The benefits and order of Isolation Transformers

[christarak]

I am wondering if you recommend using an Isolating Transformer? If so, what is the best order in which to link my supply voltage equipment. I have a Variac and a (home made) light-bulb current limiter (which I can bypass to an unlimited current through an ammeter-see attached). When I suspect a serious short I plug the Variac into the 240v mains and then into my current limiter into which I plug the amp. If I were to get an Isolation Transformer, where in that power supply chain would it go? (I'm thinking the last link in the chain into which I plug the amp?)
Is there any negative trade-off in using an isolation transformer to either my safety, my test equipment, or the customer's amp?

[Greg_L]

I am wondering if you recommend using an Isolating Transformer? If so, what is the best order in which to link my supply voltage equipment. I have a Variac and a (home made) light-bulb current limiter (which I can bypass to an unlimited current through an ammeter-see attached). When I suspect a serious short I plug the Variac into the 240v mains and then into my current limiter into which I plug the amp. If I were to get an Isolation Transformer, where in that power supply chain would it go? (I'm thinking the last link in the chain into which I plug the amp?)
Is there any negative trade-off in using an isolation transformer to either my safety, my test equipment, or the customer's amp?

[ATTACH=CONFIG]49562[/ATTACH]

In my non-expert opinion there is no downside to using an iso transformer when working on an amp. Uncle Doug just did a very nice series on iso transformers on youtube. Check it out.

I think you'd want it like you said - last in the chain. Wall > Variac > light bulb limiter > iso transformer > amp.

Check out Uncle Doug's videos and see what you can get out of them.

[Helmholtz]

Typically the amp's own power transformer is an isolation transformer.

[Enzo]

There is no problem using an iso for everything, but the only things I work on that require it are the SMPS. AS Helm says, the power transformer in most things provides isolation.

[J M Fahey]

True.

That said, more and more stuff comes with an SMPS so it´s a good idea to have one.

Not that many in the Guitar Amp World, specially in Classic Tube ones or old SS stuff but on newer ones? .... just think of all those brick sized 500W Bass amps everybody is using now, powered PA cabinets, powered mixers, or even those Quilter thingies, etc.

If you ever go beyond repairing Tube amps or old Peavey and Crate ones, you will need one.

I don´t have one yet, so I´ll probably wind a custom one, adding selectable primary taps, for over a year now my street block is suffering from low voltage , 200V and even 189V or 185V brownouts instead of expected 220V because they added 250 shops to the 60 or 70 we already had (I am in a very Touristic area) and when all turn Air Conditioning on the buried corner transformer simply can´t cope with that.

So a rotary selected 220/210/200/190/180V primary will be very useful.
And isolation will be the cherry on the cake

[Sowhat]

In general I don't use an isolation transformer for most amp work but as Enzo says: except for SMPS work. I have two mains feeds to my bench coming from a 1800watt power conditioner to stabilize the mains a little. From there it splits to a 5 amp, limiter/variac source and to a 2.5 amp isolation/variac source. That plus a few variable DC supplies is about it for me.

Now... I've been using my isolation transformer before my variac for decades and not visa versa as suggested above. I may be greying out here but why does that matter? or does it?

[Greg_L]

In general I don't use an isolation transformer for most amp work but as Enzo says: except for SMPS work. I have two mains feeds to my bench coming from a 1800watt power conditioner to stabilize the mains a little. From there it splits to a 5 amp, limiter/variac source and to a 2.5 amp isolation/variac source. That plus a few variable DC supplies is about it for me.

Now... I've been using my isolation transformer before my variac for decades and not visa versa as suggested above. I may be greying out here but why does that matter? or does it?

I think you only want to isolate the part being tested? Your scope and all the other stuff still needs mains ground. The part being tested needs to float? I think that's the logic behind it.

[christarak]

Thank you gentlemen for your advice. Based on your excellent suggestions I will get an Iso and:
1. Get into the habit of using it on every amp I work (so I don't have to think about when I definitely have to use it)
2. Use the order of: Wall > Variac > light bulb limiter > iso transformer > amp

[christarak]

Thanks Greg. That was my understanding, too. I always clipped my scope to the amps earthed chassis and I haven't had a problem. I guess I was nervous about cutting myself free from the safety of having an earth of the amp I was working on.

[Greg_L]

Thanks Greg. That was my understanding, too. I always clipped my scope to the amps earthed chassis and I haven't had a problem. I guess I was nervous about cutting myself free from the safety of having an earth of the amp I was working on.

My understanding is that as long you're grounding your probe to amp chassis ground, then it's no big deal. No iso transformer necessary for that.

But since the probe's ground is actually grounded to earth through the scope, that makes it unsafe to use the probe's ground clip on anything other than chassis grounds. Whatever you attach the probe ground clip to BECOMES GROUND since it's grounded through the scope. That makes an iso transformer necessary for using the ground lead as a secondary probe when measuring across points that can't be grounded.


But....don't take my word for it. My understanding is rudimentary at best. That's just my way of understanding the role of iso transformers and when you need one.

[Enzo]

It is even more important when servicing SMPS, on the primary side, the negative side of the main power supply is at -170v with respect to ground, not ground. Clip a scope probe to that and you short out the supply.

[christarak]

It is even more important when servicing SMPS, on the primary side, the negative side of the main power supply is at -170v with respect to ground, not ground. Clip a scope probe to that and you short out the supply.

Oh, That's not good.

[Enzo]

But as Greg L pointed out, if your SMPS is on the isolator, then you CAN connect a scope ground to that point. Just remain aware of that as you work.

In the case of many SMPS, there will be a control IC, and it has a ground reference at that -170v point. The IC runs on either 5v or 12v usually, and so the +12 the IC sees is really -158v. (12v more positive than the "ground"reference.)

[cerrem]

It's typical to use DIFFERENTIAL probes when working on SMPS....
I use them all the time, especially when measuring signals in guitar amps... to avoid ground loops which can give erroneous measurements......
SMPS come in many flavors of topologies....
For MIL stuff I design isolated ones.... For Electric vehicles have design Bi-Directional non-isolated...
As for ISO-Transformer.... Keep in mind that there is no more NEUTRAL on the secondary of the ISO... It becomes HOT L1 and HOT L2 and GROUND...
ie, 120V in.... out is 60V to GND to 60V ...
So amps with chassis NEUTRAL or "DEATH CAPS" need chassis reference to GROUND..by way of NEUTRAL..
The winding capacitance of the Power Transformer is different since the voltage gradient across the windings is no longer 0V to 120V....so sometimes you can a slight difference..

[Enzo]

Differential probes are great, but the average guy doesn't have that equipment.