Are there any adverse effects from isolating the AC power supply on my electronics bench?

Beast is right. Look at the coils for the transformer! And it is bolted down so that it will not fly away under load!?!

Yes, Clem, Hehe, that HUGE BEAST of a toroidal transformer is what sold me too!  I mean, how can one not love such a massive chunk of electromagnetic joy?

Come on people!  I'm looking for HELPFUL input on how to ISOLATE power to the bench, but do it SAFELY.  Don't let me do something STUPID!  Ok, well, please don't let me do something STUPID.

Lets just make this even MORE interesting!    What if I decided that I wanted to isolate my ENTIRE bench INCLUDING my test equipment!  What then?

What PROTECTIONS should be in place to prevent DAMAGE to myself and my babies (test equipment, soldering station, power supplies, frequency generator, etc.)?

Any issues to be worried about with SUDDEN load changes on the secondary?

4ringfan

I'm guessing you don't want to listen to your father's advise  ... I'm sure he has been able to offer some good advise.

Isolating transformers are really only designed to work with double insulated appliances.

On a class 1 appliance any fault current is passed through the earth and trips the breaker.

If you isolate your bench, then you remove that potential difference and hence the protection.

Since this PowerVar maintains the Class 1 by connecting the earth to the sockets, how is the transformer wired?

.. does it actually float the secondary or is it just providing a 120v/120v transformer and the neutral is joined both sides.?

Also inside the device has a black box which contains ??

The other issue I see if you isolate your bench is the phase voltage (and neutral) can float to anything, rather than it never exceeding the 120/240v.

Switchmode type supplies have high harmonics, and its quite likely that the impedance of this device will not remove/smooth that component, which could then pass to other loads.

It may also cause the secondary voltage to rise either as an AC or something closer to DC.

Bench Isolation

If you are trying to ensure you don't get a shock, then I recommend looking at RCD's. (You call them GFCI)

A Residual Current Device detects an inbalance between the phase and neutral currents and trips which interrupts the power.

Residual-current device - Wikipedia, the free encyclopedia

They are available as portable, socket outlet, or switchboard mounted, and the medical/children version will trip at 10mA instead of the normal 30mA.

They are mandatory on all NZ homes now. (except for some special circuits).

Mark

Mark,

My 17 year old son wants to buy a motorcycle.  I've tried to convince him that it is a bad idea.  Death, dismemberment, high insurance cost, in ability to use it year round, weather, road hazards like animal carcasses, etc.  Nothing seems to be deterring him from his quest to do something potentially stupid.  I keep hoping that someone else other than me may convince him that it is a bad idea because God forbid he listen to his old man.  I think we all have a little rebellious teenager in us.  I'm no different.  My father is very intelligent and has a few more years of experience on me, so I should generally bow to his knowledge and wisdom.  Sometimes, however there is more than one way to skin a cat.  That is why I'm coming to the educated engineers on E-14 for alternative insight as to what should or should not be done.  He can't always be right... OR CAN HE?  Eeek!

I will be modifying the PowerVar.  All primary filters, protection, etc will be left in place.  The secondary will be stripped down to basically the transformer windings.  I do not know what is in the black box as I do not have the PowerVar in my hands yet, but I would guess that it is a noise filter of some sort or other.  I don't want either of the secondary wires to tie to ground in any way.  I want to be able to stand in a puddle of water and touch either lead from the secondary.  My dad, John, mentioned possible capacitive coupling to ground just because of the proximity of the wires to the ground.  I will need to do some dielectric testing to see if that is a problem.  I suspect that I will also carry a good earth ground through to all of the equipment for shielding.

You mentioned that the bench voltage could float to anything, possibly exceeding the 120VAC.  If the transformer is a 1:1 ratio, should the secondary floating voltage not track the incoming primary voltage fairly closely?  Where do you see this increase coming from?  Is the increase not an increase in total voltage, just that the output voltage can float higher than actual ground due to capacitive coupling within the transformer?  http://electronics.stackexchange.com/questions/150540/connecting-center-tapped-transformer-to-earth-ground-or-why-am-i-b…

The collapsing magnetic field may also cause a voltage increase when the unit is turned off, but I believe there may be a way to mitigate that problem, but I'm waiting to see if anyone can tell me what that is before I divulge the idea.

As far as the GFCI goes, could I just put that on the secondary?  It would be non-invasive and just detect any increase in current on one leg over the other.  I wasn't thinking about having the monitored ground resistor as part of the circuit, however.  Something similar to this:

Hi Mark,

I appreciate your insights on this question. There are so many side effects to each of the choices that we make that it is always good to get input from the experts. I was not aware of any potential problems with switch mode power supplies when used on a isolated supply. These are good things to know.

John

4ringfan

Mike

Let him buy a motorcycle .. he will anyway.

While we don't have the same weather problems, I brought one just after I got my licence.

