Putting the variac first, and then the transformer does make a lot of sense. And indeed, some isolation transformers do have that electrostatic shield between primary and secondary.
BUT still, carrying the green-wire ground through to an item under test with isolated mains connection does not increase safety.

 

OK thx. I was all for cutting the ground/neutral tie to the mains but how about supplying a dedicated earth ground to the secondary. That is where it gets a bit fuzzy for me. Having a grounded chassis is a good thing for the DUT.

 

What other choices are there?
Have no idea what that is.
Always a good idea.
Also handy.

https://www.amazon.com/s?k=miniture+residual-current&ref=nb_sb_noss

 

after reading everyone's thought's. Should I wire the trans step down or step up, variac before or after ISO. Little confusing, all roads lead to rome. feed the variac 115 or 215?

If I use 220v, I was going to pull a 12/2 w/g 20amp 220v over to the work area. So many different option, just like to make a decent power box. if I do 220v into varica to step down iso, or 120v varica to a step up iso trans? Or back to orginal thinking, iso first, could go step up or down. trans only puts out 2.1amp as a stepdown, if I go step up does power factor double or no?


Programable fuse, never heard of one.. interesting.

 

Iv'e seen circuit for main e-fusing, but I think the industry is providing DC IC's. See https://www.st.com/en/power-management/e-fuses.html/

A 3A isolation transformer doesn't give you squat. It may not be useful.

Like I said, I never gave 120 and 240 capability a thought, but for both variac before isolation.

Your transformers would say have a 0-120 V primary and a 120-0-120 (240 CT) secondary.

What will you use an isolation transformer for? Do you need 240? Do you need 240 with a Variac?

I had to work on 15 kV regulated power supplies at about 1.5A. Input was 208 3 phase 60 or 90 A. I can't isolated that
That's where differential probing comes in, but I could not use that either.

Most of the work was done unpowered. The HV section required a key to access. A 1 Meg 200 W resistor was really odd in this device. So was a BIG transformer with 3 primaries.

It had the functionality of a giant TV with a 30 kW electron beam.

 

OK thx. I was all for cutting the ground/neutral tie to the mains but how about supplying a dedicated earth ground to the secondary.

When you analyze the problem, that's exactly what you have to do.

For power conditioning, I purchased an isolation xformer and a ISOBAR surge suppressor. One system saw a 17 year life before decommissioned. No instrumentation failures. In 17 years the SCSI hard drive was still working. Juat dust and floppy drive failures. Another totally different system did suffer power supply failures and fans.

 

For general bench use, what amp rating would you recommend for an isolation xfmr? I can see using it for aligning a 100W xcvr and smaller amp rated equipment.

 

My 3A system takes up about 8" x 10" w x 4" h, maybe less. The transformer is something like this https://www.automationdirect.com/ad...z-240-z-208-z-120_vac_to_240x120_vac/c1fc50we and sits out of the way to maximize bench space.

I also have 10A or so of isolation only.

Check surplus options. Maximize bench space. Do include an ON indicator.

 

OK thx. I was all for cutting the ground/neutral tie to the mains but how about supplying a dedicated earth ground to the secondary. That is where it gets a bit fuzzy for me. Having a grounded chassis is a good thing for the DUT.

If the power feed to the DUT is entirely isolated from the mains and from ground, then just what is the mechanism of any benefit from grounding the body of whatever is being tested? The only possible shock capability at that point is between the two power feeds to the device, since neither of those lines has any potential relative to ground.

 

just what is the mechanism of any benefit from grounding the body of whatever is being tested

Cutting down on noise perhaps. That is why I asked. Does it have any real benefit otherwise?

 

After watching Mr Carlson I seen he has his variac feeding the transformer. I thought he had it the other way around.

what happens to current when you step up 120-220. Transformer rated 215v - 115v 2.1amp out, when you feed the primaries with 120v and get 240v out, current get cut in half?

I was thinking 2 amps might be more then enough for what messing around I will currently do, probably 1. If I need a larger one can build that down the road. Don't plan on using 220v, but if its there might as well make use of it.

 

See https://www.se.com/us/en/faqs/FA123947/ and www.powervar.com especially their youtube videos.

 

After watching Mr Carlson I seen he has his variac feeding the transformer.

