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Author Topic: Isolation transformer  (Read 6744 times)
K1PJR
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« on: August 28, 2009, 11:27:40 AM »

I have been restoring old tube radios and decided I should finally get an isolation tansformer. I picked one up on ebay, Tripp Lite IS250. Wanting to understand what these things do I called tech support. I expalined what I was using it for and the tech person said it only cleans the line of any interference but does not solve the hot chasis issue.

Is he correct or maybe he didn't understand what I meant about a hot chassis. Do I keep this unit or resell it?

Also, what does the transformer actually eliminate? The chassis will still have 120v so how does it eliminate the "shock hazard"? Is there a way to measure this with my DVM?

Thanks,
Phil
K1PJR
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KL0S
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« Reply #1 on: August 28, 2009, 01:50:48 PM »

Phil -- there own spec sheet describes the isolation method being "transformer" and that indicates to me that the unit should function as you want it to.

You may want to consider a unit like the old Heathkit IP-5220 Isolated Variable Power Supply.  They show up on eBay frequently...that's where I got mine.  Then you'd have both isolation and the ability to vary the AC voltage as needed.

Dino KL0S
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AA4PB
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« Reply #2 on: August 28, 2009, 06:00:15 PM »

From the Trip-Lite web site: "Internal low-impedance isolation transformer component with Faraday Shield offers 100% isolation from the input AC line."

There wouldn't be any point in using a Faraday Shield if they were going to tie the primary and secondary neutrals together.

"Secondary neutral to ground bonding eliminates common mode noise, providing an isolated ground reference for sensitive equipment..."

NEC wouldn't permit them to bond the secondary neutral and ground unless it was completely isolated from the building electrical system.

I think its a keeper - whoever you talked to didn't understand his product.
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AA4PB
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« Reply #3 on: August 28, 2009, 06:09:49 PM »

The "shock hazard" occurs if a non-isolated radio is connected such that the hot lead (rather than the neutral) is connected to the chassis. That means that there would be 120VAC between the chassis and the building electrical ground.

When you use an isolation transformer, neither side of the line (on the secondary of the transformer) is referenced to the building electrical ground or neutral. The "shock hazard" is eliminated because you could have one hand on a cold water pipe and the other on either side of the secondary line because neither side is connected to building ground.

Now there still is 120VAC between the chassis and the other power lead inside the radio and you can get shocked if you get across that. But, you don't have to worry about standing on a concrete floor or attaching a grounded piece of test equipment to the chassis.
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W5FYI
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« Reply #4 on: August 28, 2009, 06:26:24 PM »

"Also, what does the transformer actually eliminate? The chassis will still have 120v so how does it eliminate the 'shock hazard'?"  The transformer separates the primary winding from the secondary with non-conducting insulation, so there is no direct electrical contact between the AC grid and the radio. Consider the primary winding as a load that converts electricity into magnetism. That magnetism crosses the insulation to the secondary winding, where it is converted back into electricity. As far as the radio is concerned, the transformer's secondary winding is its power source, not the AC grid.

Without the isolation transformer, it is possible for electricity to flow from the grid's hot wire to the chassis and back to the grid via the neutral wire. If you were to become part of the circuit, say your feet were grounded, and you touched the chassis, part of the current could then flow from the hot grid wire through you to ground--and you would certainly feel it!! However, with the isolation transformer acting as the AC source (without direct connection to the grid) then there is no way for the grid AC to get to the chassis and from there through you to ground. In effect, you're isolated from the AC grid by the transformer's insulation.

"Is there a way to measure this with my DVM?" Certainly. You should check that the primary winding terminals do not show continuity to the secondary winding terminals (ohmmeter check - infinite ohms). And, once the transformer is plugged in and operating, you should use the AC voltmeter to measure voltage between the secondary winding terminals and ground. There should be zero volts if the transformer is doing its job.

Be aware, though, that sometimes primary turns will short to secondary turns when a transformer goes bad. If that were to happen, the transformer would no longer be safe. It's always good practice to run a voltage check from the secondary terminals to ground whenever you use an isolation transformer, and it's a better idea to use a GFI outlet to plug the isolation transformer into in the first place. My dad's advice to me about high voltage electronics (and many other things) was "ABC--Always Be Careful."  GL
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W8JI
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« Reply #5 on: August 28, 2009, 07:21:38 PM »

Phil,

The proper way to do this is to ground the chassis of the radio directly to the mains safety ground if you are sure it is a good connection.

Then you use the isolation transformer.

The Faraday shield won't do anything unless it is run out to a safety ground that is totally independent of the chassis ground for the isolation transformer. The insulation does it all.

This all assumes the secondary of the transformer floats.

Using a transformer alone does NOT mean you won't get a nip off the chassis, or perhaps blow up sensitive test gear. That's because even a small amount of capacitance (that cannot be measured by an ohmmeter) might allow a few mA of current. Or you could have a voltage breakdown issue in the system someplace.

You do understand that if there is a bad component in a radio, and IF that radio has any other connection to the external world without a safety ground to the chassis, the chassis can become hot even though you have an isolation transformer.

Ground the chassis when using an isolation transformer, always. Never ground it without one on an AC-DC radio. :-)

Tom
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K1PJR
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« Reply #6 on: August 28, 2009, 07:26:19 PM »

Claude

Thanks for the expalantion. I understand it now. I ran the test as you mentioned and here is what happened. If I put one lead to the output of the transformer (hot side) and the other to the ground side of the outlet it reads 120v. (?). But if I run a wire from the cold water gorund in my basement to the hot side of the transformer the reading is 29v.

Not 120v but certainly not zero. I understand the removal of the ground potential so why the different readings? Sorry but I'm a CPA and I'm trying my best to understand the technical side of radio.

