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eHam Forums => Elmers => Topic started by: K8AXW on July 18, 2019, 11:56:39 AM



Title: Isolation Transformer Recommendation
Post by: K8AXW 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?


Title: RE: Isolation Transformer Recommendation
Post by: AA4PB 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.


Title: RE: Isolation Transformer Recommendation
Post by: AC2EU 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.


Title: RE: Isolation Transformer Recommendation
Post by: WA7ARK 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...


Title: RE: Isolation Transformer Recommendation
Post by: WA3SKN on July 19, 2019, 03:55:32 PM
What are you "isolating" for?
Power?
Audio?
RF?

-Mike.


Title: RE: Isolation Transformer Recommendation
Post by: W9IQ 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


Title: RE: Isolation Transformer Recommendation
Post by: K6BRN 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


Title: RE: Isolation Transformer Recommendation
Post by: W9IQ on July 20, 2019, 03:10:44 AM
Well, that is annoying - but good to know.

- Glenn W9IQ


Title: RE: Isolation Transformer Recommendation
Post by: KX4QP 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).


Title: RE: Isolation Transformer Recommendation
Post by: WB4SPT 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...


Title: RE: Isolation Transformer Recommendation
Post by: KX4QP 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.


Title: RE: Isolation Transformer Recommendation
Post by: W9IQ 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


Title: RE: Isolation Transformer Recommendation
Post by: VK6HP 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.



Title: RE: Isolation Transformer Recommendation
Post by: VK2TIL on July 22, 2019, 08:47:10 PM
This is an illustration of different transformers, showing how the earth/ground operates;

(https://i.postimg.cc/85PCxzKp/Bild-04.jpg) (https://postimages.org/)

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


.


Title: RE: Isolation Transformer Recommendation
Post by: WB4SPT 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?


Title: RE: Isolation Transformer Recommendation
Post by: AC2EU on July 24, 2019, 04:04:23 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

When do you need to isolate the safety ground?

any time that the device under test has a chassis potential that is not ground. That applies to ac/dc and some military stuff that runs on battery.
When in doubt use isolation else things go poof or bang.


Title: RE: Isolation Transformer Recommendation
Post by: KX4QP on July 24, 2019, 04:16:20 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.

I'm not completely sure I follow this.  The "neutral" on the output side of the 1:1 transformer has the same potential to ground as the "neutral" on the input side, does it not?

Regardless, given I work (in my shack) on a plastic table, sitting on a plastic chair on a carpet (over engineered lumber sub-floor), the only ground danger I have is the case of the rig itself -- and the isolation transformer won't help me there, because that case "ground" is the same as the transformer output "ground" -- or it's ungrounded, which means there's no protection from, say, worn insulation energizing the rig's case (though I still don't have an external ground accessible anywhere within reach of the work space).


Title: RE: Isolation Transformer Recommendation
Post by: AA4PB on July 24, 2019, 05:10:43 PM
With an old AC/DC type radio, the chassis was connected to one side of the power cord. The other side went to the rectifier and the series connected tube filaments. There was no transformer. The plugs had the same sized prongs so it could be inserted into the outlet in either direction. Plug it in one direction and the metal chassis was connected to the hot side of the AC line. Plug it in the other way and the chassis was connected to the neutral side of the AC line. It didn't matter so much for the user because the metal chassis was installed in an insulated case and they had plastic knobs on the control shafts. For the service tech who had the radio out of the case, a hot chassis was not very healthy. It's especially bad if you tied to connect a grounded scope probe to the hot chassis! Shops used an isolation transformer to power these radios so that the chassis was never connected to the hot side of the AC line, regardless of which way the plug was inserted.


Title: RE: Isolation Transformer Recommendation
Post by: VK6HP on July 24, 2019, 05:50:59 PM
I'm not completely sure I follow this.  The "neutral" on the output side of the 1:1 transformer has the same potential to ground as the "neutral" on the input side, does it not?

No, it does not.  Unless you connect the secondary circuit to ground, it is "floating" and you'll be unable to measure any potential difference between either secondary conductor and ground with a VOM.  There are some second-order effects to do with leakage and capacitive coupling but let's not complicate the safety discussion.

In the workshop situation you're describing, perhaps referencing something like your SB-102 with a "proper" double-wound power transformer, the safest thing to do is to always ensure that the chassis is connected to your mains safety ground.  With a transformerless radio, the safest thing is to use an isolation transformer.  Don't ever rely on the incidental insulation you may get from footwear and floor coverings, unless you're working in a properly equipped HV lab with associated protocols.

73, Peter.


Title: RE: Isolation Transformer Recommendation
Post by: KX4QP on July 29, 2019, 04:17:20 PM
How are any of us over the age of 50 alive?

I remember when I first encountered a polarized plug -- because I couldn't plug it in at all, in the unpolarized, ungrounded outlets in the house we lived in around 1972.  We had a couple grounded appliances (refrigerator, as I recall, and a space heater -- in the bathroom) that were plugged in with those orange ground adapters that connected the ground to the screw holding the face plate on the outlet -- except these outlets didn't even have boxes, never mind grounded boxes.

Does "unable to measure any potential difference" mean the neutral is at ground potential on both sides, or the voltage between them is "undefined" like what you get when you divide by zero?  Or does it mean a meter with a needle will read zero, and a digital one will jump all over the place?