So did Sencore in their PR570.

Ideal transformer: VpIp=VsIs where p = primary and s=secondary.

naturally, there are some losses. The primary and secondary can be switched, but the losses are higher.

Having 240 in the shop is a good idea.

In some scenereos, it makes sense to have an equipment "kill switch". Real good for high school shops and kids.
It's also good to have the panel for the "shop" in the shop.

Lights. Make sure they don't go out at the same time or different times. e.g. Sawing and you loose lighting or soldering any you lose lights. Been there done that for the last one.

==

My bench is 3' x 8' and covered with a replaceable masonite top. The surface is a Neoprene mat.



Aside: I found "cord issues" to be a problem. In our machine shop at work we had one cord reel cord for hand power tools.

At home my floresent light above the bench is naked with an aluminum foil reflector.

I built my bench as a teen. Basically a 3 x 8; Sit down with a stool or stand up. Covered with a replaceable sheet of masonite.
Large sheet of neoprene for the work area.

Lighting is naked florescent tubes and aluminum foil reflector in the rafters. There is an incadesent light on a string to get into the shop. There are times you want reduced lighting.

There is a short shelf for part of the bench only about 4" high, This contained my Variac and an HP200C audio oscillator.
There is a wiremold outlet strip behind it. There's a 10A 12 V supply (selectable between 0-32 V) for doing car radio work. Bought for $15.00 ABOUT 2' UP. There was also a Blaupunct car radio that got stolen by a utility meter reader.

The bench was initially designed with 4 duplex outlets cut into the 2x4 face with a dual gang 4-switch for each one. I had planned on a circuit breaker for the whole bench. This was before stuff that had to be plugged in all of the time. e.g. VCR

So, turn off 4-switches and bench was dead.

I added a thermostat and a space heater under the bench.

Cords were an issue:
The front mounted cords got caught in my feet. The back of the bench was for the most part OK. I'd like to add a hanging cord from the ceiling for the Variac outlet. At work we had a cord reel for hand power tools in the machine shop and that worked out well. Before I had a soldering station, the cords got melted.

In some shop applications it's nice to have a kill switch and have all of the shop breakers in the shop.

I think it's best to have a self-contained de-soldering gun. The station I have has hot air which is available all the time, but for solder/desolder you have to plug them in separately to the same port.

 

So did Sencore in their PR570.

Ideal transformer: VpIp=VsIs where p = primary and s=secondary.

naturally, there are some losses. The primary and secondary can be switched, but the losses are higher.

Having 240 in the shop is a good idea.

In some scenereos, it makes sense to have an equipment "kill switch". Real good for high school shops and kids.
It's also good to have the panel for the "shop" in the shop.

Lights. Make sure they don't go out at the same time or different times. e.g. Sawing and you loose lighting or soldering any you lose lights. Been there done that for the last one.

==

My bench is 3' x 8' and covered with a replaceable masonite top. The surface is a Neoprene mat.



Aside: I found "cord issues" to be a problem. In our machine shop at work we had one cord reel cord for hand power tools.

At home my floresent light above the bench is naked with an aluminum foil reflector.

I built my bench as a teen. Basically a 3 x 8; Sit down with a stool or stand up. Covered with a replaceable sheet of masonite.
Large sheet of neoprene for the work area.

Lighting is naked florescent tubes and aluminum foil reflector in the rafters. There is an incadesent light on a string to get into the shop. There are times you want reduced lighting.

There is a short shelf for part of the bench only about 4" high, This contained my Variac and an HP200C audio oscillator.
There is a wiremold outlet strip behind it. There's a 10A 12 V supply (selectable between 0-32 V) for doing car radio work. Bought for $15.00 ABOUT 2' UP. There was also a Blaupunct car radio that got stolen by a utility meter reader.

The bench was initially designed with 4 duplex outlets cut into the 2x4 face with a dual gang 4-switch for each one. I had planned on a circuit breaker for the whole bench. This was before stuff that had to be plugged in all of the time. e.g. VCR

So, turn off 4-switches and bench was dead.

I added a thermostat and a space heater under the bench.