In those days you weren't restricted to 250cc so I brought a 400cc 4 cyl, which I still own.

On the other hand most 100cc pocket rockets have more power and speed now.

Maybe you could help him by having lessons in advanced riding, or a few racing lessons, and buy him some good leathers in case he does meet the pavement.

I'd also recommend a larger bike as they tend to have to be riden more sensibly and can't be thrown about as much.

Floating

Because the secondary is floating, you can make it rise to anything you want ... try it with a 12v transformer, and add a supply to ground, you'll see that both sides of the secondary rise to the supply voltage.

If you used a high value 'ground resistor' on the 'neutral' side it would stop it floating, but would not stop it being pulled by a lower impedance source ie an un isolated voltage source.

Using a RCD on the secondary will work and if you really want 5mA just use that rather than 2 of them.

It will trip if the 'phase' current is higher than the 'neutral' current by 5mA. (or the neutral is higher than the phase)

The only way for that to occur is if there is a leak to ground ..... ie you, or a piece of wire.

Collapsing

The collapsing magnetic field is something that generally occurs with DC or large inductive loads.

The ac field collapses 50/60 times a second, so its no different when you switch it off..... it just doesn't come back on, but it might stop part way through the cycle.

The ultimate would be to have the switch off controlled to only occur at the zero point, but this tends to add a potential problem around safety.

The only other thought is to remove the power and dump a resistor across the secondary (2x 150w lamps would work).

You have been thinking about it, so the next thing is to add some loads (scope/other test objects) to see where you might have an issue.

I'd also plan on an easy way to kill the secondary power in an emergency.

I use one of these when hooking up unwired appliances

http://cpc.farnell.com/cliff-electronic-components/cl1853/quicktest-block-with-1-5m-lead/dp/PL10638

Mark

Yes  these quicktest devices have been around for years they are very good and I use one. They save a lot of bother putting plugs on to stuff for test and therefore discourages you from taking on dangerous shortcuts like bodging wires into sockets and that sort of stupidity !

Hi Mike, why do you want to do this? Mains powered equipment needs an earth for safety and emc filters. Portable equipment Safety Transformers generally have a centre taped "isolated" secondary (i.e. not an auto-transformer) but the centre tap is earthed...Peter

Hi Peter, Mike asked me to respond as I am helping him set up the bench and the isolation transformer is my idea. Learning is part of any activity and I always keep my mind open and try not to let my own ideas become hardened. Both Mike and I use our benches for servicing electro-mechanical equipment, specifically dental equipment, as well as experimenting with electronics. This includes x-ray controls, timers, motor and actuators control circuits and electronic interfaces with the real world. I originally installed an isolation transformer on a series of outlets in my own shop so that I could run a couple of Variac power supplies without the danger of having direct connection to the mains. I also from time to time like to be able to connect my scope probe ground at a higher potential point in a circuit. In addition to the isolated bench outlets I also have standard outlets that can be used if there is a need. I am not an engineer but rather a self educated technician and I decided it would be a good idea for Mike and me to throw our isolation transformer idea out on the forum so we could learn from members like mcb1 and yourself who are engineers. Mark has already given us some good insights and options that will cause us to modify our original plan. Any additional insights you can add will be appreciated.

John

If this isolated bench were in a corporate lab you would have to do a risk analysis and you would be restricted to who had access to the bench. It is dangerous to run live equipment with no earth and it would be treated as using live equipment. You would be restricted to using safety connections and told to attach connection with the power off unless there was some other protection in place. Earth is your protection and electrocution is painful or/and deadly. Equipment with emc filters will charge up the chassis if it is not earthed and will be ineffective.

When you say "run a couple of Variac power supplies without the danger of having direct connection to the mains", what is less dangerous, you should ask yourself? It sounds more dangerous to me as you may be lulling yourself into a false sense of security.

Now when you say "I also from time to time like to be able to connect my scope probe ground at a higher potential point in a circuit" you hit on a real issue. The solution is best to use a isolated/differential input scope. These are available from Tektronix eg. Digital Signal / Digital Phosphor Oscilloscopes (DSO/DPO) | Farnell element14 ...these are the scopes you would find in a corporate lab...you can buy second-hand.

...or at lower bandwidth but more affordable from pico scope eg. PICOSCOPE 3425 - PICO TECHNOLOGY - PC OSCILLOSCOPE, DIFFERENTIAL | Farnell element14 ...this is the scope I use at home  as I need isolated channels.

Floating an ordinary scope off earth is not safe, the isolated scopes come with safety leads.

Also you say "In addition to the isolated bench outlets I also have standard outlets that can be used if there is a need." If you mix earthed and non-earthed equipment, you may get paths to earth back through test equipment earth, and potential for smoke! I've seen that happen.

What you are suggesting is a step back in time to the 1980s or early 90s when the only way was to use these risky non-earthed methods of working. And re-read what mcb1 has said...@mcb1