Isolated AutoTransformer - Page 1
Forum section: electronics / testgear

author: tvl
author_rank: Newbie
timestamp: February 01, 2019, 09:22:56 pm
content: Remove

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 01, 2019, 09:32:50 pm
content: You are correct. No transformer can be both an AUTO-transformer AND an ISOLATION transformer.The two types are mutually exclusive.When I look at the product page forhttp://www.bkprecision.com/products/power-supplies/1653A-150v-2a-ac-power-supply.htmlI see that it claims to be an isolation transformer.But I don't see anywhere where it claims to be an auto-transformer?

author: wraper
author_rank: Unknown
timestamp: February 01, 2019, 09:38:09 pm
content: Isolated autotransformer is impossible. Because autotransformer's definition is transformer with single winding.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 01, 2019, 09:50:01 pm
content: Click the manual. Looks like it's just two transformers back to back.

author: rsjsouza
author_rank: Super Contributor
timestamp: February 01, 2019, 10:04:14 pm
content: The picture tells me this is an isolated variable transformer (Variac). The "variable" part is an autotransformer, which is followed by a proper isolation transformer.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 01, 2019, 10:43:51 pm
content: The output should still be sinusoidal. Obviously since the output is adjustable, the amplitude could be different. No transformer is perfect, so you might see a tiny bit of distortion or roll-off (the magnitude of which should be described in the documentation.)Since it seems like this device is designed for use with devices on utility power, it will likely be optimized for 50-60 Hz operation.I'm not sure what you mean by clean up, but since transformers pass AC, any anomalous waveforms will pass through, unless the device has some filtering (check docs).

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 02, 2019, 01:02:50 am
content: Alas, it is not unusual to find that a vendor has garbled or obfuscated the description, specs, or even photo or name of something. If there is any doubt go directly to the manufacturer's information, preferably on their corporate website.Remember that copywriters (people who write "copy") are not necessarily experts or even have the slightest understanding of what they are writing about. They may not know an "Autotransformer" from an Aardvark.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 02, 2019, 01:08:32 am
content: What are you trying to say Richard Crowley?We have already determined that the device is an auto-transformer with an isolation stage.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 02, 2019, 01:28:32 am
content: Quote from: timelessbeing on February 02, 2019, 01:08:32 amI am saying that calling something an "Isolated Autotransformer" is confusing at best.Everyone here agrees that there is no such animal as an "Isolated Autotransformer".The word "Autotransformer" appears nowhere in any of the manufacturer's official documentation.So I maintain that Test Equipment Depot did nobody any favors by spuriously adding the word "autotransformer".Using the word has no positive benefit and only confuses everyone who reads it.IMHO, this is a perfect example of a silly/stupid vendor flub.If they wanted to highlight the fact that this is a variable gadget then they should have used the word "variable".The word "Autotransformer" does NOT imply that it is variable.I would imagine that somebody along the line may have called it a "Variac" which is a protected brand-name.So somebody who doesn't know any better thought that "autotransformer" means "variac". (Which it does not).Not all Variacs are autotransformers. And not all variable AC power transformers are autotransformers.And certainly not all autotransformers are Variacs (or variable, either.)

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 02, 2019, 01:29:56 am
content: I think it depends on the characteristics of the so called distortion and noise."Conditioning" implies deliberate suppression of unwanted elements, so no, a naked transformer doesn't condition. Does thisdevicecondition the power? I don't know but the link you provided has the manual.A transformer has inductance, though, so yes it should attenuate higher frequencies to some degree.Quote from: tvl on February 02, 2019, 01:08:42 amdefine faithful. For what purpose? How can we answer this question without knowing what you are trying to achieve?

