best, a Grundig RT-5; it is both a variable autotransformer (Variac) and an isolation transformer.


.
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WB4SPT
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Posts: 777




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« Reply #14 on: July 24, 2019, 09:40:19 AM »

Something to be aware of....

Many new "isolation" transformers bridge the safety ground from input to output due to regulations and liability issues.  You have to cut this link yourself to provide true isolation of the ground, as well as the hot and neutral lines.

Brian - K6BRN

When do you need to isolate the safety ground?
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