Variac question

How does the isolated one work? (I've never heard of one.)

George H.

There's no reason I can think of why that might happen, unless you don't really have an isolated variac and instead have a triac/SCR circuit on the primary of a transformer that provides the actual isolation.

Some Navy gear I worked on had that (back in the 70's) and it was noisy.

They do make them. Primary winding is close to the core, and the variable tapped winding is on the outside and isolated - bulkier than the usual auto-transformer style of variac, but otherwise identical. They can also be step up or step down at max output voltage.

I never took the thing apart. I bought it used so I don't have a schematic of the insides. Looks like I should be taking it apart. That's OK, I love looking under the hood. Thanks, Eric

IIRC AutoTransformers can do about 2X the VA of a regular transformer for the same core size. ...Jim Thompson

I'm curious now -- report back if you find out.

could be a saturation issue

I was thinking that, but triacs sound like a better candidate. It seems like the saturation would have to be BAD to make a follow-on device noisy.

Yeah. The transformer in this case is in a battery charger. I also, just to see, plugged my regular non- isloated variac into the isolated one. BIG hum. I have been too busy the last couple weeks to take the isolated one apart but I'll let everyone know what it looks like inside later this week. Eric

** If the OP's unit does use a triac, the likely cause is triac misfiring leading to "half waving" of the AC voltage.

Half waving is more likely with inductive loads ( lagging phase angle) and two transformers in series qualifies for that.

Only happens with pulse firing rather than continuous or hard firing.

.... Phil

Humming noises involve using too little copper on the load side (skimping). I'd try increase the diameter of the white or grey wires there.

I'm not sure you need to take it apart. If it is similar to an autotransformer but with an isolated primary, it will have a largish knob in the center of the transformer that is obviously turning a wiper across the coil. If it is a potentiometer driving a triac type circuit the knob will likely be a small knob or slider. If the distinction in your case is not clear, then I guess you will need to take it apart.

Took it apart last night. As you said it has the isolated primary. No semi-conductors inside, just wires and wipers. Eric

semi-conductors inside, just wires and wipers. Eric "

If you actually have an isolated variac hold on to it. Most of them are simple autotransformers. They work but hot ground is still hot. An isolated one allows you to do more, like work on the primary side of a power supply without floating the scope.

With all these old amps around here I am pretty sure I could dig up something that would put out 120 VAC isolated, but being the lazy asshole I am I just float the scope.

Variacs are actually losing their usefulness. Some modern SMPSes won't start if the voltage comes up slow. There are actually some circuits that will self destruct if brought up slow on a variac. But I certainly would not throw it out.

It sez it's isolated. And I am not going to get rid of it. I have found uses for both of my variacs over the years. One function they serve is to remnd me to pick things up. Hitting one with your little toe doesn't have as near as much effect on the variac as it does on your toe. Eric