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 02, 2019, 01:42:24 am
content: Quote from: Richard Crowley on February 02, 2019, 01:28:32 amOutrageous, I know. You should write a letter to them.Do you often purchase technical equipment based solely on their name? So you would add more words to the name to me itlessconfusing? Or what would you call it?It seems to me that it is in fact an auto-transformer, and it has been isolated.Quote from: Richard Crowley on February 02, 2019, 01:28:32 amAnd yet ... here it is.Rarely can you pack each of a device's features into the name. The very first feature listed in the description is "variable"I don't know what copywriters have to do with it. It was probably typed in by some minimum wage data entry clerk.Quote from: Richard Crowley on February 02, 2019, 01:28:32 amMy search of the manual revealed 13 instances of "auto-transformer"

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 02, 2019, 02:28:18 am
content: Quote from: tvl on February 02, 2019, 02:13:10 amThat is the intended design of this unit, and BK Precision is a reputable company so no worries there.Quote from: tvl on February 02, 2019, 02:13:10 amIf you power the inverter from a battery, then the output will be isolated and therefore safe to probe with your scope.Quote from: tvl on February 02, 2019, 02:13:10 amJust remove the ground clip since it's not needed, and probe away.Quote from: tvl on February 02, 2019, 02:13:10 amAgain, define faithful. 1% distortion? 10%? If you look close enough then of course you'll find some. Please go read the manual as it will certainly be specified there.But utility power (at least in my home) is pretty far from a pure sine wave to begin with, and high power appliances will distort the power in your home more than this power supply will.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 02, 2019, 02:48:32 am
content: Yes

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 02, 2019, 03:21:21 am
content: Quote from: Richard Crowley on February 02, 2019, 01:02:50 amIs there a difference? If you can fit the same plug?

author: nctnico
author_rank: Super Contributor
timestamp: February 02, 2019, 09:33:56 am
content: Quote from: tvl on February 02, 2019, 03:19:08 amWhat are you going to use it for? If it is for measuring mains powered devices then buy a differential probe. Much safer. You really shouldn't use an isolation transformer nowadays for measuring on mains. For starters: You likely have a GFI which gets defeated when you use an isolation transformer. And there are many scenarios where using an isolation transformer is unsafe.

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2019, 10:17:00 am
content: Quote from: timelessbeing on February 01, 2019, 09:50:01 pmI see no reason or advantage to doing this. What would be the point when you can use a single isolating transformer? Could we be missing something?Autotransformers save copper and will be smaller and cheaper than a transformer with isolated windings but, as has been said, they do not provide galvanic isolation.When I was a kid power line voltage stability was bad and many devices, like TVs, needed more stable voltage so a small autotransformer was set. As the voltage variation required was small a very small autotransformer with a selector switch could handle much larger power than if it had two isolated windings.For testing mains-powered devices I have a 500 VA transformer I got cheaply in China. Primary has 400 and 230 volts and secondary has several taps all the way to 230 Volts. By connecting 230 volts to the 400V primary I can get 130 volts on the secondary. I usually have it with a 100 W incandescent bulb in series with the primary which protects against shorts and mistakes. (Just yesterday I went to measure voltage on the secondary but forgot I had the tester leads in the 10A current position and so I shorted the secondary. That lightbulb saved a blown fuse and a blown multimeter. The lightbulb lit up and let me know I needed to pay attention.)

author: wraper
author_rank: Unknown
timestamp: February 02, 2019, 01:01:00 pm
content: Quote from: soldar on February 02, 2019, 10:17:00 amProbably it's cheaper to add separate isolation transformer instead of using variac with isolated primary winding. Also variac winding must be flat on wiper side. Probably it's difficult to do if there is primary winding sitting underneath.

author: Specmaster
author_rank: Super Contributor
timestamp: February 02, 2019, 02:34:21 pm
content: This is in effect as has already been described, an variable auto-transformer to provide the variable voltage aspect of the power supply, which is then fed into a 1:1 ratio isolating transformer for your safety when using the power supply in conjunction with equipment that may either have a live (hot) chassis or a grounded chassis. The voltage control only operates on the isolated socket outlet, into which you should plug the equipment that your working on.The other voltages are fixed but you can vary the current.

author: soldar
author_rank: Super Contributor
timestamp: February 02, 2019, 03:46:23 pm
content: Quote from: wraper on February 02, 2019, 01:01:00 pmOh, OK, I see. Yeah, I've never really had or used a Variac. I've always just used transformers with taps.