Thanks
Phil
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AA4PB
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« Reply #7 on: August 29, 2009, 11:08:46 AM »

The 29V reading is probably caused by leakage through the small amount of capacitance between the transformer windings. It is that high because the volt meter doesn't draw much current. If you had any amount of load resistance across the meter then the reading would likely drop very close to zero.

In my opinion the leakage doesn't speak well of the transformer's isolation, especially with a Faraday shield. You might try the 29V reading again with a 10K resistor across the meter.
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KE3WD
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« Reply #8 on: September 08, 2009, 01:24:14 PM »

Here's an old testbench trick:  

You can construct an isolation transformer for this kind of work relatively inexpensively using parts you may already have or may find in someone else's junkbox.  

All you need are two power transformers of the same size.  

Connect them back to back using thier filament windings.  6.3V of one to 6.3VAC of the other, for example. Ignore any high voltage secondaries, just tape them up so there won't be a shock or shorting hazard.  The filament windings will carry the most current anyway.  

Two doorbell transformers can work for this task and will power the smaller radios and stuff.  If they get hot, you are trying to power more than they can handle.  Furnace transformers, 24VAC typically, can also do the trick.  These may be found at hardware stores and the like if you have no source of parts of junkbox.  Old tube gear will also contain power transformers, many times it is likely the only thing still working right in a rusty old chassis.  

Now the first transformer represents the primary and its primary coil is attached to the AC mains as you normally would.  

The second transformer's primary 117VAC coil is now the secondary, that is where you attach the socket where you plug the DUT (Device Under Test) in this casse, your antique radio.  

Isolation is actually better because there are TWO magnetic field changes involved.  

Size of transformer is important for reasons of VA capacity.  Larger the transformers used, the larger the load you can use them on.  Rule of thumb is to get the largest two transformers that match each other that you can find.  

-KE3WD
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W8JI
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« Reply #9 on: September 09, 2009, 04:09:50 AM »

Phil,

The problem is impedance ratios.

A Faraday Shield really does not eliminate or reduce leakage current or capacitance unless it is 100% complete and grounded to a good independent ground. Otherwise it can just as much increase leakage as reduce it.

What you are reading is normal, and why I told you you have to ground the chassis to a good ground if you really want a zero voltage chassis to protect  sensitive test equipment or to guarantee no shock hazard from the chassis.

You are reading stray capacitive coupling or very high leakage resistances. This does NOT mean the isolation device is defective. It just means you have to connect things properly if you want zero leakage.

Did you know it is possible, with no ground on the chassis, for a chassis to have line peak voltage PLUS the highest voltage at any point inside the device on the chassis? The isolation transformer offers a limiting impedance, but it is not perfect infinite impedance isolation. Unless you somehow ground that chassis to a proper earth reference ground voltage will never be zero.

Tom
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KE3WD
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« Reply #10 on: September 11, 2009, 05:40:07 PM »

Do not ground the chassis side of an isolation xformer on the test bench unless you like blowing up test equipments that have their ground referenced to chassis.  Like your oscilloscope.  

A lot of these older radios may or may not have a polarized line plug on them, replacements over time, etc. but even if the plug is polarized, still there and wired correctly (you may be surprised as to how often they aren't wired correctly when grandpa replaced the linecord or plug) there still is that darned "death cap" to consider, one end going to AC side that is supposed to be the neutral and the other end going directly to chassis.  Often, this cap can be shorted, too.  

Grounding the UUT defeats the main reason I have to use the isolation transformer on the bench.  

Poof.  

The second you attach your scope's ground clip to the chassis.

That is WHY we need the isoformer on the testbench.  


--KE3WD
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AA4PB
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« Reply #11 on: September 11, 2009, 07:28:03 PM »

No, we used an isolation transformer to isolate the whole unit from both sides of the AC line (hot and neutral). As long as the transformer is doing its job you can't hurt anything by attaching a ground to the chassis. The scope ground clip lead is attached to the AC power ground on the scope AC plug which is why we need to make sure that the equipment chassis is NOT connected to the AC hot lead (thus the isolation transformer). It won't hurt a thing for the scope ground and the radio chassis to be tied to the same ground.

The problem with these AC/DC radios was that if you inserted the plug the wrong way then the chassis would be tied to the AC hot, putting it at 120V in reference to the building ground and the scope ground.
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KE3WD
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« Reply #12 on: September 13, 2009, 06:31:46 PM »

It is your test bench and your test equipment.  

I won't subject mine to the possibility of up to the full AC wave riding on the bench's ground line, through the chassis' of the test gear.  

Because I've had to troubleshoot scopes and replace logic chips in them from just such a situation.  

I'll leave the UUT floating.  Works for me, going on a few decades now anyway.
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HFRF
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« Reply #13 on: September 17, 2009, 12:21:26 PM »

Man, some people make things complicated.  All you need to do to test a transformer to see if the primary is totally isolated from the secondary is to connect an ohmmeter between each primary wire to each secondary wire.  If it is an isolation transformer, you should see infinite resistance between any primary and secondary wire.

You might call the company who made it or look on their web site  for more info about the  product.
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AA4PB
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« Reply #14 on: September 17, 2009, 04:00:06 PM »

Well, there just isn't any way that grounding the chassis is going to make the whole test bench ground rise to 120VAC. If the isolation transformer is working then the chassis and the test bench will be at the same ground as the test equipment ground. One purpose of using the isolation transformer is to prevent "arcs and sparks" if you connect a the ground lead of a grounded piece of test equipment to the chassis.

Connecting an AC/DC piece of equipment to an isolation transformer is NO DIFFERENT than plugging in (without an isolation transformer) a more modern device that has an internal power transformer or connecting a power supply that contains a transformer. The power transformer IS an isolation transformer.
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