[R-390] Isolation transformer question

Sun Jul 8 20:40:15 EDT 2007

Sure we do. WE would have to use shotguns to get our beef and hot wings
would be enormous.

Tracy
KE4ODD

-----Original Message-----
From: r-390-bounces at mailman.qth.net =
[mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Patrick
Sent: Sunday, July 08, 2007 9:39 PM
To: Gary Gitzen; r-390 at mailman.qth.net
Subject: Re: [R-390] Isolation transformer question

And if cows had wings......... no one knows what could happen.
----- Original Message -----=20
From: "Gary Gitzen" <r390a at uwave.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, July 08, 2007 3:54 PM
Subject: [R-390] Isolation transformer question

> Hi R-390,
>
> Paul: many thanks for your thoughts on the problems using
> isolation transformers (IT). Please consider extending
> your thoughts to the following.
>
>
> Paul suggested a few issues and/or problems with using
> isolation transformers, as well as a possible "solution".
>
> His first comment was that in theory the floating secondary
> could be floating at KV levels above ground due to static
> electricity. A second, unmentioned but much more dangerous
> possibility, is a power supply fault in the powered device
> causing possibly high voltages to appear on the IT secondary.
>
> Paul further suggested using small 110V "globes" (presumably
> light bulbs for us here in the US) from each secondary
> leg to ground. These would glow dimly under normal operation,
> and one would glow brightly if either secondary leg shorted
> to ground. I think this is a Very Bad Idea, and will get to
> that shortly.
>
> First, let's take the case of static electricity on the secondary.
> This is easily dealt with by connecting 1-10 meg 2W resistors
> from each leg to ground. Static problem solved.
>
> Second, Paul's suggestion of a "leg shorted to ground" indicator
> is an excellent idea, but might I suggest instead two neon bulbs,
> each in series with say 220K/2W from each leg to ground?
> Under normal conditions, they would both be dark, and conduct
> zero current. If either turns on, one leg is shorted to ground.
> If they both turn on, some fault in the powered equipment
> is putting high voltage on the secondary.
>
> Paul further claimed that the two "globes" would create
> a virtual center tap on the secondary, with "safe" voltages
> of 55-60VAC on each leg. I must respectfully disagree with
> "safe". My understanding is that under the wrong conditions,
> as little as 10-20V at 10MA or so can prove fatal.
> Paul further claims that any fault condition, such as a shorted
> leg, would cause at least one bulb to glow brightly. That would
> be true for a shorted-to-ground leg, but what happens if
> A: That bulb burns out?
> B: A fault puts high voltage on the secondary, blowing both
>   bulb filaments?
> In either case, both bulbs would be dark, but that might not
> be noticed when the "normal" condition is a dull glow.
> The neon bulbs appear to address both issues. If either/both
> neons come on, a fault exists.
>
> Paul's suggestion of a virtual center tap on the secondary,
> resulting in 55-0-55 is excellent, but there may be a better
> way of getting it, and then adding even more protection for us
> mortal users.
> If you take a small transformer with a 120/240 split or
> center tapped primary, connect each end to one of the IT legs,
> then ground the middle of the winding(s), you have created
> a 60-0-60 voltage source, ideal for powering an R-390X rcvr.
> But we're not done yet. For added protection, add a GFI
> to the secondary. Ground the GFI and we now have a 60-0-60
> voltage source with GFI protection in case the user happens
> to connect him/herself between either leg and ground.
> If you happen to have a pair of 130-150V varistors and connect
> them from each leg to ground, you've just added protection
> in case a power supply fault in the powered equipment ever
> tries to put high voltage on its AC power feed from the IT.
>
> Does anyone else have any opinions, insights, or suggestions
> for further improvements?
>
> Aside: a 60-0-60 AC feed is now popular with the professional
> audio crowd (not to be confused with audiophools) because
> it reduces hum.
>
> Just my $0.02
>
> Gary, San Jose CA
>
> O  /
>  >< ------------------------- cut here ----------------------
> O  \
>
>> From: "Paul Galpin" <galpinp at absamail.co.za>
>> To: <r-390 at mailman.qth.net>
>> Date: Sun, 8 Jul 2007 21:22:54 +0200
>> Subject: [R-390] Isolation transformer question
>>
>> Certain assumptions are being made about the secondary of the I.T. In =

>> fact,
>> there are several unknowns. What is the secondary live-to-earth=20
>> resistance,
>> neutral-to-earth impedance? How do these change when a piece of =
equipment

>> is
>> connected? The secondary is floating and could even be standing up at =

>> kVolts
>> of static (very unlikely, but theoretically possible in a very dry=20
>> climate)
>>
>> IMHO, it would be best to relate the secondary to earth in a known =
way.
>>
>> My suggestion is to connect small 110V globes from each side to =
earth,
>> giving visual indication that the supply is now 55-0-55, very safe if =
you
>> insist on connecting yourself to the power cables!  If either lamp =
goes=20
>> out
>> and the other goes bright, then you've got a short somewhere between =
one=20
>> leg
>> and earth. You might want to put a shorting switch across one leg to=20
>> earth
>> to create this condition, maybe, for certain tests
>>
>> Of course, down here it would be a 110-0-110 supply, but that's still =

>> safer
>> than the full 220.
>>
>> Incidentally, ELT's (GFI to you) also go out on a low impedance
>> neutral-to-earth short.
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