Planning to build a low voltage ac supply

[saturated]

I'm thinking that using my ugly 120v variac to get like ten volts AC is a pretty bad idea 


So here's my latest plan: 

I have another autotransformer and a few transformers

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So how about mains to transformer (24v) and transformer to autotransformer 

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However my other question(s) is could I not just run the transformer output to a potentiometer to adjust voltage?

(Yeah dirt daubers are bad here)

[J M Fahey]

It depends on what will that variable AC feed.

If very little current (a few mA) a rheostat (heavy duty wirewound potentiometer) will do, but for any significant load, a transformer is better.

IMPORTANT: A Variac does NOT isolate you from Mains, so secondary is still very dangerous/deadly (pick one), **even if you set it to low voltage**

Now through a 120 to 24 Vac *isolating* transformer as shown, it becomes safe.

[Tassieviking]

I would try using the 240v as the primary tap and then measure the 24V and see what you have.
It should be safe to put 110V on the 240V primary winding, you will just get a lot less on the secondary side.

Transformers work on a ratio so drop the primary voltage by half you should get about half the secondary out.
Just don't put a higher voltage on a transformer then what it is rated at.

The other way is to use the variac to supply the transformer so you could regulate it that way
Cheers
Mick

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Yes sir thanks guys 

I put the big ugly 120 (140) volt variac together as an enthusiast but it has gathered dust.  Even though it has two fuses in it I'm looking for a smaller safer tool.

I'm not that guy rebuilding amps and bringing up full voltage on a variac so it's pretty useless for me...or I'm useless to it 

I was using the big variac one time and it threw my breaker  I was surprised to see both fuses still good and checked all my wiring for shorts 🩳 to no avail and have used it since.....idk.

I just plan to use this for my breadboard textbook experiments when I need some ac like 15v or whatever 

Btw I have read ad nauseum about isolation transformers and floating etc and it makes my head spin     

[saturated]

I would try using the 240v as the primary tap and then measure the 24V and see what you have.
It should be safe to put 110V on the 240V primary winding, you will just get a lot less on the secondary side.

Transformers work on a ratio so drop the primary voltage by half you should get about half the secondary out.
Just don't put a higher voltage on a transformer then what it is rated at.

That's really cool damn I wouldn't have thought about that in a million years 

Unfortunately I would receive another slap upside the head guessing the secondary would be >24v   

I really like these 🥼🧪 experiments

I'm curious to see the outcome though seems like I should be able to calculate it 

I'm not gonna try it but I wonder what would happen if I connected mains to the secondary  (600V ?) ¹

[g1]

If you put 120VAC to the 24V secondary, you risk exceeding the voltage ratings and possible arcing or other damage.
Aside from that, it would be stepped up at the primary side, either by a factor of 5 at the 120V tap, or a factor of 10 at the 240V tap.
So your 600V is theoretically correct, and there would be 1200V at the 240V tap.  Also, it would not be able to deliver much current at those voltages.

[Tassieviking]

g1 has it 110% correct, you should never apply a higher voltage to a transformer then what it is rated at, the only exception is if you have the full specs and  it states what class the insulation is on the winding. ( max volts it can handle )
Often the primary windings will have a different class of insulation then the secondary windings, they do that to save money as the higher the voltage the more or better quality the insulation has to be.