Isolated Variable Transformers? Or Separate Iso and Variac? Plus a Newbie Tech Question

[ThomOfYorke]

What do people recommend for the bench? One top contender I've gathered for a combined iso-variac is the Sencore PR57 (one for $400 and one for $600 on eBay, though the $600 one is modified, with the front plug disconnected and a plug added through the back). Others I've found are the Global Specialties 1504 ($400 on eBay) and the B&K Precision 1655/1653 ($200-300 on eBay).

Do people recommend any of these, or would it be best to get a separate ISO and Variac?

I also had a newbie tech question. Is the PR57 fully or truly isolated? I was watching a video on the PR57 and a commenter noted that the schematics for the PR57 show that the earth-ground for the PR57's power plug is shown to chassis ground (first pic) and the earth-ground for the PR57's isolated output is also shown to chassis ground (second pic). The commenter says: "They are effectively connected. On your household power socket the earth-ground and AC-neutral (long slot) are tied together at the breaker box. Basically what ever you plug into the ISOLATED OUTPUT of the PR57 will share the same chassi ground (potential) as anything else that's plugged into your household socket. The PR57 will not protect you or your scope from a Hot chassis or high side of a SMPS."

Another commenter followed up on what this person said: "[Above commenter] is correct: The issue is that the ground is not isolated (check for continuity between the input ground plug and the isolated output socket and you will see). Most American household electrical panels have the ground and neutral bonded, therefore the Sencore's "isolated" output ground pin is effectively connected to the input neutral as well. The simple way to solve this is to float the ground - do so at your own risk...."




So this leaves me a bit confused as to how this unit is an isolation transformer, if the above is correct. It also leaves me unsure as to what I should get for an ISO. Basically I want an ISO that I can use to help prevent me from accidentally blowing up my scope.

Thanks everyone for your help.

 

[George S.]

I use a B&K TR-110 isolation transformer as seen on the rear of the bench.
I'm a hobbyist with no post grade school electronics training so I got it to play it safe with the scope.
I don't work on anything old enough to have a hot chassis. Having it gives me confidence to experiment and learn.
I HAVE blown things up by accident occasionally, but am getting better.
Additionally get a Variac and build a DBT(dim bulb tester).

 

[mr_rye89]

I have a ONEAC 500 va iso transformer I picked up for hot chassis radios/TVs, a cheapo variac I bought for my WOPL builds, and the dim bulb tester. Good to have all three with all the questionable crap I bring home.....

 

[wattsabundant]

I don't think an isolation transformer is necessary for bench testing. Any consumer electronic gadget has either an outboard power supply that is isolated or an internal transformer. The worst thing that can be done is to start lifting power line grounds.

Any decent 10 amp Variac will suffice. 15 or 20 amps is nice but necessary. I use a rack mounted chassis with variac, series light bulb, meters for volts and amps and switching between them all built in.

 

[nakdoc]

I recommend an isolated variac. You never know when someone has miswired an AC plug. The isolation protects your 'scope. If you want, you can isolate the scope and not the variac. I have a used Sencore PR57for sale for $250. nakdoc77@gmail.com

 

[WOPL Sniffer]

I swear by the Good ol Variac. Plus they look cool when resto'd

 

[ThomOfYorke]

@nakdoc: Oh shoot, I just saw this. Is your PR57 still for sale? I'd be interested. I just bought a Tripp-Lite IS1000HG and was going to get a Staco variac but that's only because I couldn't find many PR57s for sale on eBay, and the ones that are for sale are suspect: one is 'modded' to have a plug going through the back of it, but the hole the guy made in it looks like jagged metal and the plug doesn't look safe (and he's asking $600), and the others either have broken meters or have no photos of the unit powered on.

 

[mlucitt]

My understanding is that an isolation transformer magnetically couples the AC voltage to an "isolated" output jack. The ground is connected to the chassis on the input and the output to shield the transformer and still provide a path to Ground in the event of a device failure. Typically, there should be no current flowing through the Ground line.
In the breaker box, the Ground and Neutral are connected to provide a path for device current only in a device failure. Normally, the current returns on the Neutral Line as the complete circuit to the electric utility.
Did I get that right?

 

[J!m]

That’s right.

And shared ground throughout you house also share noise. Isolated ground is a direct line to ground from that outlet only. That way you don’t pick up the SMPS noise from your phone charger on your phono preamp…