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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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14th Jul 2019, 4:13 pm | #1 |
Triode
Join Date: Jul 2019
Location: Harwich, Essex, UK.
Posts: 39
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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 |
14th Jul 2019, 4:30 pm | #2 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Isolation/isolating transformers
Quote:
Cheers, GJ
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14th Jul 2019, 4:37 pm | #3 |
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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. |
14th Jul 2019, 4:49 pm | #4 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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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. |
14th Jul 2019, 4:50 pm | #5 |
Triode
Join Date: Jul 2019
Location: Harwich, Essex, UK.
Posts: 39
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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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14th Jul 2019, 5:23 pm | #6 | |
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Re: Isolation/isolating transformers
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14th Jul 2019, 5:26 pm | #7 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,510
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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 |
14th Jul 2019, 6:23 pm | #8 | |
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Re: Isolation/isolating transformers
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14th Jul 2019, 6:30 pm | #9 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Isolation/isolating transformers
One thing's for sure, folks won't be blaming wrong advice if they get electrocuted.
Lawrence. |
14th Jul 2019, 7:39 pm | #10 | ||
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,510
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Re: Isolation/isolating transformers
Quote:
Andy |
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14th Jul 2019, 8:26 pm | #11 |
Heptode
Join Date: Apr 2016
Location: Cheltenham, Gloucestershire, UK.
Posts: 872
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Re: Isolation/isolating transformers
Lawrence! Does your Mother know you're out [and about]? !!
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14th Jul 2019, 8:31 pm | #12 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Isolation/isolating transformers
Quote:
Lawrence. |
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14th Jul 2019, 8:33 pm | #13 | ||
Heptode
Join Date: Apr 2016
Location: Cheltenham, Gloucestershire, UK.
Posts: 872
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Re: Isolation/isolating transformers
Quote:
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14th Jul 2019, 8:38 pm | #14 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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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 |
15th Jul 2019, 12:16 pm | #15 |
Nonode
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,495
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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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15th Jul 2019, 1:48 pm | #16 | |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
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Re: Isolation/isolating transformers
Quote:
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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15th Jul 2019, 2:18 pm | #17 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
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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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15th Jul 2019, 2:44 pm | #18 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,549
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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. |
15th Jul 2019, 3:33 pm | #19 |
Nonode
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
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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 |
15th Jul 2019, 4:01 pm | #20 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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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. |