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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 14th Jul 2019, 4:13 pm   #1
jhockridge
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Default Isolation/isolating transformers

Good afternoon. Could anyone help me source a 400-500w isolating transformer ? I think it would be a really sensible idea to plug my projects into when aligning/troubleshooting. I was also wondering if a variac is a good plan as they used to use them a lot in the lab I worked in.

Thanks for helping me out. James
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Old 14th Jul 2019, 4:30 pm   #2
GrimJosef
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Default Re: Isolation/isolating transformers

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Originally Posted by jhockridge View Post
... I think it would be a really sensible idea to plug my projects into when aligning/troubleshooting ...
Are you certain ? We shouldn't let them lull us into a false sense of security.

Cheers,

GJ
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Old 14th Jul 2019, 4:37 pm   #3
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Default Re: Isolation/isolating transformers

No point penny pinching, buy a new one if you think it is necessary (RS, Farnell etc. do them for less then 200). Isolation transformers are not for safety (they can reduce it because you think you are safer) but for convenience, such as 'scoping a "live" chassis set or debugging large earth leakage problems. To the variac, handy but can still supply currents large enough to damage, the classic lamp limiter is better for vintage stuff (as long as we can still get classic lamps).

A bit OT, I have a "fan heater limiter" at work for the odd huge PSU with "it blows the mains" faults. Got a bit of bother from the safety people until I asked "how is this unsafe", no (sensible) reply.
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Old 14th Jul 2019, 4:49 pm   #4
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Default Re: Isolation/isolating transformers

Someone like Farnell has a good range:

https://uk.farnell.com/c/transformer...r-rating=100va

I've got one of these: https://uk.farnell.com/block/uste100...0va/dp/1177119

which has the advantage that the secondaries can be wired in series or parallel depending on whether you want 115 or 230V.
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Old 14th Jul 2019, 4:50 pm   #5
jhockridge
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Default Re: Isolation/isolating transformers

Interesting stuff chaps. I like the look of lamp limiters as well - they are certainly cheaper. Lots of reproduction 'vintage' filament bulbs around at the moment as well.
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Old 14th Jul 2019, 5:23 pm   #6
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Default Re: Isolation/isolating transformers

Quote:
Lots of reproduction 'vintage' filament bulbs around at the moment as well
Forgot about those, as they run at a low temperature should last for ages.
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Old 14th Jul 2019, 5:26 pm   #7
M0FYA Andy
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Default Re: Isolation/isolating transformers

Whist accepting that an isolation transformer could reduce safety because you think you are safer, that is a matter of 'human factors' not physics. From the physics viewpoint, surely safety is improved because the mains input to your equipment is now no longer referenced to earth, so you have to contact two places rather than one and earth, which you may already be in contact with.

Also remember that a Variac is normally an autotransformer and doesn't provide isolation.

Andy
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Old 14th Jul 2019, 6:23 pm   #8
merlinmaxwell
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Default Re: Isolation/isolating transformers

Quote:
so you have to contact two places rather than one and earth
Still two places.
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Old 14th Jul 2019, 6:30 pm   #9
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Default Re: Isolation/isolating transformers

One thing's for sure, folks won't be blaming wrong advice if they get electrocuted.

Lawrence.
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Old 14th Jul 2019, 7:39 pm   #10
M0FYA Andy
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
so you have to contact two places rather than one and earth
Still two places.
True, but I meant two places internal to the equipment as opposed to one of them being through your feet or the hand you are resting on an earthed piece of equipment which may be the unit under test or something else on the bench.

Andy
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Old 14th Jul 2019, 8:26 pm   #11
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by ms660 View Post
One thing's for sure, folks won't be blaming wrong advice if they get electrocuted.

Lawrence.
Lawrence! Does your Mother know you're out [and about]? !!
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Old 14th Jul 2019, 8:31 pm   #12
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Default Re: Isolation/isolating transformers

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Originally Posted by Tractionist View Post
Quote:
Originally Posted by ms660 View Post
One thing's for sure, folks won't be blaming wrong advice if they get electrocuted.

Lawrence.
Lawrence! Does your Mother know your out [and about]? !!
Mum sadly died some years ago, not sure what your post is referring to.

