Suggestions for an isolation transformer

[Cliff Matthews]

Yup that's a thermal fuse (commonly known as a TCO or 'thermal cut-off). It will be wired on the mains side, since the 4.5 amp overload breaker seems to be on the secondary side.

[Spork Schivago]

"Wouldn't I need two circuit breakers?   I was thinking one like this:

Make sure if you use breakers they are rated for the proper voltage. The rating on the one you showed is 32VDC for auto/RV use.

Also think about how long an overload will take to trip the breaker. Thermal type breakers like the one shown in the link or on most power strips can take some time with a moderate overload to trip. If you've ever used an electric space heater on a power strip, you may have had it trip after several minutes, then you had to wait a few minutes for it to cool down so it can be reset.

A fast acting fuse or a thermo magnetic trip breaker might be better.

https://thegrid.rexel.com/en-us/knowledge/product-faqs/w/wiki/323/how-circuit-breakers-combine-thermal-and-magnetic-protection

Thanks for that info!   Yes, I was worried about how long it'd take to trip.   I like the idea of a thermo magnetic trip breaker.   There's currently fast acting fuses in there on the primary side.   The ones I linked to I believe are rated for 32VDC or 125VAC, aren't they?   That's the way I read them.

[Spork Schivago]

I can only upload 2MB of pictures at one time, so this will be in multiple posts.

You could reduce the pixel size and file size as a consideration to those of us with slow connections. There is no need to post such huge sizes.

Yeah, I was using some free on-line jpeg compression utility, but it was only cutting the size in half, or a little bit more, enough to allow me to upload them.   Then I downloaded IrfanView and I'm just using that now.   I think it's much better anyways.   I had forgotten that IrfanView could reduce sizes until a member mentioned it.   Otherwise I would have used that from the beginning.   I will try to keep them small, especially for people with slower connections.

[Spork Schivago]

Here's the specs for my Craftsman 82357 DMM, just in case anyone wants them:

Code: [Select]

Basic Accuracy DC voltage 400mV - 1000V ±(0.1% of reading)
AC voltage 4V - 700V ±(1.0% of reading)
DC current 400µA - 20A (fused) ±(1.2% of reading)
AC current 400µA - 20A (fused) ±(1.5% of reading)
Resistance 400½ - 40M½ ±(0.5% of reading)
Capacitance 100nF - 100µF ±(2.0% of reading)
Temperature 0 to 1999°F / -20°C to 1370°C ±(3.0% of reading)
Frequency 500mHz - 10MHz ±(0.1% of reading)
Duty Cycle 0.1 to 99.9%


[Cliff Matthews]

Another as the site says, "rescued from the dustbin of history" if it works, don't fix it..    Site says the meter was made for Sears by Wens Precision Company and it seems their DMMs are all portable data loggers now. Interesting, never heard of them.. but it seems 5-years ago they were sold in AU. 

[SoundTech-LG]

Ok, finally got around to popping the top off of this beast again. The other photo is the Buck wired autoformer to drop my 240 Volt service down to roughly 208 Volts for this 5KVA server room step down isolation Xformer. It has a surge suppressing delay ckt. in it, and 2 fans to keep it cool. Overkill.

[Spork Schivago]

Should I add a fan to keep my isolation transformer cool?

[Cliff Matthews]

If not used 24/7, I'd vote no. It's UL approved, has large aluminum case, full-load is rare, it's ~90% efficient, fans irritate, the list goes on..

[Spork Schivago]

I think we're done with this thread then.   Thanks for all the help!   Gonna save some cash and order those C14 to NEMA 5-20R adapters.   They where a bit hard to find.   I could find the NEMA 5-15Rs all over the place, but the NEMA 5-20Rs are harder to find.   I only found one company who makes them.

https://www.stayonline.com/power-cords/iec-320-c14-plug-adapters-9958.asp

10$ each, and I want 11 of them.   9 for the transformer, 2 for my PDU to plug in my ONT and MoCA adapter.   I have business class internet for my datacenter, however, I feel the hardware my ISP has provided is more residential than business.   I do have the option of having a dedicated fibre line ran directly to my router but that is extremely expensive.   Hopefully, if my business takes off, I'll be able to afford that sometime.   That's not related to this thread through.   Thanks everyone for all the help and suggestions!!!!!   I understand a lot more now than I did when I started.

[Ian.M]

An IEC C14 plug is only rated for 10A (15A intermittent duty in North America), so you only need a couple of those C14 to NEMA 5-20R adapters for bench use so you don't have to change out NEMA 5-20P plugs on equipment under test.  Everywhere else you need an IEC C14 to NEMA 6-??R adapter for  permanant use the equipment  should *NOT* have a  NEMA 5-20P plug (if it has, its probably going to overload the C14 plug), so you can use the cheap and readily available the IEC C14 to NEMA 5-15R adapters.