author: BravoV
author_rank: Super Contributor
timestamp: February 02, 2019, 03:55:56 pm
content: Quote from: soldar on February 02, 2019, 03:46:23 pmHere the photo to give you an idea why the winding must be flat, the exposed copper winding and the carbon wiper that contacts that area, its like transformer with fine adjustable tap. Photo of my decades old Japanese made variac.

author: Specmaster
author_rank: Super Contributor
timestamp: February 02, 2019, 04:03:06 pm
content: I doubt that you can get a isolated variac for partly the reason already mentioned about the carbon brushes but also the lack of space to put a secondary winding.

author: madires
author_rank: Super Contributor
timestamp: February 02, 2019, 05:36:40 pm
content: I'm wondering why BK placed the auto transformer first and then the isolation transformer. Most designs I've seen so far have the transformers the other way around.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 02, 2019, 08:37:47 pm
content: Quote from: madires on February 02, 2019, 05:36:40 pmI've seen units where the variac provides both a full range output and a LV output, which is available as AC or DC with switchable filtering. Maybe it is based on such a unit, but downsized.

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2019, 02:36:23 am
content: Autotransformers by definition only have a single winding however variable transformers with a single core, two windings, and primary to secondary isolation do exist and used to be very common. Below is a photograph of the internals of a Lionel train transformer which has 4 low voltage outputs. The high voltage primary can be seen at the bottom of the O-shaped core where it is wrapped in insulation.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 03, 2019, 03:33:42 am
content: Interesting transformer.Yet I think that the power supplies of which I thought (EA, can't recall, but one other for sure) are build the other way round. They have a large central variac, from which various are made:1.) 0...(250...300)V without isolation (typically receptacle, maybe binding posts in addition)2.) 0...(250...300)V with isolation (typically PE-less receptacle, maybe binding posts in addition)3.) 0...30V either both AC and DC separately, or with a selector, with another switch adding the smoothing(typically binding posts)In addition, various fixed voltages or even a additional regulated lab. PS are build in.In the unit which I had open, the size relations between the transformers clearly pointed to that.

author: edavid
author_rank: Super Contributor
timestamp: February 03, 2019, 05:17:57 am
content: I can't see that these B+K units had high enough volumes to justify making a custom transformer, unlike a Lionel train set.BTW I'm very happy with my 1655 and use it all the time. Definitely worth the $30 I spent on it

author: soldar
author_rank: Super Contributor
timestamp: February 03, 2019, 08:20:36 am
content: Quote from: nctnico on February 02, 2019, 09:33:56 amI disagree with this.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 03, 2019, 08:37:40 am
content: Indeed. They both have their uses.

author: soldar
author_rank: Super Contributor
timestamp: February 03, 2019, 10:20:28 am
content: If I am working on the PSU of any mains-powered device you can bet your scope that I am working through an isolation transformer. You always should.Medical equipment, operating rooms, etc.also use isolation transformers. It goes to show doctors also have a heart and it is not totally true they do not care if their patients live or die.

author: nctnico
author_rank: Super Contributor
timestamp: February 03, 2019, 03:48:30 pm
content: Quote from: soldar on February 03, 2019, 10:20:28 amDefinitely no and all the application notes of every major oscilloscope manufacturer agree with this.There are many ways your device under test can become grounded again through other equipment or even the bench you are working on. The isolation transformer's only function is then to defeat safety by grounding and the GFI. And thus make your work environment completely unsafe.An isolation transformer is only somewhat safe in an environment which has been setup for using an isolation transformer and used by people who had proper training.Nowadays a differential probe is the way to go. They aren't very expensive to buy either.Your analogy with medical equipment is totally wrong. Medical equipment uses isolation transformers to get a clean supply without HF leakage currents. Look at the thick ground straps on medical equipment used at a hospital. If the isolation transformer was used to get a floating supply, then the grounding wouldn't be necessary. Even if medical equipment is using a floating supply, then is must be fitted with a warning system indicating the equipment isn't floating.Using an isolation transformer for measuring mains powered equipment is extremely dangerous and should not be promoted as a safe working practise because the oposite is true.