Cords were an issue:
The front mounted cords got caught in my feet. The back of the bench was for the most part OK. I'd like to add a hanging cord from the ceiling for the Variac outlet. At work we had a cord reel for hand power tools in the machine shop and that worked out well. Before I had a soldering station, the cords got melted.

In some shop applications it's nice to have a kill switch and have all of the shop breakers in the shop.

I think it's best to have a self-contained de-soldering gun. The station I have has hot air which is available all the time, but for solder/desolder you have to plug them in separately to the same port.

funny you are talking about cords. I noticed I am tangled in cords, feet, bench and hanging. Power and test cords all over, little mess, spend a lot of time screwing with them. Right now I have to share this space with other actives so its more complicated. Been thinking about ground loops on the bench, etc. and all the cords, lack of outlets.. Time to add a few power strips that are dedicated to test eq, maybe other source for stuff. Looks like my iso/var will have its own source, which is good.

Still tossing plans around for iso/var. I will probably go with original 220v feed to step down 115v. I will get a 5amp variac, most likely, and wire it like several have done including mr Carlson. Test only on a little bit of current and is it looks like a go, switch to line voltage to variac. amp.vlt meter, chicken switch.. and will need to fues everything. I wonder what the duty cycle is of the transformer? if its a stated load, I'm thinking 100%? Earth post/connect on variac box for grounding?

I like the light bulb load idea, cool to look at too, but worried about them getting broke. Thinking about other similar ideas and even a remount light rail that could be plug and unplugged. Right now thinking portable box, but if it stationary bulbs would not matter if they hung off it.

 

funny you are talking about cords. I noticed I am tangled in cords, feet, bench and hanging. Power and test cords all over, little mess, spend a lot of time screwing with them. Right now I have to share this space with other actives so its more complicated. Been thinking about ground loops on the bench, etc. and all the cords, lack of outlets.. Time to add a few power strips that are dedicated to test eq, maybe other source for stuff. Looks like my iso/var will have its own source, which is good.

Still tossing plans around for iso/var. I will probably go with original 220v feed to step down 115v. I will get a 5amp variac, most likely, and wire it like several have done including mr Carlson. Test only on a little bit of current and is it looks like a go, switch to line voltage to variac. amp.vlt meter, chicken switch.. and will need to fues everything. I wonder what the duty cycle is of the transformer? if its a stated load, I'm thinking 100%? Earth post/connect on variac box for grounding?

I like the light bulb load idea, cool to look at too, but worried about them getting broke. Thinking about other similar ideas and even a remount light rail that could be plug and unplugged. Right now thinking portable box, but if it stationary bulbs would not matter if they hung off it.

I would place some of these on the ends of the bench. Those or similar as the Pomona stuff comes at a cost.They come in assorted sizes with different gaps.

Next, I would go to Lowes, Home Depot or any home improvement store and invest in some outlets and matter of fact if your bench is not on a GFCI breaker I would get some GFIC outlets and then piggy back some good quality outlets. I would get several good quality double gang outlet boxes and metallic outlet covers. I would also get a few sections of EMT and all the needed EMT fittings and couplings. Then nicely mount all of this hardware. Life will be good.

Ron

 

Earth post/connect on variac box for grounding?

You do realize that if isolation is in effect, the earth ground post/conductor will be ineffectual for safety purposes if it is not referenced to a power conductor.
Max.

 

You do realize that if isolation is in effect, the earth ground post/conductor will be ineffectual for safety purposes if it is not referenced to a power conductor.
Max.

someone brought that up in a earlier post and just tossed that out there. I can always have a earth grd by my bench, that might be a better choice.

More talking and think, better the results

 

someone brought that up in a earlier post and just tossed that out there. I can always have a earth grd by my bench, that might be a better choice.

But from the safety aspect, will it be ineffectual for any power-isolated outlets.
Max.

 

I found this: http://www.stagecue.com/variac.html which makes things pretty compact. The light bulb brought out only when needed and those incandescent lightbulbs are likely scarce.

There is no binding post output which I did find useful. The bulb doesn't have to be there all the time.

 

Leakage current: There has generally been circuits describes in old service manuals for a leakage test.

This
https://iaeimagazine.org/magazine/columns/ul-question-corner/ul-question-corner-leakage-current/

Says that you have to do a ground lift to do the main measurement and also with reverse polarity.