Lawrence.
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Old 14th Jul 2019, 8:33 pm   #13
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by Tractionist View Post
Quote:
Originally Posted by ms660 View Post
One thing's for sure, folks won't be blaming wrong advice if they get electrocuted.

Lawrence.
Lawrence! Does your Mother know your out [and about]? !!
Mum sadly died some years ago, not sure what your post is referring to.

Lawrence.
Sorry for your loss - I was actually referring to your arid wit.
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Old 14th Jul 2019, 8:38 pm   #14
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Default Re: Isolation/isolating transformers

Yeah, look before you leap.....same goes for live chassis.

No worries.

Lawrence.

Last edited by ms660; 14th Jul 2019 at 8:47 pm. Reason: addition
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Old 15th Jul 2019, 12:16 pm   #15
SteveCG
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Default Re: Isolation/isolating transformers

Would it be fair to say that a low wattage isolating transformer is a more appropriate thing to use with low wattage loads if safety is the prime consideration?
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Old 15th Jul 2019, 1:48 pm   #16
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Default Re: Isolation/isolating transformers

Quote:
Originally Posted by jhockridge View Post
Interesting stuff chaps. I like the look of lamp limiters as well - they are certainly cheaper. Lots of reproduction 'vintage' filament bulbs around at the moment as well.
Be very wary of these for use in lamp limiters.
Some types use long thin LED arrays laid out so as to somewhat mimic an old style filament bulb. These are NOT suitable for use as lamp limiters.

Other vintage style lamps use a tungsten filament, these ARE suitable for use as lamp limiters.

I would stick to ordinary GLS lamps to avoid any risk of getting the wrong sort. The clear amber fireglow or light blue daylight lamps are suitable.
60 watt is the most common size but others exist.
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Old 15th Jul 2019, 2:18 pm   #17
Diabolical Artificer
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Default Re: Isolation/isolating transformers

I'll admit to being a bit confused about isolating tfmr's, does it stop you getting a belt if you touch live or not? I just got my DMM, set to AC volts, connected live to the red lead and stuck the black test lead tip into the concrete lintel of my workshop and read 60v AC RMS this is with my isolating tfmr/variac set at 200v. Is the meter reading wrong? Sometimes the meter will give a low reading when not directly connected to an AC source.

If an isolating tfmr doesn't protect against mains shock then what's the point of using one? I'm also not 100% clear whether it is safe to connect a scopes probe black ground lead anywhere but on the ground/chassis when the DUT is powered from an iso tfmr. If the DUT is sitting on a wooden workbench with no insulating mat then it's earthed to some extent yes? Why are scopes X IP circuit ground connected to earth anyway, why not left floating?

Some clarification for us neophytes would be handy. I've read several threads on the subject here ont forum non of which clarify the situation much. Some sort of list of rules for numptys would help.

Andy.
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Old 15th Jul 2019, 2:44 pm   #18
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Default Re: Isolation/isolating transformers

An isolating transformer allows the DUT to float so that one point in the DUT can be earthed for example the negative of a mains rectifier.
With the DUT connected and nothing else you will get live mains through any capacitance between the windings in the isolating transformer. This will show on a DMM.
If you connect the chassis to a scope earth and then touch a high voltage point in the DUT it will bight you as you have effectively created a local earth in the DUT.
Isolating transformers became a must have item in the days of live chassis TVs and normally accompanied the scope in TV workshops.
Your amplifiers all have isolating transformers and earthed chassis in them.
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Old 15th Jul 2019, 3:33 pm   #19
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Default Re: Isolation/isolating transformers

As we are all aware playing with electricity profesionally or for pleasure can be hazardous and even fatal. For me the 'safe' approach is to objectively understand the risks and to act in mitigation of those risks. Unfortunately, risk mitigation is more of a subjective activity which is why the topic always seems to cause much debate with sometimes conflicting views.

With regard to isolation transformers I find the following article helpful from a hobbyist's perspective. Others may or may not find the same.

https://sound-au.com/articles/iso-xfmr.htm

Alan
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Old 15th Jul 2019, 4:01 pm   #20
ms660
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Default Re: Isolation/isolating transformers

"A service tech who routinely uses one will assume that it's perfectly safe to handle any part of the circuit"

Oh really.....Not met many service techs then.

Lawrence.
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