author: wraper
author_rank: Unknown
timestamp: February 03, 2019, 03:52:19 pm
content: Quote from: soldar on February 03, 2019, 10:20:28 am

author: madires
author_rank: Super Contributor
timestamp: February 03, 2019, 04:36:14 pm
content: Don't we have enough threads discussing pros and cons of isolation transformers?

author: ArthurDent
author_rank: Super Contributor
timestamp: February 03, 2019, 05:47:02 pm
content: Here is a photo of the insides of my older 120VAC input B&K 1653. They have made this supply for some time and newer ones may look a little more modern than mine but they work the same.The power comes in through a fuse holder and on/off switch to the variac (powerstat) adjustable autotransformer. The zero to max output of the line referenced autotransformer goes to an isolation transformer and the isolated secondary is connected to a 3-prong outlet. The two power pins are floating with respect to ground but the 3rd prong is connected to case and input power ground. There is an analog meter with a momentary switch that displays output voltage or when pushed displays output current derived from a 1 ohm power resistor in series with the output.You still have to be careful and know what you're doing when using this type of supply and have a good reason for an isolated output. Never float a scope or any other piece of test equipment. If you feel you have to do that there is probably a flaw in how you are approaching the problem.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 03, 2019, 10:49:26 pm
content: Sounds about right to me.Do some isolation transformers actually carry the ground though? To me that seems to defeat the purpose.

author: nctnico
author_rank: Super Contributor
timestamp: February 03, 2019, 10:55:24 pm
content: Quote from: timelessbeing on February 03, 2019, 10:49:26 pmMany do. The purpose is to get rid of leakage currents. It also requires the equipment to be properly earthed. Medical equipment which sits behind an isolation transformer has thick grounding straps.GFIs are mandatory nowadays because they safe lives (now someone will chime in saying that GFIs may fail -sigh-) but an isolation transformer defeats a GFI. Once you connect one of the hot leads from an isolation transformer to ground (through the ground of an oscilloscope probe for example) the circuit you are working on is grounded again and since the GFI is defeated you'll have no protection against receiving a deadly electric shock.

author: rsjsouza
author_rank: Super Contributor
timestamp: February 04, 2019, 03:06:41 pm
content: Quote from: nctnico on February 03, 2019, 10:55:24 pmExactly. My mom would have surely wanted one when I, as an infant, almost died of electric shock when my tongue twirled all the way back to my throat and I couldn't breathe (her account, I don't remember).My girls certainly were much safer than I ever was when surrounded by electricity.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 04, 2019, 08:52:48 pm
content: How did you get shocked?

author: rsjsouza
author_rank: Super Contributor
timestamp: February 04, 2019, 09:23:08 pm
content: Quote from: timelessbeing on February 04, 2019, 08:52:48 pmI really don't know, and my mom only heard me gasp for air (she was outside of the room). My guess is that I stuck something metallic to the phase wire and licked it.(I've been into other life/death scenarios similar to that as well, but that one is the only electricity related).

author: Electro Detective
author_rank: Super Contributor
timestamp: February 05, 2019, 10:35:50 am
content: I've ploughed through all the comments and can only state and or parrot what a few have saidthat an 'Isolated Auto Transformer' can be either a variac plus an isolation transformer (in whatever series order you prefer, before the DUT)or a fixed isolated 1:1, or 1:Pick A Number,or stepdown or stepup 240v to 120v transformer (as an example) would/should be classed as an 'isolated auto transformer',because that's what it is, a fixed lazy persons 'auto' voltage converter, or 1:1,and not a 'manual variable auto transformer'Either way, and whatever you use, or how many pages this post tallies upif there is ANY earth/ground continuity to neutral or Line 2 on a bench setup that should not have it,you can expect breaker trips, sparks, hospital stays, seeing eye dog choices and training, funerals, séancesand no payouts from insurance companies..If you're real lucky to escape some of those extreme scenarios, you may expect a bit of credit card mayhem

author: coppercone2
author_rank: Super Contributor
timestamp: February 05, 2019, 05:55:09 pm
content: I made one out of a isolation transformer and a torroidal variac. I put the isolation transfor.er before the torrodial autoformer with the idea that it would reduce inrush current being a ei core rather then a ring core.Also the order was chosen for human interface safety reasons.