Isolation Transformers - Page 1
Forum section: electronics / testgear

author: Unknown
author_rank: Unknown
timestamp: April 21, 2014, 01:47:50 pm
content: Hi all ive been looking at getting an isolation transformer and they seem to be difficult to get a hold of over here in the UK unless im not looking properly. So i have found this:http://uk.rs-online.com/web/p/safety-site-transformers/4368805/And was wondering if this has the same princeaples as say a tripp lite. i would be using it to plug in tvs and other smaller electronic devices to test with my oscilloscope.I know the princeaples on why you would need one and understand it doesnt make you 100% safe and you still need a bit of savvy.Also not sure what the difference between one phase and three phase or if its something i dont need to worry aboutAny help would be much appreciated.

author: mariush
author_rank: Super Contributor
timestamp: April 21, 2014, 02:44:48 pm
content: You can find loads of transformers here :http://uk.farnell.com/isolationPick 1 x 220v/230v/240v or 2 x 110v/115v/120v (you can put them in series to get 220-240v) ... there's also some that have multiple windings in primary that in series will get you 220-240v, for example 100v+100v+10v+10v or something like that.Filter the list and see what options you get for secondary windings.. pick something that gives you 220-240v (one winding or several in series/parallel)Then look at the VA rating... I guess a good suggestion would be to get one that has about 25% more than the watts your devices will require.See alsohttp://cpc.farnell.com/they're somewhat of a "retail" version of Farnell, if i remember correctly fewer items, but cheaper shipping to uk addresses, or they allow smaller orders (though an isolation transformer is expensive enough that I don't think it would be a problem ordering it from farnell directly)

author: Unknown
author_rank: Unknown
timestamp: April 21, 2014, 04:06:51 pm
content: Yes I was looking into those and making my own isolation transformer with a 3 pin british socket on it. But obviously would prefer to buy a shop manufacturer one being a beginner. There are loads of schematics online. So will look into it further

author: DmitryL
author_rank: Regular Contributor
timestamp: April 21, 2014, 06:26:52 pm
content: Quote from: NateTronix on April 21, 2014, 04:06:51 pmIf you really want to make your own isolation transformer, you can try using 1 or 2 transformers from microwave ovens. There were some instructions on "instructables.com".On the other hand, microwave ovens transformers are built to a price: they have primary winding made from aluminium, they are designed to work in saturation without a load; I don't think that you can load them to specs (600 or 800 watts) infinitely. You can rewind them, impoving winding, but everything really depends on what you need.

author: nctnico
author_rank: Super Contributor
timestamp: April 22, 2014, 12:44:13 pm
content: Better stay away from isolation transformers. Too much can go wrong. Better get a good GFCI and some differential probes. That is way more safer than an isolation transformer. An isolation transformer is a cheap solution from the old days when GFCIs where not common.

author: ganzuul
author_rank: Contributor
timestamp: May 11, 2014, 07:39:02 pm
content: Quote from: nctnico on April 22, 2014, 12:44:13 pmCould you please expound on the failure modes? I too had the idea of assembling an isolation transformer from old MOTs, simply inspired by the immense fun I had in school with these mix-and-matchable spools and laminated cores that we played with in physics class. Back then, the most dangerous thing a friend I came up with when supplied with 24V was a surprisingly powerful electromagnet.

author: tobor
author_rank: Contributor
timestamp: November 15, 2014, 02:12:33 am
content: I've a question on this topic... I recently re-watched Dave's "How NOT to blow up your oscilloscope" and then was looking at my BK Precision TR-110 Isopack isolation transformer. According to my ohm meter, the schematics, and indeed, the internal wiring... (photos available here:https://www.eevblog.com/forum/buysellwanted/bk-precision-tr-110-isopack-variable-voltage-isolation-transformer/msg76298/#msg76298) The 'isolated' outputs share the common mains and chassis ground. So the only things actually isolated are the hot and neutral lines.My question is, aside from the ability this unit gives me to vary the voltage from 90V to 140V, what good does isolating only the hot and neutral do when powering a DUT?

author: Napalm2002
author_rank: Regular Contributor
timestamp: November 15, 2014, 03:59:47 am
content: I believe that you are getting the isolation transformer and variable confused.

author: coppice
author_rank: Super Contributor
timestamp: November 15, 2014, 04:46:04 am
content: Quote from: nctnico on April 22, 2014, 12:44:13 pmCan you explain how a GFCI is going to provide the galvanic isolation we need for a wide variety of test work, like probing inside the primary side of power supplies?

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2014, 02:15:59 pm
content: Quote from: tobor on November 15, 2014, 02:12:33 amThe way I usually use my isolation transformer is to isolate hot and neutral while connecting the neutral to ground at the point where my probe ground is attached.The problem I usually encounter when working with off-line circuits is that the neutral is elevated from ground because of the return current flowing through it. The voltage difference is not much but the impedance is very low so if the probe ground is connected to neutral, a ground loop is created and if neutral opens for whatever reasons, like because of a line side short, the entire neutral return current including the current from the short will flow through the ground of the test instrument possibly damaging it.In deference to nctnico's recommendation about differential probes, when I use my differential probe on off-line circuits, I usually use my isolation transformer also. This allows the differential probe ground to be connected to neutral which is already grounded improving high frequency performance.

author: m100
author_rank: Regular Contributor
timestamp: November 15, 2014, 07:24:37 pm
content: Cheapest new UK source I've found for isolation transformers is Airlink230v-230v 30VA-5000VA GBP12 - GBP192 +VAT & Delivery500VA works out at GBP60 all inhttp://www.airlinktransformers.com/chassis_mounting_toroidal_transformers/chassis_mounting_toroidal_transformers_standard_range/?filter_input_voltage=230&filter_output_voltage=230&filter_va=&submit=Filter+ResultsNo connection other than as a satisfied customer