Grounding the chassis of the SB-102 doesn't give me a warm, confident feeling when I'm about to try to poke a short meter probe into the chassis to reach a line expected to carry 800 V.   :o


Title: RE: Isolation Transformer Recommendation
Post by: VK6HP on July 29, 2019, 06:07:54 PM

Does "unable to measure any potential difference" mean the neutral is at ground potential on both sides...


It means that if you were dumb enough to touch either one of the isolation transformer secondary conductors while standing on a grounded surface there'd be no current through your body.  (Again: very important not to ground either side of the isolation transformer secondary if you want to use it in this safety role).

The reason I put it in terms of a VOM measurement is that those second order effects I mentioned can be measured with the right equipment, but I'm not going to dilute the essential safety message.

Re the 800 V.  You're a responsible adult who understands the need for caution, are you not?  The 800 V connections are not chasing you around the room; you know where they are and you're going to do it right, right?

Grounding the SB-102 chassis means that if you do accidentally get between active mains conductors and the chassis, your house ground fault devices (such as the residual current detectors we are obliged to have in Australia) activate.  I can't answer for the vagaries of earlier North American mains distribution but it's always a good idea to verify what you're actually working with at a given property, wiring code notwithstanding.



Title: RE: Isolation Transformer Recommendation
Post by: AA4PB on July 29, 2019, 06:15:25 PM
Grounding the chassis of the SB-102 doesn't give me a warm, confident feeling when I'm about to try to poke a short meter probe into the chassis to reach a line expected to carry 800 V.   :o

The other side of that 800V supply is connected to the chassis so if you get between the 800V line and chassis you are going to feel it whether the chassis is grounded to the mains supply ground or not. At least if the chassis is grounded then you know that you won't get a shock if you touch the chassis while some other part of your body is touching some other grounded object.


Title: RE: Isolation Transformer Recommendation
Post by: KX4QP on July 30, 2019, 04:25:03 PM
Grounding the SB-102 chassis means that if you do accidentally get between active mains conductors and the chassis, your house ground fault devices (such as the residual current detectors we are obliged to have in Australia) activate.

If you live in a house that wasn't built recently, GFCI outlets or breakers are where you put them yourself.  The house I'm in is a mobile home approximately twenty years old, and there's precisely one working GFCI device in the house -- in the outlet above the sink in the bathroom we don't use (just worked out that way -- and the breaker box is chaotic enough that swapping the protection device or installing more is an unavoidable exercise in restarting and resetting stuff, not to mention costly).  The grounds are good, but as OSHA says, "a ground won't protect you."  In the case of grounding the chassis of a rig like my SB-102, a ground may actually cause an accident, because the space is tight enough that it's almost impossible to make a measurement inside without touching the chassis -- and if you also touch a hot conductor, you will get a shock.  None the less, the HP-23A has a grounded cord (and early one, no doubt), which grounds the radio chassis through the power cable.

I've got a pretty good idea how to protect myself -- I repair power tools for a living, in a shop that has GFCI devices only because I personally insisted on them after a "near miss" where an exposed brush wire in a tool under test electrified the entire row of 4 steel work benches.  It's almost impossible to get the boss to actually replace these devices when they fail, too ("just get me the part number and I'll order what you need" -- except he never does; one bench has a failed device that's been out of service for more than a year), so I've gotten quite "enlightened paranoid" about mains voltage.


Title: RE: Isolation Transformer Recommendation
Post by: VK6HP on July 30, 2019, 06:11:36 PM
I'm glad to hear the HK and its grounding arrangements might yet save you from yourself.  

I don't know of anything that will save you from poor technique in terms of measuring the HV within the radio but you really don't have to be contacting the chassis while you make a measurement - if you find yourself doing that, you're doing it wrong, no matter how small the space. For example, if you get some insulated clip adapters for your test leads you can solidly attach the common lead to a chassis point and an HV point prior to switch on. Usually I just use the chassis clip but spending a few minutes finding orientations and positions that work is always worthwhile.

Having just taken a pretty big hit financially extending a switchboard and associated breaker/RCD system I'm familiar with some of the practicalities.  If it's not possible to upgrade the switchboard right now, why not grab a portable RCD for the radio bench?  If you want to progress you clearly need to find ways forward instead of arguing how the status quo blocks you at every turn.

In that vein, this prattling on the internet is not getting my 630 m reflectometer finished so, with the isolation transformer questions answered, I don't propose to add more.

73, Peter.





Title: RE: Isolation Transformer Recommendation
Post by: KX4QP on July 31, 2019, 04:26:45 PM
This is the first I've heard of a GFCI having any chance of protecting me from the HV -- which is DC with filter capacitors the size of a beer can, near enough.


Title: RE: Isolation Transformer Recommendation
Post by: VK6HP on July 31, 2019, 05:15:13 PM
As it's a safety matter, I again note that protecting yourself from the radio HV and other supply rails is your responsibility, per my previous post. In the hands of a responsible individual, and done properly, testing and repairing classic radios is an entirely tractable challenge.

The mains ground fault device protects you in the event of contact with the mains. In the scheme of electrocution statistics that's a worthwhile contribution to safety and, if the shack power circuit doesn't already have an RCD or similar, the suggestion is to consider adding one to the test bench.