[GTA]

I'm bringing back this old post to say that I have three available isolation transformers for sale to anyone who may be interested. They're Toroid Corp's ISB-060M, 600VA medical grade iso-transformers and I've listed them in the Buy/Sell/Wanted section: https://www.eevblog.com/forum/buysellwanted/fs-(ca)-isb-060m-600va-medical-grade-isolation-transformer/

[Manny Sparks]

If you want a secondary side GFCI to function you need to bond secondary neutral to ground before the GFCI which means the secondary circuit is no longer floating, which in many applications defeats the purpose of having an isolation transformer in the first place.

What if the ground is a floating ground i.e. a wire that connects the chassis of the appliances together and then to neutral before the GFCI, but is not connected to earth? My country's electrical code specifically requires that the ground not be connected to neutral, but it doesn't say why. I suppose the reason is a path to earth could appear if, say, a washing machine leaks water, bridging the gap between its chassis and the ground.

[Zero999]

If you want a secondary side GFCI to function you need to bond secondary neutral to ground before the GFCI which means the secondary circuit is no longer floating, which in many applications defeats the purpose of having an isolation transformer in the first place.

What if the ground is a floating ground i.e. a wire that connects the chassis of the appliances together and then to neutral before the GFCI, but is not connected to earth? My country's electrical code specifically requires that the ground not be connected to neutral, but it doesn't say why. I suppose the reason is a path to earth could appear if, say, a washing machine leaks water, bridging the gap between its chassis and the ground.

The neutral of an isolation transformer is not the same as the mains neutral. Your isolation transformer is floating, whilst the mains neutral is already connected to earth, normally either at the building entrance, or substation. Your country's electrical code will not be violated by connecting the neutral on the secondary side of an isolation transformer to earth. Indeed, there will be some situations when it's required or recommended, but those won't apply to a bench top transformer for test purposes.

[p.larner]

can i homebrew one of these?,i have two big 220-240v transfos from old farnell psu's,can i not just wire them back to back?,if so what happens wrt to earth/ground?.

[watchmaker]

I just went through this.  Then I asked about it on the TEA subgroup.  The relevant posts are bookmarked below.  The bottom line is that isolation transformers are no longer recommended.  It turns out that adds complication to the setup and the safest procedure is to use a differential probe. 

And since DUT is on your bench because it is defective, what if that defect is related to a shock hazard that a GFCI would detect?

https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg5267169/#msg5267169

I spent hours trying to digest the conflicting info on the internet. I became totally befuddled  because my experience with iso xformers in the shop class during the 60s and 70s predisposed me to believe in them.   Much has been learned in the last 50 years!  THANKS for those on TEA who got me straight.

I found that differential probes are like DMMs, as tech requirements changed, industry had to upgrade and dump perfectly good obsolete probes.  I bought a Yokogawa produced until 2017 for $75.  You do not need to spend $200 for a new but questionable probe!

This is cheaper than the Jameco Iso xformer I returned.

After that discussion, I found this Youtube which provided visual and empirical demonstrations that echoed what was said in those posts.

[madires]

I just went through this.  Then I asked about it on the TEA subgroup.  The relevant posts are bookmarked below.  The bottom line is that isolation transformers are no longer recommended.  It turns out that adds complication to the setup and the safest procedure is to use a differential probe. 

Be aware that in some specific situations an isolation transformer is still very useful, e.g. when repairing old TVs or radios with a hot chassis.

[watchmaker]

I think I learned the differential probe takes care of this.  But what you say is what I did in fact learn 50 years ago.

If taking VI measurements, use a battery DMM.  The issue as I came to understand it has very little to do with personal safety other than having an earthed probe come into contact with the wrong wire and exploding in your hand.  You do always need to follow the one hand in pocket rule.

The diff probe and battery DMM avoid this.  At least this is how I changed my behavior.

Regards,

Dewey

[madires]

Hot chassis means that the chassis is connected to mains. With a polarized mains plug it should be connected to neutral, but with a non-polarized plug there is a 50% chance it's the line/phase. You could check that before plugging in the DUT and turn the plug accordingly. Or you could use an isolation transformer. The problem with a hot chasis is that one can easily touch it by accident, even with one hand in the pocket (-> rubber gloves).

[Ian.M]

That was true in the hot chassis tube radio era as a half wave rectifier tube could supply enough current direct from mains, and the cost of implementing a bridge rectifier was prohibitive but transistorised and later integrated circuit hot chassis TVs typically had a bridge rectifier feeding the DC bus, making the chassis hot with half-wave rectified mains no matter which way round you connected the power plug.

There were also oddities like Salora's Ipsalo II  which omitted the usual switching transformer, and instead had a line output stage with an isolated primary, so the line output stage and line scan coils were 'hot' but most of the rest of the circuit wasn't

Working on such equipment was a specialised trade requiring extensive training, usually formally taught at a trade school or by one-to-one mentoring. The bench setup was quite particular as well, mostly in terms of removing as many grounds as possible from the bench and nearby area, and minimising the risk of contact with those that couldn't safely be removed or insulated (e.g. the CRO).  The problems with learning it from the internet are: finding reliable information in the first place that isn't either dangerously out of date or dangerously wrong; and also you've got nobody watching your back and correcting you if you start to develop dangerous habits.